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Much work has already been done on this subject, though little of a quantitative character. Many observers have described the curious our sensations which rapid alternations of light and darkness can cite under certain conditions, admirably exemplified in the “spectrum tops.” Foremost amongst those whose experiments and writings have led to the present very general interest in the subjects of flicker, and of the sensation of light and colour, may be mentioned Helmholtz, vanus Thompson, Shelford Bidwell, Henry, Charpentier, and tod; whilst the first to try experiments on the relative sensitivess of the eye to flicker in light of different colours, seems to have been J. Plateau, who, however, employed pigments, and not the colours of the spectrum.

(1898). XX. On a method of viewing Newton's rings. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science: Vol. 46, No. 279, pp. 245-252.

Since the Iast papers the research has been continued at intervals through the nine years. Lome numerical errors on pp. 325 and 327 in the last paper have been detested by the author, but nothing which in the least decree alters the main results of that paper, viz., that the number of revolutions per second which a disc having a white sector w , and a black sector 360° — w , must make in order that the flicker may just vanish, under an illumination I, is given by an equation of the form n = a+b log w (360- w ), a being a constant, and b of the form c+d log I, where c and d are constants. If the disc is considered to be under unit illumination when lighted by a standard stearins candle, burning 8·273 grm. of stearins per hour, at a distance of 4 metres; that is, if I is taken to be 1 under these conditions, then, from the combination of the series of observations described in papers I and II, with man others made since, it follows that the numerical value of n is determined with a very fair decree of accuracy to within at most threes revolutions per second, and generally within one, from the equations n = 10·7+12·57 log I, for discs half white, half black, and n = -27·83+(8·57+20·79 log I) log w (360 - w ), when tbs angle of the white sector is w °.

General Objects:— To examine— ( a ) The height of the reflecting layers of the atmosphere. ( b ) The structure of the corona. ( c ) The polarisation of the corona. ( d ) The coronium ring. ( e ) The shadow bands. ( f ) The meteorological conditions of the eclipse. Our station was near Philippeville, about 1·5 kilometres to the west-north­-west of the village of Saint Charles, on the long and broad summit of a grassy hill rising at a moderately steep slope. The position of our camp, as given by the large-scale survey map, was:— N. Lat, 36° 45' 38"·7. E. Long., 6° 51' 59"·T, or in time, 0 h. 27 m. 27·6 s. Height above sea-level, 690 feet or 210 metres. These values agreed well with independent observations made with sextant and telescope.

For many years the writer has, from time to time, been engaged in studying the effect of a powerful magnetic field upon crystals in the act of their formation and growth. It seemed to him probable that if the molecules of substances have magnetic poles, they might group themselves differently when under the influence of a powerful magnetic field, thus producing an orientation of the crystals, or even an alteration in the form or optical characteristics of the crystals themselves. It seemed also possible that if the supposed polar properties of the molecules were the result of atomic polarity, a powerful external magnet might produce some appreciable effect in the chemical combination of the atoms, changing the rate of chemical reaction, if it did not change the character of the compounds formed.

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In the first paper, curves are given which show the rate at which a disc half white half black must be rotated in order that flicker may just vanish when the disc is illuminated by the different colours of a diffraction spectrum. Also curves showing how the rate of rotation alters with the alteration in the angular width of the coloured sector when placed in the different colours of the same spectrum. These last are reproduced with some additions and corrections in the present paper (fig. 4).

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THERE is an error, for which I am responsible, in my letter of July 24 (p. 31). The words kathode and anode should be interchanged in one sentence, which should then read thus:—“The same reasoning would indicate that it would be well to make the anode convex towards the kathode, and fairly small.… ”

IN my two papers of June 4 and June 18 respectively, I proved the existence of non-homogeneity in the X-rays, and gave a simple method of strengthening and maintaining the discharge of these rays from an ordinary focus tube by means of a conductor wrapped round the part of the tube level with and behind the kathode, and separated by a small sparking gap from either an earthed wire or the kathode's external wire loop, or the wire leading from the induction coil to the kathode itself. Since then my attention has been chiefly turned to the most remarkable results which can be attained with a Wimshurst machine, with which this paper is concerned. My Wimshurst has two 15-inch plates, and, not being of the latest type, has the old-fashioned metallic sectors and buttons, instead of the plain varnished glass disc, though I do not imagine that the results will be found materially different with the simpler, and, probably better, form.

