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PHOTOGRAPHY IN NATURAL COLOURS 235
thickness of the film is only reached when it is no longer practicable to flow and dry a thicker one. A point to be remembered, however, is that the greatly increased exposures necessary on account of the opacity and slow speed of thick films makes it much more difficult to obtain satisfactory results when working with them.
Up to the present we have been supposing the plate to be illuminated by monochromatic light ; let us now see what is the result when the source of light is such that when analysed spectroscopically it either gives two or more distinct lines, or else broad bands of colours. In such cases there will, of course, be regions in the film where the waves will by interference strengthen the effect, and in others the effects will be much diminished.
Lippmann has developed the theory of the reflecting elements of the film by assuming that minute reflecting particles are distributed throughout the film. On his theory, white is produced owing to the continuous irregular distribution of the particles. Experiments which have been carried out by Ives indicate that when a plate with a fairly coarse grain is used, if over-exposure be carefully guarded against, and the plate is subsequently developed with pyrogallic acid, there is apparently a very close approach to a condition of separate reflecting particles. On the other hand, with complex radiations, and ilso with over-exposure, there must in either case be a certain amount of fusing together and a consequent loss of luminosity.
Schutt * brought forward the idea that the action of light merely causes a periodic change in the refractive index, and this idea more nearly fits in with the reflection obtained when the plate has been developed with hydroquinone and then bleached.
From such considerations as these it appears quite evident that when the object is to keep the luminosity as
1 Ann. d. Physik, 57, p. 533, 1896.