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Although dry paper is widely used as a supporting and insulating material in telephone cables, the published data with regard to its specific inductive capacity (or permittivity) appear to be very meagre. For this reason Mr. Gavey, C. B., Engineer-in-Chief of the Post Office, asked us to investigate the matter, and sent for test a large number of samples of paper obtained from four different cable manufacturers. All the samples consisted of what is known as “chemical wood paper,” presumably free from lignified fibre. This type of paper, according to the Society of Arts Report on the Durability of Papers, is better in lasting quality than “mechanical wood paper” or paper made from straw, jute or esparto grass. Mr. Gavey, in addition, has kindly supplied the results of some tests on actual cables and further data for some of the samples of paper. This information is embodied in Part II of the present paper. Part I.–Tests on dry paper. One of the main difficulties in the testing of paper lies in the fact that it absorbs moisture so readily; and the presence of moisture has a large effect on the specific inductive capacity and an enormous effect on the insulation-resistance. The nature of these effects is well illustrated by the curve in fig. 6 (Part II, p. 204), which shows how the capacity increases and the resistance decreases as a well-dried cable is allowed to absorb moisture from the atmosphere.

1. In the paper recently communicated to this Society, Dr. Duffield has described a promising method of determining gravity at sea, but a somewhat fuller discussion on the theoretical side is required before an opinion of its capabilities and limitations can be formed. For this purpose it is more especially necessary to investigate the effects of the forced vibrations which may be imposed on the mercury in the apparatus by the vertical acceleration of the ship. 2. The problem of determining the period of free oscillation in a barometer tube is very simple, but in the present case three tubes are connected with each other, and at least two of the tubes are not of uniform section, but contain a portion with a narrow bore introduced to damp the oscillations.

The principal object of this paper is the connexion of the results deduced in a former paper from the observations at the Royal So­ciety’s Apartments, with the observations at the Royal Observatory at Greenwich, in order to determine mean numerical values, and to establish the laws of periodic variation from this long series of obser­vations ; the two series of observations are here induced to one and the same series. The observations at the Royal Society having been discontinued between the years 1781 and 1786, it was necessary to supply this link in the series, more particularly as these years were distinguished by very severe weather, and their omission would have a sensible effect on the results. The deficient observations have been supplied by a comparison of the observations which were made at Somerset House, with the observations during the corresponding years made by Mr. Barker at Lyndon in Rutlandshire, from 1771 to 1799, cor­rections being thus obtained for reducing the Lyndon observations to those at Somerset House.

The author states that, from a microscopical examination of the blood-vessels and circulation in the ears of the long-eared bat, he has ascertained that, different from what he had discovered to be the case in the wings, the veins of the ears are unfurnished with valves, and are not endowed with rythmical contractility, and that the onward flow of blood in them is consequently uniform.

The author inquires whether the peculiar coloured dispersion of white light intromitted into a solution of sulphate of quinine, is the result of an analysis of the incident light into two distinct species, or merely of a simple subdivision analogous to that which takes place in partial reflexion, as exemplified in the colours of thin plates. He endeavours to ascertain the laws which regulate this singular mode of dispersion, which for brevity he terms epipolic , on account of the proximity of the seat of dispersion to the intromitting surface of the fluid. It might have been expected that by passing the same incident beam successively through many such dispersive surfaces, the whole of the blue rays would at length be separated from it, and an orange, or red residual beam be left: but the author establishes, by numerous experiments, the general fact, that an epipolical beam of light , meaning thereby a beam which has been once transmitted through a quiniferous solution, and undergone its dispersing action, is incapable of farther undergoing epipolic dispersion. There were only two liquids, out of all those examined by the author, namely oil of turpentine and pyroxylic spirit, which, when interposed in the incident beam, act like the solutions of quinine in preventing the formation of the blue film: and the only solid in which the author discovered a similar power of epipolic dispersion, is the green fluor of Alston Moor, and which by this action exhibits at its surface a fine deep blue colour.

1. No systematic investigation on this subject has yet been attempted. The efforts made to group these materials in a series according to the charges generated when they are rubbed two at a time are of uncertain value because of (1) impurities of substance and surface, (2) indeterminate initial surface strains. Moreover, the general practice has been to rub the solids when held, one in each hand. A more reliable method is to mount the bodies in a mechanism which ensures that the same parts of them come into contact at each fresh stroke. In the present research, the elements, mostly metals, are those specified in the preceding paper. The textiles are made and cleansed as described in an earlier paper. Silk, cotton and linen in our experience behave reliably when fully cleaned. The other great textile material, wool, is more oily and cannot be readily cleansed; so is less trustworthy. We therefore at present confine ourselves to one animal and one vegetable textile, choosing the pure samples of silk and cotton specified in the above paper. Also filter paper, another convenient fabric, is used after being boiled in several lots of distilled water. As the silk and cotton are treated with boiling chloroform, we tried exhausting the filter paper in this solvent to see if such treatment affected the cellulose in any way; but no effect on the properties of the paper could be found.

The difficulties attending continuous registration of electrical potential gradient of the atmosphere are very great; but it is not my intention in this paper to discuss these. My object is to call attention to the measuring part of an electrograph, and more especially to experiments made with an instrument that promises to be of considerable service in work of this kind. A recording voltmeter ought to have a uniform scale value over the range for which it is intended to be used, it must possess a high degree of insulation, and it ought to be capable of acting efficiently for long periods without having to be taken to pieces for cleaning. The general excellence of the Dolezalek electrometer and its high degree of mechanical symmetry suggested to me its use as a recorder, although I do not know that it has been tried before. The instrument was designed to measure very small differences of potential between the quadrants with a potential of, say, 100 volts on the needle. The first thing to ascertain was whether, with a fixed difference due to a single Weston cell, the sensitiveness could be reduced so that the movement of the needle would record, on a suitable scale, the potential applied to it. A trial suspension made of phosphor bronze proved successful, and a scale value of about 200 volts per centimetre was obtained on the photographic paper, which is carried on a drum 1 metre from the mirror attached to the needle Up to 500 or 600 volts the behaviour was excellent, but for higher potentials the needle began to tilt, and if a sudden change was made the needle generally discharged to the quadrants. Experience showed that it would have frequently to carry over 1000 volts. The needle was therefore loaded by prolonging the vertical axis to about 3 cms. beneath the needle, and adding at the end a small brass nut of about 1½ grm. It now carries 1100 volts with perfect safety and stability.

This paper embodies the results of experimental investigations, carried out during the past five years in the Fuel Laboratories of the Imperial College of Science and Technology, South Kensington, into the resinic constituents of bituminous coals and their supposed determining influence upon the coking propensities thereof. The property of “coking,” when carbonised at 900°-1000°C., is not inherent in all “black” coals, but varies from zero up to a high degree. Although, to some extent, it appears to be connected with the amount of volatile matter yielded by the dry ashless coal substance at such temperatures, the connection is by no means so direct as is sometimes supposed. Thus, whilst it may be said to be generally non-existent in coals yielding below 15 or above 45 per cent, of “volatiles,” and to be at its maximum in those yielding between 15 and 30 per cent., and whereas also it gradually diminishes as the yield of “ volatiles” rises above 30 per cent., yet there is no corresponding gradual diminution as the yield falls below 20 per cent. Indeed, in the region of 15-20 per cent, of “volatiles” there is an abrupt failure of coking properties; for in this region—and particularly at about 15 per cent. —are to be found not only some exceptionally strongly-coking coals, but also many that are quite devoid of the property. The reason for such an apparently sudden disappearance of coking properties in the said region remains one of the unsolved questions in the chemistry of coal.

