A note on the influence of the optical system of a microphotometer on the plate calibration curve

The influence of the optical system of four microphotometers viz. Zeiss, Leeds and Northrup, A.R.L. and Optica Milano on the plate calibration curve of a photographic plate is described. The following general conclusions are reached: (1) That, when the Zeiss and Optica Milano instruments are used for the blackening determination of spectrum lines the slit width of the detector must at least be a sixth of the width of the projected image of the spectrum lines, which are being measured. These instruments show little stray light effects, when measuring weak lines but for intense lines stray light effects must be taken in to consideration. (2) When using A.R.L. instruments it is very important to use a slit which is about a third of the width of the spectrum lines which are being measured. (3) Of the instruments tested the Leeds and Northrup instrument has the least amount of stray light. This means that a very limited amount of stray light is dispersed in the optical system of a microphotometer when (a) the light source is a strip filament of restricted diameter; (b) the image of the light source formed in the gelatine layer of the photographic plate is at least a third of the absolute width of the weakest spectrum lines.