The advance of photography : its history and modern applications (1911)

166 THE ADVANCE OF PHOTOGRAPHY neighbourhood for a much longer period of time than would be the case if the dye were absent. If this explanation were correct, then the action of the dye would be purely a physical one. On the other hand, Abney considers that the action must be looked upon as purely chemical. By his theory the colouring matter is supposed to undergo photo-chemical oxidation, and it is further assumed that it acts as a reducing agent, thus assisting in the reduction of the silver salts. According to Vogel's theory the best sensitizers for the red and orange rays should be found among the green and blue dyes. While many of the dyes in common use help to bear out the truth of this theory, there are several wellknown exceptions to the rule, for red, yellow and violet dyes or colouring matters, as well as certain resins and colourless organic substances, have been found useful in this respect. Even when plates are thus specially prepared it is still found that the light of the violet end of the spectrum has relatively too great an effect upon the film, hence it is that some suitable screen must be used with the lens when such plates are exposed. These screens are specially prepared by the various makers, and although any lemonyellow screen which is not too dark answers well, it is better to obtain the screen recommended with the plates. It can be seen that if too much of the blue light is cut off, those parts which are coloured blue will be represented as too low in intensity, and the photograph will then err in the opposite direction to those obtained with ordinary plates. It must be remembered that the exposure must be greatly increased when using a screen. Thus, if a fourfold yellow screen is used, the exposure should be four times the length that would be allowed with plates of the same rapidity, when not using a colour screen. Of course when the sun is near the horizon, so that the light received from it has a