1. When reading a recent paper before the Royal Society, and also in the Press, Dr. Edridge Green has stated that he can find no connection between the luminosity and the colour sense of persons having either normal or abnormal colour sensations. Since I feel that to allow such a statement to go unchallenged might be interpreted as meaning that no such connection could be shown to exist, I propose in the following paper to place before the Society some of the evidence which indicated that there is in reality a very intimate relation between luminosity and colour sense. The results given include a small part of those which have been obtained in a series of experiments which have occupied the last two years and form part of investigation which is still in progress. The term “luminosity” as used in this paper has the following meaning: Suppose that light from some source, such as the electric arc, is admitted to a spectroscope by means of which a real pure spectrum is produced, and that a slide in the plane in which the spectrum is formed carries a slit of fixed width. Light of sensibly one wave-length, i. e. monochromatic light, will pass through this slit, and by means of a lens placed in the beam of this light an image of the first face of the prism which is used to from the spectrum can be formed on a screen. In this way a monochromatic patch of light is obtained, the brightness of which depends on the nature of the source of light, the width of the collimator slit, the width of the slit placed in the spectrum, which for short will be called the movable slit, and the dimensions of the lenses employed. Further, if alongside this coloured patch is formed a white patch of light produced by light which proceeds from the same source but has not undergone dispersion, and that by some means or other the intensity of this white light is altered till the coloured and white light, measured in any arbitrary units, measure the luminosity of the light of that colour which is passing through the movable slit. Since the unit in which the white light is measured is arbitrary, we are not concerned with the absolute intensity of illumination of the white patch, and may use any device we like to alter the quantity of white light which falls on the screen so long as we are able to measure the ratio of the quantity of light employed in different experiments. It will further be observed that for any given person the measurement of the luminosity of a given coloured light in the spectrum involves the comparsion of the brightness of the coloured patch as it appears to him with the brightness of the white patch as it appears to him.

In the first section of this paper it is pointed out that one and the same metal may cause a great variety of different colours, just as the colour of gold vapour differs from the colour of the light reflected from gold as well as from the colour of the light transmitted by gold leaf. While the ultimate cause of the colour of a metalliferous medium is to be found in the structure of the metal molecule itself, the arrangement of these mole­cules, according to any fixed law, causes them to affect one another’s free periods in a definite manner, and thus gives rise to corresponding optical properties. The object of this paper is to discover, by means of these optical properties, the molecular arrangement (microstructure) of various metal glasses, of colloidal solutions of metal and of metallic films. Expressions giving the refractive index and the absorption coefficient (the optical constants) of a compound medium consisting of metal (1) in small spheres (granular), and (2) in discrete molecules (amorphous), diffused through an isotropic non-dispersive transparent medium (the solvent), are next investigated in terms of the corresponding optical constants of the normal metal.

Robertson and Fox in their fundamental memoir have published a comprehensive account of the precautions necessary for obtaining reliable results with a prism spectrometer in the infra-red between 1 and 17 μ. The authors feel, however, that some advantage to other workers may accrue if they in turn outline their own particular difficulties with the manner of their resolution. it may be stated at once that it is how possible to make scale readings to an accuracy of 1 mm. during the day , with a galvanometer sensitivity of 10 -10 to 10 -11 amp. per millimetre deflection at 1 metre distance. If it is realised that the work is carried out on the top floor of the Chemistry Department of this Institution, in a building which houses some dozen electrical generators and a large liquid air plant with a stream of heavy bus traffic 10 yards from the building and a tube railway at 100 yards, it will be seen that the usual experimental difficulties have been completely overcome. Preliminary experiments were made in a constant temperature room in the basement; the humidity was high and had a deleterious effect on the rocksalt portions of the apparatus, whilst the discomfort of night work in such a room was extreme. It has consequently decided to re-approach the problem from the beginning, but not before trials had shown that where the motion of the building as a whole is small, comparatively simple expedients serve to eliminate most of the mechanical disturbance. This was cut out by placing the galvanometer with its Paschen shields (total weight about 250 lb.) on a large block of wood supported by for pairs of inverted small porcelain crucibles placed at the corners. Eight of the thinnest available crucibles were selected; the two in a pair were placed one on top of the other and separated by a small piece of 5-ply wood and a few pieces of filter paper. If the galvanometer and shields are not too heavy, the replacement of the rubbershock absorber in the ordinary anti-vibration spider by a crucible and one or two layers of filter paper was found very efficient.

In a previous paper by the writer an examination was made of the torsional vibrations occurring in a reciprocating engine shaft. The case studied was that of a shaft driven by a six-cylinder engine, all parts except the shaft being considered rigid. Certain interesting results, arising from the action of the reciprocating parts, appeared. On account of the importance of the subject of torsional vibrations in engineering practice, and on account of the unexpected nature of the analytical results, it was considered of some interest to examine in greater detail the effect of the reciprocating parts in producing or modifying the vibrations. For this purpose a model is proposed which, it is considered, is as simple as can be conceived, and which, at the same time, involves the characteristics of reciprocating motion. The model is described and the analytical results obtained in Part I of this paper. To verify the analysis, the model was set up in the laboratory and the vibrations observed. Further, to simplify the experimental work, it was thought sufficient for the purposes of verification to arrange it in the first place so as to give only a qualitative agreement with theory, and therefore a precise quantitative agreement was not aimed at, and should not be looked for. An experiment on a much more refined method is being proceeded with to examine minutely into the numerical accuracy of the theory. The details of the present experiment and the numerical results are given in Part II.

In a previous communication to the Society, an account was given of an investigation of the magnetic shielding of a large space, by the use of a series of spherical iron shells. The basis of this investigation, which is still in progress, is strictly quantitative, and the work has been retarded, to a great extent, by the fact that several processes adopted commonly by various investigators of magnetic phenomena have never been examined accurately on the theoretical side, and in several cases even on the experimental side, to an order of precision which is sufficient for delicate measurements. One of the main objects ultimately in view, in the work described already, is the production, throughout the volume of a large iron specimen, of a magnetic field so small, in comparison with that of the earth, for example, as to render possible some accurate conclusions regarding the behaviour of iron under indefinitely small magnetic forces. This behaviour is at present a matter of doubt, and it remains to be shown that the indications of its real nature, obtained in earlier papers by one of us, continue to point in the same direction when the force is decreased below any values which have been employed hitherto. In the experiments described in the former paper, a field so low as 0·0033 C. G. S. unit was obtained in a spherical space of radius 32·5 cm. by the use of four concentric shells. It was accompanied, however, by a leakage field of 0·006 C. G. S. unit, and the shells still showed some traces of permanent polarisation after continued treatment with many reversals of a slowly decreasing current. This method of demagnetisation is used extensively, and in its application coils have sometimes been wound somewhat loosely on the iron to be demagnetised, and the constant field given by Maxwell’s formula for a spherical internal space surrounded by a very closely wound coil assumed to exist. But if a thin shell is wound in this manner, although the more internal parts may be subject to this field, the results must be widely different in the outer material of the shell. Further experimental work on the improvement of magnetic shielding beyond the order obtained already— in fact by such methods as the employment of a very small magnetising current to remove, in the internal space, the small part of the earth’s field which may remain after the operation of the shielding effect of a set of shells —requires a precise knowledge of the value of the magnetic induction in a shell, or one member of a set of shells, wound by a spherical helical coil. For the condition of indefinitely close packing is found to be essential to the success of Maxwell’s formula, which—but only from this particular point of view—does not appear, either from an a priori theoretical consideration, or from the experimental results described in the present paper, to have the large range of validity which has been assumed for it hitherto by practical workers on magnetic phenomena.

