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PHOTOGRAPHY IN NATURAL COLOURS 233
B. Gelatine, 2 grams; potassium bromide, 0*25 gram;
water, 50 c.c.
C. Silver nitrate, 03 gram ; water, 5 c.c.
A and B are heated until the gelatine melts, after which it is allowed to cool to 40° C. C is then added to A, which in turn is added to B, the latter process being done slowly, the mixture being stirred meanwhile ; the sensitizer is then added, and the resulting mixture carefully filtered. After the plates are coated and the film has set, they are washed for fifteen minutes, and then allowed to dry.
His experiments tend to show that whether the reflected light is viewed from the film or the glass side of the plate, no increase in the purity takes place for any increase in the thickness of the film used beyond a thickness of about thirty wave-lengths.
It is found that the coloured light reflected by the layers of silver deposited in the film increases in intensity with an increase in the length of exposure, until a stage is reached when the diffusely reflected light can no longer be noticed ; after that stage very little change in the intensity takes place, although the exposure may be increased considerably.
In the less exposed parts the film appears when viewed by reflected light like an ordinary negative — that is, a positive can be seen, caused by the diffusely reflected light. With further exposure the film tends to lose this appearance and become black, and behaves in much the same way as unsilvered glass.
When viewed by transmitted light the plate appears greenish in the very slightly exposed parts, a kind of yellowish brown in parts which have received a moderate amount of exposure, and in those parts where the film has been sufficiently exposed to cause a disappearance of the diffuse light produced by reflection, the plate appears quite clear, just like a piece of yellow glass.
Therefore Ives concludes that the behaviour of the silver