HAVING read in your number for March 24 (p. 484) a letter on the above subject, I thought it might prove interesting to try a similar experiment with the carbon filament of an ordinary incandescent lamp. That used was an Edison Swan 16 candlepower 80 volt, and the method employed to heat it was to pass a gradually-increasing current (supplied from accumulators), using a water resistance gradually diminished by the addition of very dilute sulphuric acid in sensibly equal portions. The room in which the experiment was performed was very carefully darkened, and the observers were kept in darkness some minutes before the current was switched on, the dilute acid being added, so that, after visibility had been reached, five additions should bring the lamp to dull redness (by diffused daylight. The number of the experiment being called out, each observer wrote this down, together with his impression of the colour, in the dark, so that the retina was not affected by any extraneous light throughout. Each observer closely inspected the filament till he felt satisfied as to the colour, and then rested his eyes in the dark till the next observation. There were twenty-five observers. The result seems worthy of notice.

ON Sunday, August 7, 1898, being in Norway, I was climbing, with a friend, the upper slopes of the Horntind, above Skogstad, in the well-known Valders route between Christiania and Laerdal, lat. 61° 15′ 30″, long. exactly 6° E. We had reached a height of about 4000 feet above sea level when we saw the very remarkable halo of which I send you the photograph of the copy of a very careful drawing, made on the spot. I first caught site of the halo at 11.30 a.m., on lying down for a short rest on a large flat horizontal stone, but I have no reason to doubt that it had been visible for some time before. The early morning had been brilliantly fine, the air still, and the sun very hot; about 10.30 a.m. a very light breeze from almost due south began to blow, with intervals of dead calm. When the halo was seen, the sky was completely covered with a thin white haze. There was, however, no rain that day, though the weather on the next and succeeding days was not good. The sky outside the circles seemed everywhere brighter than inside them; the sun shone through the haze scarcely brightly enough to throw a distinct shadow, and his rays aroused no sensation of warmth. The inner edge of all the rings was fairly sharp, and of an orange-red colour, brightening into yellow, which grew paler towards the outer rim, where it faded into a bluish-white radiance, which in turn became imperceptibly blended with the white misty sky. The width of the rings was from one-and-a-half to two degrees.

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THE shadows to which Mr. Cyril Crossland refers in his letter to NATURE of May 30 have straight, fairly well-defined edges, and are therefore certainly cast by the sun itself, which would be still visible to anyone in the high reflecting layer, whether to east or west of the observer. They are certainly not cast by light “reflected from the glowing clouds in the west”, as Mr. Crossland thinks. The convergence of these rays towards the east, which the present writer has often seen, is purely a perspective effect. The rays themselves are in reality practically parallel, but seem to converge to east and west just as the parallel track of a straight railway seems to converge in both directions to anyone standing between the rails. The effect in the east soon after sunset is sometimes so striking that anyone might well believe that the sun had set there, were there no other circumstances to judge by.

MESSRS. A. VOSMAER AND F. L. ORTT, in a paper kindly sent to me by the editor of NATURE, have arrived at the conclusion that the X-rays are more or less perfectly discharged particles.

AFTER examining the fluorescent and photographic action of the rays (X2) emitted on strongly heating a “focus tube,” and finding them different to the rays which have been hitherto noticed (X1), in that the relative transparency of flesh, bone, aluminium and glass differs for the two kinds of rays, it seemed desirable to try the effect of cooling the tube. Solid carbon dioxide and ether, and then solid carbon dioxide alone were employed, with the result that in both cases the fluorescence of screen and tube very rapidly died out and the current apparently failed to pass through the latter; as the tube gradually grew warm again, the fluorescence in it returned, not gradually, but very suddenly, at a temperature not very far below that of the room, the glass lighting up brilliantly, and the shadows of the bones showing on the screen with increasing distinctness. The emission of X1 rays reaching a maximum at about 12° C. (a rough guess). On further heating X2 rays begin to be evolved, judging from the increasing opacity of the flesh, whilst at the same time the fluorescence excited on the screen grows rather brighter, until the state recorded in my paper of June 4, is reached. As the condition for the maximum of X1 rays probably varies to a certain extent with the different forms of tube, and even different specimens of the same kind of tube, with the degree of exhaustion, c and from the accounts of various operators such would seem to be the case, though, as will be seen in the light of subsequent experiments, the particular method of heating or cooling is an important factor in the result. Solid carbon dioxide seemed very opaque to the rays when its low density is considered, but the effect may have been partly due to the frost condensed upon it from the air.

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SINCE the publication in NATURE of the letters on these three phenomena, my attention has been directed by Prof. S. P. Thompson to the “Memoire sur les Halos,” by M. A. Bravais. The first of the above three halos, recorded by Prof. E. E. Barnard, is apparently new, unless the radius of one of the two circles was in reality considerably smaller than that of the other; if this was the case (which from the account seems scarcely likely) this halo might be part of the halo depicted on Plate iii. Fig. 98, and described on pp. 87, 88 and following, and of which Bravais says that the different parts are by no means always visible together.