1. In previous papers I have investigated the wave-making resistance of a ship by comparing it with a certain simple type of pressure disturbance travelling over the surface of the water. In a recent paper on the effect of form and size on the resistance of ships, by Messrs. Baker and Kent of the National Physical Laboratory, reference is made to this point of view. The main work of these authors consists in the examination of model results and the deduction of empirical formulæ of practical value. In addition, they connect the wave-making properties with the pressure distribution and have obtained graphs of the latter for various ship forms under certain conditions; these curves show a range of negative pressure, or defect of pressure, between the positive humps of excess pressure corresponding to the bow and stern. The authors remark that this will have an effect upon the wave-making, but conclude that it is sufficient for their purpose to be able to state that such pressure disturbances, as they have shown to exist when a ship is in motion, will produce waves which will vary more or less in accordance with the theory referred to above.

In a former paper I showed that when a barrier fits tightly against the sides and across the bottom of a vessel of oscillating water (1) a vortex forms against each side of the barrier in turn as it becomes the lee side; (2) this vortex never forms while the water is attaining to the mean level, but only while it is falling below and rising above that level. I suggested that such vortices were due to the conjunction of the mainstream flowing over the barrier with opposing local streams created by local differences of pressure set up in the neighbourhood of the barrier by the oscillating water. Exception was taken to this explanation, and also, later, to experiments made with a box partially covered with thin gutta percha diaphragms, for the purpose of proving the formation of the local differences of pressure alluded to. The object of the present paper is to give an account of further experiments carried out for this same purpose, with pressure indicators which are free from the objection urged against the diaphragms. viz: that they themselves might cause variations in the distribution of pressure.

In a recent paper in these Proceedings I described a method by means of which the mariner is enabled to foretell risk of collision at sea, assuming that be knows the course and speed of each of the vessels concerned and that he is at intervals able to ascertain tbe distance separating them. For an account of the principles involved I must refer to my former paper. A simple geometrical construction enabling them to be applied is therein given. This construction informs the navigator whether there is risk of collision or not, and also enables him to ascertain the instant at which the danger is greatest. However simple in character, it is probable that, to the average seaman, the operations involved in a geometrical construction would appear less practical than a mechanical or instrumental mode of interpreting the obser­vations. It will be an additional attraction to an instrumental method if time be saved by its use. For the time available for the solution of the problem involved may be short. This might well be the case if the earlier signals for any reason escaped notice. Accordingly I have endeavoured to reduce the work of interpreting the observations to the simplest form. Of various types of apparatus for the interpretation of the geometrical principles involved, that which I now describe is, I believe, the easiest both to work and to construct.

In the development of the study of crystals by X-rays the methods used divide themselves naturally into four types : the Bragg Ionisation Spectrometer method, the Laue method, the Powder method of Debye and Scherrer, and the Rotating Crystal method of Rinne, Schiebold and Polyani. The techniques of the first three of these methods are fully explained in such books as ‘ X-Rays and Crystal Structure,’ by W. H. and W. L. Bragg, ‘ The Structure of Crystals,’ by Wyckoff, and ‘ Krystalle und Rontgenstrahlen,’ by Ewald, as well as in original papers. On the other hand, the rotation method is only slightly touched on in these works, the literature is scattered in a great number of papers, and the technique has not so far been described at any length in a convenient form. Particularly in English, references to it are scanty. In this paper the author has tried to give a concise account of the method, together with various types of charts and tables as it is used in the Davy Faraday Laboratory. The methods described differ in certain respects from those used on the Continent,* but they have been found to be rapid and sufficiently accurate.

This paper arose out of a discussion with Mr. Glauert on the validity of certain results on wind-tunnel interference given in a recent paper by Theodorsen, and I am greatly indebted to him for several very valuable suggestions. In his paper Theodorsen investigated, on the lines of the approximate theory laid down by Glauert, the interference factors due to rectangular tunnels in the following five conditions: (1) tunnel entirely enclosed; (2) free jet; (3) horizontal boundaries only, the vertical sides being free surfaces; (4) vertical boundaries only, the horizontal sides being free surfaces; (5) bottom boundary only, the remaining sides being free surfaces. Apart from one or two minor errors, Theodorsen's paper has to face a more serious objection on the score that the approximate method employed is not always valid. This objection to the approximate method was raised rather briefly in a previous paper, but at that time all the cases discussed, using the approximate method, were those due to Glauert, and they were such that the approximate method did not come into conflict with the exact theory. The more complicated method was, however, of some practical importance as it enabled additional refinements to be introduced into tire numerical evaluation of the interference factors, and it enabled Glauert to show that there is an appreciable drop in the interference factor in the Duplex Tunnel (height/breadth equal to one half) as the span of the aerofoil increases. Some of the cases discussed by Theodorsen are such that they must be considered according to the exact theory. The crux of the matter is as follows:— The basis of the theory of wind-tunnel interference is due to Prandtl, and he investigated several cases himself. On the assumption that trailing vortices spring from the aerofoil and extend downstream without distortion, Prandtl showed that the whole problem can be converted into one dealing with the flow in a transverse section of the wake far behind the aerofoil, the necessary boundary conditions being that the velocity potential is constant over the surface of the free jet, and the stream function constant over the rigid boundary. The theory was extended by Glauert and others, and applied particularly to the case of the small aerofoil in a rectangular tunnel. When the aerofoil is very small the exact distribution of vorticity is not of extreme importance as, to a first approximation, they all give the same result.

This paper is supplementary to one by the same author published in the 'Proceedings of the Royal Society' in 1899, "On Flapping Flight of Aeroplanes," communicated by the late Prof. George F. FitzGerald, F. R. S. Towards the end of that paper, the author refers to the subject of hovering, and points out that the method of his paper does not apply, in any satisfactory way, to that case. The paper of 1899, in fact, considered the force supporting an aeroplane as due to its motion with nearly constant speed in a nearly horizontal, but slightly waved path, to which its plane was inclined at a small variable angle.

