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234 THE ADVANCE OF PHOTOGRAPHY
deposit is in all respects just as if the particles of silver were* first of all separate and so produce a scattering of the light, but with longer exposure become fused together into a homogeneous mass.
Ives also found that the effect of varying the length of time for development depends upon whether the plate is viewed from the film or glass side. Greatly increased development causes fog, and this decreases the purity if the plate is viewed from the film side. When viewed from the glass side no effect whatever is noticed in cases of long development unless the films used are thin, in which case there is a diminution in the purity of the colours. He recommends that plates for this work should be developed until a fog just begins to appear.
The small effective number of laminae (twenty to thirty at most) appears due not to the small number actually formed, as has often been assumed, but to the mode of action of the developer. Some developers, such as ferrous oxalate,. or hydroquinone, can be used without any trace of fog until development has proceeded with great uniformity quite through the film. The deposit obtained with these developers is black and opaque, so that the reflected light is such as to render the colours very dull. Hence it becomes necessary to bleach with mercury chloride, and the resulting loss of reflecting power is more than compensated by the increase in effective lamina?. The deposit in such cases is so transparent that absorption becomes practically negligible, and all the lamina? ma}' be considered as acting with equal effect. On this account, when monochromatic light is photographed, and the plates, are treated as described, the result is that the reflected light from the plate is a narrow bright line when examined spectroscopically, and not, as is usual in such cases, a diffuse band.
In such plates increased thickness of film results in a. larger number of laminae being formed, and the limiting,