It is sometimes necessary to count physical events which are repeated very rapidly. An example of this occurs in problems connected with radio-activity, where it is desired to count the number of α-particles entering a collecting chamber. One method of doing this is to amplify (by means of thermionic valves) the minute ionisation currents produced in the chamber by single α-particles, and to pass the amplified currents—which take the form of impul­sive surges of about 10 milliamperes—through telephones, counting the “clicks” by ear. For more rapid working, the currents are passed through an oscillograph, and a photographic record is obtained, which can be analysed at leisure after development. It is often necessary, however, in addition to recording the particles rapidly, to know the result immediately, so that further observations may be made on points of special interest. In a recent paper it has been shown that it is possible to use a mechanical relay and a counting meter (similar to a cyclometer) in conjunction with a valve amplifier for auto­matic numerical counting of α-particles. Each 10 milliampere current surge (corresponding to a particle) passes through the relay windings and closes the contacts momentarily, thus completing a local circuit containing the counting meter. Such a method of counting, while extremely simple and convenient, suffers from one great drawback. However sensitive the relay and counting meter mechanism may be, a small but finite time is required by their armatures to move to and fro in the process of counting. If, therefore, a second particle arrives before the recording of the first has been completed, it will be missed. Thus, this simple method of counting is unsuitable when the counting speed is such that there is a probability of a second particle arriving within the recording period. While it is true that a probability correction could be applied, it is evidently preferable to endeavour to make the system record all particles, however rapidly they may arrive. In the present paper it will be shown how this defect may be overcome by the use of thyratron valves in place of mechanical relays. Such valves are employed in the present work as direct current relays, and can be regarded as having negligible inertia, and no moving parts. Further, they are much more certain in action than mechanical relays, and can control larger currents for operating counting meters than could be safely carried by the contacts of delicately adjusted mechanical relays. The principle underlying the circuits which are described in the paper is that the process of counting is shared by a group of inter-connected thyratrons, which automatically switch one another into operation as need arises, so that however rapidly particles are arriving, there is always a thyratron available for each particle. The switching process is entirely electrical , and, as it involves no mechanical operations of any kind, it can be made to function extremely rapidly. The auxiliary mechanical apparatus which may be employed ( e.g ., counting meter) is given an oppor­tunity of operating comparatively slowly, and is not necessarily the governing factor which imposes a limit to the counting speed. A suggestion is also made at the end of the paper as to the possibility of constructing a “counting meter,” the dials of which consist entirely of thyratrons, and contain no moving parts of any description.

In previous communications various experimental methods of examining the characteristics of downcoming wireless waves have been described. Of these methods by far the most useful has proved to be that which involves a small continuous change of transmitter wave-length, by means of which there is produced at a receiving station a succession of interference maxima and minima between ground waves and waves deviated by the upper atmosphere. By using loop and antenna receiving systems in several different ways it has been possible to estimate, from a comparison of the interference “fringe” amplitudes, the relative intensities of the ground and atmospheric waves and also the angle of incidence of the latter at the ground; while a comparison of the phase differences between the two sets of waves, as deduced from the interference “fringes” recorded on different types of aerial systems, has yielded information relating to the polarisation of downcoming waves. In all such investigations, however, the number of “fringes” produced by a known wave-length change has been found, even when the determination of that number was not the primary object of the experiment. In this way the equivalent path difference for the ground and atmospheric wave tracks has been calculated as a routine measurement from which the equivalent height of the ionised layer could be deduced. In the course of such investigations carried out at the Peterborough Radio Research Station, much evidence has therefore accumulated relating to the diurnal variation of the equivalent height of those regions in the atmosphere which are responsible for the deviation of wireless waves. As was pointed out in the first communication dealing with the work of the station, considerable interest is, in the study of wireless wave propagation, attached to the transitional periods of sunrise and sunset. As the experiments under discussion were usually carried out on wave-lengths of from 300 to 600 metres ( i. e ., on frequencies of from 1000 to 500 kilocycles per second) the study of the sunset period was, except on rare occasions, impossible, because of interference from broadcasting transmitters. The majority of the tests therefore took place during the sunrise period when, in addition to the advantage of the low level of artificial interference, there was also that of the early morning minimum of natural atmospheric disturbances. In carrying out such early morning runs it was soon found that, in the few hours before sunrise, the signal records showed the presence of both primary and secondary interference maxima and minima, indicating the simultaneous reception of two or more downcoming rays. After sunrise the phenomena became simpler and smooth “fringes” were recorded. This latter particular period was therefore especially suitable for the determinations of the characteris­tics (intensity, angle of incidence, polarisation, etc.) of downcoming waves already described. The present communication, is in part an attempt to elucidate these pre-sunrise phenomena and in part an introduction to two papers which are to follow, since it was the study of the significance of multiple downcoming rays which led to the investigation of the subject with which these two papers respectively deal, namely, the simultaneous reception of downcoming waves at several receiving stations and the use of shorter wave-lengths.

It has been shown in the previous paper that, where there are four refractive centres arranged dissymmetrically in a molecule, there is a definite rotatory power depending on the dimensions of the molecule. In the simple active molecule of four different radicals we seem to have been provided directly with four different refractive centres. It is, however, necessary to examine if a chemical radical fulfils all the properties which were assigned to the hypothetical particle in the general calculation; but, if this true, we can substitute dimensions of such a molecule in the general formula to device a simpler formula for the rotation. If the radical is to have the same properties as the polarizable particle, it will have a definite refractivity for a fixed wave-length and its electric field in the polarized state must approximate to that of a dipole of equal value situated at its centre. A chemical radical has a definite refractivity for a given frequency, but it is only if the radical is small and fairly compact that its ield in the polarised state may be taken as that of a dipole concentrated at a single point. Consequently this paper will be limited to the calculation for molecules containing radicals of only a few atoms in length. The refractive properties will be regarded as concentrated at the geometric centre of these small radicals.

Investigations on the initial directions of emission of photoelectrons produced by X-rays show that the distribution depends on the direction of propagation of the rays, whilst in the case of a polarised beam it depends also on the direction of the electric vector of the incident radiation. The distribution of the initial directions of the photoelectrons relative to the direction of the incident X-ray beam is usually called the “longitudinal distribution,” whilst that relative to the plane containing the electric vector and the rays, in the case of a polarised beam, is termed the “lateral distribution.” The present paper will deal only with the longitudinal distribution of the photoelectrons. In the first place a survey of the experimental situation will be given, from which it will be seen that the results of previous observers are not in a satisfactory state of agreement. Secondly, it is the purpose of this paper to give an account of the authors’ own experiments on the longitudinal distribution in which it is hoped that some of the possible errors giving rise to the discordant results of previous investigators have been eliminated. The main interest of the present work lies in the results obtained for the asymmetry, which show that the average forward momentum of a photoelectron is nearly 40 per cent, greater than that of an absorbed quantum. This result of our experiments was first announced in a thesis published in April, 1927.

In a recent paper, Ward, Wynn-Williams, and Cave have described a method of counting accurately the number of α-particles emitted per second from a radiactive source. This method,-the practicability of which was first shown by Greinacher,-involves the amplification by means of thermionic valves of the small ionisation current caused by an α-particle in a few millimetres of its path in air. The amplification can be made linear, so that the final current which actuates the recording instrument is proportional to the ionisation produced by the particle in the ionisation chamber. Compared with that produced by an α-particle, the ionisation produced by an β -particle is too small to be recorded under working conditions. In their work on the determination of the number of α-particles emitted per second from a gram of radium, the magnified current was passed through an Einthoven galvanometer, and clear photographic records were obtained when the α-particles were entering the chamber at a rate of several hundred a minute. Under the conditions of the experiment, the β and γ-radiations were too weak to cause any disturbance, thus enabling the determination to be carried out with accuracy. For many purposes, however, it is seecential to count particles in the presence of strong γ-radiation, and to achieve this end special attention had to be paid to the amplifying and recording system. An account will be given in a later paper by Wynn-Williams and Ward, of the arrangements employed for this purpose, but it suffices to say here that the ions produced by a particle in the detecting vessel should be driven on to the collecting electrode in a time of the order of 10 -3 sec, and that the process of amplifying and recording should be sufficiently rapid to give a trace in an interval of less than 10 -3 seconds. For this purpose, a special oscillograph was constructed which has been found to be very effective. With these modifications it has been found possible to count α-particles at the rate of several hundred per minute even in the presence of strong γ-radiation.

In the ‘ Proceedings of the Royal Society,’ A, vol. 110, p. 64 (1926), the authors have described an investigation of the critical potentials corresponding to the values of the electron energy associated with some of the softer X-ray levels of the elements chromium, manganese, iron, cobalt, nickel, copper, and zinc. In this investigation a greater number of critical voltages were found for each element than had been anticipated. An attempt was made to deter­mine which of the critical potentials corresponded to the M I and M II. III levels for the element in question, by extrapolating the appropriate Moseley curves, and also to account for all the critical potentials in terms of single or multiple electron displacements from the M group. The significance of the various critical potentials obtained experimentally has also been further discussed in a subsequent paper by two of the authors. The results recorded in the present paper have been obtained from a continuation of the experiments, in which more convenient methods of measuring the photoelectric current produced by the soft X-rays have been employed. The experimental tube in which the electron bombardment of the various metals took place, and in which the resulting radiation was detected, has been described in previous papers. In the present research, the photoelectric current, which served as a measure of the radiation produced by the bombard­ment, was usually measured by observing the steady deflexion of an electro­meter needle instead of by timing its motion. This method has the advantage that it allows of a series of observations being taken more rapidly than is possible by the timing method, so that the chances of variations occurring in the con­ditions of the apparatus during the experiment are considerably reduced, and series of observations can be made over larger voltage ranges and at smaller voltage intervals. Moreover, it was found possible to make the electrometer measurements more sensitive by this method, an advantage which was especially useful when the slowest bombarding streams were being used.

The special interest for metallurgists of the phenomenon described in the title of this paper, particularly owing to the help it gives towards under­standing the still rather obscure nature of the causes of hardening itself, encouraged the authors to carry out further work on lines suggested by the results of our previous experiments. The latter have been recorded in a previous paper, which itself resulted from a still earlier paper by one of us, in which the original discovery of the phenomenon was recorded. This further work has added appreciably to our knowledge of the phenomenon and at the same time led to the discovery that, under certain conditions of quenching in which hardening does not take place, absorption instead of evolution of heat takes place.

The principal object of the researches which are communicated in this paper was to ascertain, with as much accuracy as possible, the differences in the transformations which modern explosives suffer when fired under gradually increasing, but considerable, differences of pressure. The first part of the paper gives a description of the varied apparatus which was designed or used in the research. Although the author has made experiments with many other explosives, those to which attention is devoted in this paper are three in number: (i) the cordite known as Mark I (for which the country is indebted to the labours of Sir F. Abel and Sir James Dewar); (ii) the modified cordite known as M. D.; and (iii) a tubular nitro-cellulose known as R. R. Rottweil.

In a paper recently read before this Society, the author described some experiments on the length of the Crookes dark space under varying conditions in different gases. While the gas hydrogen was under this investigation, a slight inequality of light was noticed very close to the cathode, which, under the prevailing conditions, was not definite enough to warrant further attention at the time. When, however, helium was introduced into the apparatus, this phenomenon became strikingly clear, showing itself to be a hitherto undescribed dark space very close against the cathode and inside the Crookes dark space, possessing very different properties from the latter. By the time the measurements of the Crookes dark space in helium were completed, the behaviour of the new dark space had suggested a simple explanation, which led to a series of experiments, of which this paper is a description. Apparatus . —The preliminary observations of the new dark space were made in the “guard ring cathode” discharge tube already described, but when it was realised that an accurate knowledge of the current density was not required, this was replaced by a smaller cylindrical tube, 8 cm. in diameter, with more uniform walls and plane aluminium cathode and anode. With this exception, the apparatus was entirely as described in the previous paper.

In several previous papers, I have described experiments showing that the luminosity of the spark or arc discharge through gases and metallic vapours lasts for an appreciable length of time after the discharge has ceased to pass, and that the luminous vapour can be removed from the region where the electric current is passing, and still remain luminous, showing the same spectrum as before, though in certain cases some lines of this spectrum fade out sooner than others. The present paper is a continuation of these studies. 1. The Luminous Jet at Very Low Pressures. Effect of a Transverse Electric Field . The first experiments to be described were designed to observe the effects at very low gaseous pressures, and with small currents. The advantage of this is that the luminous particles in the jet are then comparatively free to move under the action of an external force, and their motion in an electrostatic field can be conveniently examined. Moreover, since the ions are few, they do not disturb the uniformity of the electric field applied for this purpose.

The present paper deals with the results obtained in the investigation of the atomic structure of rutile and cassiterite by the X-ray spectrometer. A detailed account of the method has been given by Prof. Bragg and his son, W. L. Bragg, in a series of papers communicated to the Royal Society. It consists essentially in allowing a narrow beam of monochromatic X-rays—in this case the rhodium rays—to fall on the face of the crystals, mounted on a spectrometer table, the axis of rotation of which passes through the face of the crystal. The beam is “reflected” by the atom planes parallel to this face, and thence passes into an ionisation chamber, containing methyl bromide in order to increase the ionisation current. The setting of crystal and chamber with regard to the incident beam corresponds to that for which ordinary light is reflected.

In previous papers the authors have described the development of experimental methods of measuring the directions and relative intensities of both the electric and magnetic forces in wireless waves received at the earth’s surface from a distant transmitting station. In this work it was seen that the detection of the arrival of waves deflected from the upper atmosphere, and polarised with their electric force in a horizontal plane, was rendered difficult owing to the relatively great reflecting powTer of the earth resulting from its high conductivity. By a suitable choice of wave-length and careful attention to detail in the design and construction of the apparatus, however, the methods employed enabled measurements to be made on both vertically and horizontally polarised waves. The results of such measurements enabled a direct proof to be given of the fact that the fading of wireless signals on a vertical aerial and the variations of bearings experienced on the closed-loop type of wireless direction-finder are due to the reception respectively of vertically and horizontally polarised waves deflected from the upper atmosphere in their passage from the transmitter to the receiver. On arrival at the receiver, these indirect or atmospheric waves interfere with the direct or ground waves, in a manner determined by their relative magnitude and phase, and produce the intensity and apparent direc­tional variations mentioned above. The results of such interference phenomena have been investigated experimentally by Appleton and Barnett and by Holling-worth. In a more recent publication the present authors have provided experimental evidence showing that the path of the indirect waves is confined to the great circle plane between the transmitter and receiver. The measurements of the quantities in the received waves as previously described by the authors were confined to observations on the transmissions from the Bournemouth broadcasting station over a short period. The object of the present paper is to describe the continuation of these measurements and their extension to the transmissions from other stations.

I.- Historical . Little exact knowledge of tribo-electricity has yet been accumulated, and this subject has certainly not been raised to the dignity of a quantitative science. In the present paper will be found an account of experiments in which the conditions of the solid bodies rubbed together have been greatly varied. Thus, temperature has been changed both before and during friction; the surfaces used have been rubbed with considerable pressure while hot; they have, wherever possible, been rubbed together when flexed; and they have been prepared before rubbing by being ground and polished in various ways. Much information has thus come to light as to the electrical surface conditions of a variety of solids. Before setting forth the methods and results of this paper, let us note the information previously available. Good summaries of the subject will be found in Wiedemann's 'Electriciät' (2) and Chwolson's treatise. (1).

By the kindness of Sir William Ramsay and Prof. R. B. Moore, we have been enabled to measure the refractive indices of krypton and xenon with much larger quantities of these gases than were available at the time of their first isolation. The method of preparation of the gases will be given by Prof. Moore in a paper about to be presented to the Chemical Society. The procedure followed in determining the indices is described in a paper lately published by the Royal Society, and need be only briefly recapitulated. Jamin’s refractometer was used, and the source of light was, for the refraction, a Bastian mercury lamp, and for the dispersion, a Nernst lamp in conjunction with a fixed-deviation spectroscope. In measuring the dispersion, one of the two tubes was filled with the gas in question, and the other with air at such a pressure that the optical lengths of path of the two interfering rays were approximately equal. The light was then continuously changed from λ = 6500 to λ = 5000, and the change in the position of the centre of a given bright band was observed in a telescope fitted with a micrometer eyepiece. The distance on the same scale between the centres of two adjacent bright bands of known wave-length (generally λ = 5461) was also noted. These observations constituted the point of departure. The pressure of the gas in one tube was then altered till a convenient number of bands of wavelength 6500 (usually 85) had passed in one direction; and, next, the pressure of air in the other tube was altered till the same number of bands had passed in the contrary direction. The wave-length of the light was then changed from the first wave-length to the second, and the movement of the centre of the bright band under observation was again noted.

When a parallel pencil of monochromatic light falls at nearly the critical angle on the plane boundary between two media of differing refractive index, and emerges in the rarer medium in a direction almost parallel to the surface of separation, the aperture of the emergent pencil becomes greatly restricted. A study of the diffraction phenomena observed in this limiting case of oblique emergence seems of interest, especially in view of the fact that in the Lummer-Gehrcke interference spectroscope we have an actual case of importance in which such obliquely emergent pencils determined the observed phenomena. Some preliminary work on this subject was carried out by Prof. C. V. Raman, and at his suggestion the author of the present paper undertook a detailed experimental investigation of the case with a view to establishing its principal features. So far as the writer is aware, though the theory of the parallel-plate interferometer in its essentials is well known, the special character of the diffraction phenomena observed with pencils very obliquely refracted at a plane surface had not been discussed previously. It is proposed in the present paper to describe the results obtained by the writer, some of the observations (to be mentioned below) having special reference to the theory of the Lummer-Gehrcke plate. 2. Experimental Methods and Results . The general features of the case may be observed on an ordinary spectrometer. It is well known that an ordinary prismatic spectroscope may be so adjusted as to give very large dispersions, by putting the prism on the table of the instrument in such a position that the light incident on the first face, after passing through it, falls at nearly the critical angle of incidence on the second face and emerges nearly grazing the surface. The lines in the spectrum then appear very widely separated, and also strongly curved, owing to the fact that pencils of light from different points on the slit of the collimator do not all pass through the principal plane of the prism. In his investigation of the optical power of spectroscopes, Lord Rayleigh has remarked that the resolving power of the prism in the position referred to above is no greater than at other positions. The width of the beam emerging from the prism tends to zero as the grazing position is approached, and the spectrum lines are much widened by diffraction. This sets off the effect of the increased dispersion on the resolving power. Owing to the great dispersion and the enfeeblement of the light in oblique transmission, a very powerful source of monochromatic light is necessary for satisfactory observation of the diffraction phenomena. A Westinghouse quartz silica lamp is most suitable, though visual observations may also easily be made with a glass Cooper-Hewitt mercury-vapour lamp. Observations are made on the green line of the spectrum. In order to secure an extended diffraction-pattern, it is advantageous to use something less than the full aperture of the prism to start with, cutting it down by one or more slits having their edges parallel to the slit of the collimator. The position of the slits is a matter of indifference, so long as the aperture of the beam is cut down before it emerges or just as it emerges from the second surface of the prism, and not after it has emerged. The simplest arrangement is to limit the aperture of the beam by a slit or slits immediately before entry into the prism. Another arrangement would be to use a hollow prism, filled with liquid, and to put the slit or slits inside the prism, so as to limit the aperture of the beam just before incidence on the second face. A third arrangement is to use a glass prism the aperture of the second face of which has been reduced by grinding down to the desired width with parallel edges. All the arrangements give identical results.

In “Colour Photometry, Part III,” ‘Phil. Trans.,' 1892, and “On the Sensitiveness of the Retina to Light and Colour,” ‘Phil. Trans.,’ 1897, a discussion on the extinction of light by a “dark-adapted” eye is given. Since the latter date, a large number of experiments have been made by myself and others on the extinction of light when the retina as a whole has been stimulated by illumination of white or coloured light. In this communication the results obtained when the stimulation is by white light are described. In the two papers above referred to the apparatus used when the eye was dark-adapted is described, but for the observations made with an illuminated retina a modification had to be made. A description of one form, which answered as well as any other form, is given. BB is a box as in fig. 1. At the end of the box is cut a hole ¾ inch in diameter and against it, but inside, is placed a 4-inch disc of white matt paper, in the centre of which is cut a ½-inch circular hole. Behind the box is a second end AA separated from the first by a couple of inches. Opposite the aperture at the first end of the box is cut a circular aperture 1 inch in diameter, against which is placed a piece of doubly-ground white glass, and if necessary a second piece can be placed behind it. The ray of the spectrum from the colour-patch apparatus can be reflected from a mirror M on to the ground glasses at d . The 4-inch white disc is illuminated by the white reflected beam of the same apparatus (or by any other light) through an aperture CC cut in the side of the box. This beam partly goes through the aperture at the end of the box and falls on G, a blackened surface, and is completely hidden from the eye end E. At the side of CC a small metal disc can be placed, which casts a sharp black image on the white disc, as shown in fig. 2. This may be taken as a measure of the blackness to be matched when extinguishing the light from the colour.

That fatigue of the retina by a colour will change the hue of the spectrum colours is well known. In this paper a method of measuring the amount of fatigue is described, showing that by fatiguing the retina colour-blindness is produced which is the same as that found when one of the three sensations is deficient in sensitiveness. In the first part of this communication the fatigue is produced in one eye, and the change in hue is registered by the unfatigued eye, and from these observations what we may call the factor of the fatigue is calculated, which is the ratio in sensitiveness of the unfatigued sensation to that of the fatigued. In the second part of the communication a method is described of measuring the luminosity of what, for short, we will call the fatigue spectrum, which is, of course, the luminosity of the spectrum as seen by the fatigued eye. In previous papers the value of the three sensations in luminosity to the normal eye has been given, and the red, green, and blue sensations have been indicated in equations as R. S., G. S., and B. S. In this paper the luminosities of the three sensations of the spectrum have to be shown as curves of equal stimulation, and the percentage values of the different colours have been derived from these curves. The curves of equal stimulation of the spectrum are the ordinary sensation curves, two of which (the blue and the green) have to be multiplied by factors to make them of equal area. Equal ordinates in these curves will then give the amount of white light existing in the different colours. The sensations of equal stimulus are indicated for the sake of brevity as R’S., G’S., and B’S.

In the appendix to a paper read before the Institution of Civil Engineers, dealing generally with the subject of the 'Electric Locomotive,' the author discussed the running qualities of locomotives from the point of view of dynamics. He based the discussion on the forces set up between wheel and rail, and these forces he referred to the creepage of the surfaces in contact due to elastic deformation of the material in the neighbourhood of the contact, defining "creepage" traversed. He later introduced two quantities, f and f ', which represented respectively the tractive force per unit creepage, longitudinally and transversly, to the rail. The quantities f and f ', which were assumed constant in any particular problem, were not determined at the time, and the present paper is primarily an attempt to compute the first of them. The area of contact between wheel and rail varies with the statr of wear of the parts. For a new rail the longitudinal dimension of the contact is in general greater than the transverse dimension; but, as the rail flattens with use, the contact area approximates in shape to a uniform strip transverse to the rail. The final state is assumed herein, the wheel and rail being conceived as cylinders having their generating lines parallel. The problem proposed is accordingly a two-dimensional one. Instead of assuming the problem to be that of a cylinder rolling on a plane, however, we implicitly assume it to be that of two cylinders of like material and of equal and opposite radii, pressed together and rolling on one another, one being subject to a torque and the other to an equal counter-torque. Under this assumption, any state of stress or strain in one member, due to tangential tractive forces only, is matched by an equal reversed state in the other, and the distribution of pressure between the members is unaffected by the traction, since the radial displacements of the surfaces in contact are complementary. We may note also that any conclusion deduced for a driving wheel is true, with reversal of stresses and strains, for a wheel undergoing braking.

In an earlier paper I showed that the light scattered by gases free of dust was not perfectly polarised, but that a residual defect of polarisation remained, which was characteristic for each gas. This was an entirely new field of investigation, and the experimental methods for detecting and measuring a very minute effect had to be worked out. As is the universal experience, many improvements suggested themselves in the course of the work, and after an interval it was decided to repeat a part of it again, with many modifications in detail, designed to give the most accurate numerical results that seem at present obtainable. This has now been done, with results which, speaking generally, confirm the earlier investigation, except in one important particular. It was decided to limit the present investigation to the permanent gases, thus excluding the vapours of organic substances. The latter are inevitably absorbed by the blacking which it is necessary to use on the inside of the vessel, and when the substance under investigation is changed they may come off and give rise to contamination. Moreover, there is the tendency of some of these substances to form fogs under the influence of light. It would have been premature to include them in a first attempt to establish accurate methods. Pioneer determinations were given in the former paper, and are believed to be substantially correct.

In the comparison of optical dispersion formulæ with experimental results, it is important to consider the meaning of the various constants, and in particular of the natural wave-lengths which occur in them. With the formula in the simple form n 2 - 1 = ƩC 1 /(λ 2 -λ 1 2 ), the quantities λ 1 are the wave-lengths of the natural vibrations and have been identified in practice indiscriminately with the wave-length of maximum absorption or that of selective rejection obtained by the method of residual rays. This interpretation is allowable if the absorption is very small. For a more general type of formula such as (n2-1)/(n2+a)=ƩC 1 /(λ 2 -λ 1 2 )the corresponding wave-length, on a similar argument, for small absorption is a certain wave-length λ 1 ', which is larger than λ 1 ; in a previous paper numerical examples were given of the difference between the wave-lengths λ 1 and λ 1 '. The present paper includes absorption more generally in the formulæ, so as to determine the position of the various maxima more accurately.

In a paper on “Polarised Röntgen Radiation,” the writer gave an account of experiments which demonstrated the partial polarisation of a beam of X-rays proceeding from the antikathode of an X-ray focus-tube, and verified the theory previously given of the production of secondary X-rays in light substances. In that paper it was shown that the secondary radiation proceeding in a direction perpendicular to that of propagation of the primary radiation from certain substances placed in that primary beam should, according to the theory put forward, be plane polarised. From gases, however, the secondary radiation was not sufficiently intense to produce a tertiary of measurable intensity, and thus the polarisation of the secondary from them was not verifiable. On the other hand, though heavy metals were found to emit secondary radiation of sufficient intensity and ionising power to produce appreciable tertiary effects, in these metals the production of secondary radiation is a more complex phenomenon, and evidence of polarisation of the secondary beam is not to be expected from experiments upon them.

The absorption and scattering of γ-rays by elements has already formed the subject of two papers published recently in these proceedings. The objects of the experiments described in this paper were (1) to extend the absorption measurements, especially in the more important direction of the extremely short waves emitted by RaC, and (2) to make a thorough investigation, with special reference to Compton’s theory, of the scattering by matter of these high-frequency radiations. It was shown in the papers referred to that the absorption of γ-rays from RaB + C, filtered through 1 cm. of lead, is closely expressed by relations similar to those which hold for X-rays. Estimates of the mean-effective wave-length of the beam placed the value at about 0.02 Å. U. Now, it is well known from the crystal analysis of Rutherford and Andrade, and the β-ray magnetic spectra measurements of Ellis, and Ellis, and Skinner, that the γ-rays emitted by radium products cover a wide range of wave-length —roughly, from 1.0 to 0.005 Å. U. It is, however, uncertain whether the very short waves obey the same general laws of absorption as the ordinary X-rays. Direct measurements on a few selected homogeneous rays, though capable of yielding most important results, are unfortunately impracticable. The problem has, therefore, been attacked by the less direct method of measuring accurately the absorption of a complex beam, successively hardened by means of five suitable filters. It will be shown that in every case the absorption of γ-rays obeys the same general law as has already been found. The measurements also throw some light on the spectral distribution of energy in the beam.

In “Colour Photometry,” Part III, XLVII, ‘Phil. Trans.,’ 1892, the results of some measures for the extinction of colour from the spectrum colours are recorded and the method of making the observations is explained. What was attempted in these observations was to reduce all the colours to a hue of the steel grey which a pure white becomes when it is of feeble intensity. During the observations then made it was found that some rays near the yellow lost their colour earlier than shown, but that the white they then matched was not completely the steel grey alluded to above. In the present results the hue of the white has been disregarded and no judgment of the hue of the white was required, making it easy for anyone to go through a series of readings. The instruments used were almost the same in principle as those employed on the previous occasion, but in some details they differed. Instead of sectors being employed to dim both colour and white, the annulus described in my paper on “The Sensitiveness of the Retina to Light and Colour” was employed, and the figure from that paper is inserted here to show the apparatus, as is also its brief description.

In this paper will be given a preliminary account of a method by which the intensity distribution in X-ray crystal photographs may be estimated by means of a simple and inexpensive apparatus. In the usual photometric procedure the blackening of a photographic plate is estimated by finding the intensity of light transmitted through it. In the method to be described the photographic plate is replaced by a carbon print, and the light and light-sensitive cell by α -rays and a simple α -ray electroscope respectively. The obvious advantages of such a system are, of course, ease of construction and negligible cost; but, as will be seen below, there already appears promise of a much more weighty advantage, namely, that with suitable adjustment the apparatus may be made to integrate the X-ray intensity of a crystal reflection after the manner of the Bragg ionisation-spectrometer. Details are given below, but briefly the method is as follows. A carbon print is first made of the photographic negative . A carbon paper (or tissue, as it is called) consists merely of a pigmented film of gelatine sensitised with dilute aqueous potassium dichromate solution. On exposure to light such a film becomes insoluble, the depth of the insoluble layer after a given time being a measure of the intensity of the incident light. On development with warm water the remaining soluble portions are washed away, and there is left a film of thickness which varies according to the variation in blackness of the original negative. This insoluble film being very tough is readily detached, and, because it is so thin, its stopping-power for α -rays is just about of the right order. The variations in its thickness are thus very easily followed by aid of a few square millimetres of foil on which polonium has been deposited, a single slit, and the α -ray electroscope described by C. T. R. Wilson, which is usually figured in text-books. The latter can be constructed from an ordinary cigarette tin, and a simple tele-microscope with eyepiece scale.

In a previous communication it was shown how a new principle in photometry could be applied to the construction of a simple “α-ray photometer” with possible integrating properties. It will be the object of the present paper to describe how the theoretical and practical development of this principle has now led to the design of an integrating photometer which has been found experimentally to give correct measurements of the total intensity in X-ray crystal reflections, and which also seems capable of adaptation to the measurement of the integrated intensity of other types of radiation. Briefly, the new method uses a carbon print of a negative instead of the negative itself, and α-rays instead of light. In ordinary photometric procedure the density of the photographic image is found by transmitting through it a beam of light which then falls on some light-sensitive receiver such as a photo-electric cell. In the procedure about to be described, the density of the negative is not measured, but, by using the “stopping power" for α-rays of a carbon tissue positive, ionisation in an α-ray electroscope is produced in direct proportion to the original X-ray intensity which formed the negative. Carbon printing-paper or “tissue” consists of paper supporting a uniform gelatine film, impregnated with an insoluble colloidal pigment, and sensitised by immersion in dilute aqueous potassium dichromate. The gelatine then possesses the property of becoming insoluble on exposure to light, so that after printing in the usual manner, development involves only the washing away in warm water of the still soluble portions of the film. It is a comparatively simple matter to obtain the insoluble film stripped from all support and free to be placed as an absorbing layer in the path of the α-particles from a small deposit of polonium. The stronger the original X-rays, the blacker the photographic image and the weaker the light which falls on the tissue during printing. This means that the stripped tissue is a film of varying thickness— during development, in fact, the relief is easily seen. It is thinnest where the original X-rays were the most intense, so that when it is used as an absorbing layer for α-rays, the ionisation in the a-ray electroscope increases with increasing X-ray intensity, and, in fact, can be made directly proportional to it. The total ionisation can thus be made a measure of the total X-ray intensity whatever its distribution over the photographic image.

Prof. McLennan and others have published in these Proceedings a paper in which they have given results of their measurements of the intensities in the secondary spectrum of hydrogen. The experimental results mentioned in the tables of that paper, however, are only densities, and no attempt was made to find from those densities the real values of intensity in the spectrum. The relations between density and intensity, however, is not so simple as McLennan supposes, as is shown in fig. 1 which follows this. In this paper it will be shown that for theoretical investigations on the secondary spectrum it would be dangerous to use the figures given by McLennan. We shall do this at the moment for a region from 4500-4900 Å.; the real intensities in this region will be shown to differ considerably from those given by McLennan. The intensities of about 230 lines have been measured by the method developed in our Institute. Further, our work shows that the source of error, already mentioned by McLennan— e. g . (p. 281), the difference of sensitivity of photographic plates in this region—can be considerable. Fig. 2 represents the sensitivity as a function of wave-length for the plates used (Ilford Special Rapid). Measurements for other regions of the secondary spectrum will be published soon. The application of the intensity rules is hardly possible at present, as the number of lines of which the classification is certain is too small, and also the wave-lengths are not known with sufficient accuracy. In this paper we give, in addition to the measurements of intensities, the wave-lengths of a number of complex lines not described or uncertain in existing literature.

In a short paper “On the Diffraction of Light by Particles Comparable with the Wave-length,”* Keen and Porter describe curious observations upon the intensity and colour of the light transmitted through small particles of precipitated sulphur, while still in a state of suspension, when the size of the particles is comparable with, or decidedly larger than, the wave-length of the light The particles principally concerned in their experiments appear to have decidedly exceeded those dealt with in a recent paper, where the calculations were pushed only to the point where the circumference of the sphere is 2·25 λ. The authors cited give as the size of the particles, when the intensity of the light passing through was a minimum, 6 to 10 μ, that is over 10 wave-lengths of yellow light, and they point out the desirability of extending the theory to larger spheres. The calculations referred to related to tho particular case where the (relative) refractive index of the spherical obstacles is 1·5. This value was chosen in order to bring out the peculiar polarisation phenomena observed in the diffracted light at angles in the neighbourhood of 90°, and as not inappro-priate to experiments upon particles of high index suspended in water. I remarked that the extension of the calculations to greater particles would be of interest, but that the arithmetical work would rapidly become heavy.

The problem of estimating distances at sea in fog or thick weather is obviously one of much importance to navigation. Notwithstanding the more perfect means of communication between ship and shore, or between ship and ship, which recent scientific advances have secured, and which are at the command of the more important ships and lighthouse stations at the present time, I can find no reference to the fact that these improved means of communication suffice to solve the problem referred to under a great variety of circumstances. In the present paper I shall confine myself to the determination of distance between ship and shore. In a subsequent paper I hope to discuss the application of the methods involved to finding the distance between ship and ship, and thereby lessening risk of collision. The method I have to propose is based generally on the differing velocities of signals in different media: (1) luminous or "wireless" signals (electro-magnetic disturbances) may be regarded as instantaneously propagated over the distance concerned; (2) submarine sounds- e. g. , bell-storkes under water-travel in the water at the rate of about 1400 metres, or 4700 feet per second; (3) aërial sounds move at the rate of about 330 metres, or 1100 feet per second. Explosive signals travel somewhat faster in water and air.

1. The study of the water waves produced by the motion of an assigned pressure distribution over the surface has hitherto been limited to the steady state attained when the system has been moving with uniform velocity for a very long time. In his latest series of papers on water waves, Lord Kelvin made an elaborate graphical and numerical study of cognate problems, and expressed the hope of applying his methods to calculate the initiation and continued growth of canal ship-waves due to the sudden commencement and continued application of a moving, steady surface pressure. In the following paper, I have not attempted any analysis of the surface elevation itself, but I have proceeded directly to the calculation of the corre ­sponding wave resistance. At present the wave resistance is known only for the steady state for certain localised pressure systems in uniform motion, and it seems desirable to attempt some estimate of the time taken to attain this state when we take into account the beginnings of the motion. One might examine the effect of initial acceleration, but I have limited the problem by considering only the case of a system which is suddenly established, and is at the same instant set in motion with uniform velocity. The work is arranged in the following order: a general expression for the wave resistance as a function of the time, an exact solution for a certain waveless system, a comparison of this solution and the group approximation, and an approximate solution for certain systems which leave regular waves in their rear.

The work described in the present paper was performed with the primary object of investigating the constitution of steel by new methods. The development. and perfection of these methods, and the standardization of the relevant experimental conditions were, at the outset, looked upon as purely incidental. During the course of the work, however, it became apparent that this secondary object was of as much importance as the results originally sought, and in consequence considerable care and attention were given to it. The paper is divided into three sections:—Electrode Potential, Electrical Resistance, and Change of Resistance during Tempering. Heat-Treatment. All heat-treatment was carried out in a wire-wound electric resistance furnace. The specimens were heated in a transparent silica tube, which was capable of being evacuated by means of a Toepler mercury pump. No difficulty was found in maintaining the vacuum even at the highest temperatures used.

In a previous paper experiments were described in which vowel-like sounds were produced by models consisting of two suitably tuned Helmholtz Resonators, connected in series, and energised by a reed of variable pitch. It was found that with these models, as also in some whose resonators were connected in parallel, the vowel-like sounds were, in general, but little affected by variation of pitch of the energising reeds, though it was evident that the resultant sound, if analysed, would necessarily be very different when the relations of the energising reed-note to the resonant notes of the two cavities was changed. In a subsequent paper, observations were described which indicated that in recognising the consonant sounds k/g and t/d, the human ear is not directly interested in the resonant changes, which constitute these sounds, and which vary very greatly according to the upper resonance of the vowel with which the consonant is associated. The ear appears rather to be on the look out for acoustic indications of the gestures of the organs of articulation, by which these consonants are produced.