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254 THE ADVANCE OF PHOTOGRAPHY
There is one other point to remember in using these plates — that is, although the film is sensitive to the action of all light, yet the action of the blueviolet is still greater than that of other colours, hence a colour filter must be used so as to diminish this action to the level of the others. A yellow filter for this purpose is supplied at a cheap rate with the plates.
The " separate " negatives obtained by this means when looked at through a pocket lens will show a definite structure for the silver deposits, those brought about by the green and red lights being in circles, while those due to blue are irregular patches of the same shape as the spaces between the circles. These negatives may be used just like ordinary negatives for obtaining bromide or P.O. P. prints, for the positives so obtained will be found not to be by any means rendered useless owing to the presence of the screen pattern on the plate.
Joiigla Omnicolore Plates. — There is yet another process very similar to the two just described by which coloured photographic plates of natural objects can be obtained. This is by means of the Jougla Omnicolore Plates. The coloured screen in this case consists of an enormous number of microscopic lines crossing one another so as to give rise to small squares. These lines are of three colours, blue-violet, green, and red, and are ruled by a mechanical process so that the squares are exactly touching each other, and there are neither blank spaces through which white light can act upon the sensitive film, nor yet do any of the colours overlap. The film used with this screen is, of course, one which is sensitive to all the rays of the spectrum. In all cases when using this kind of plate it is well to remember that the best results are obtained when the plate has been exposed to no light whatever, until such chemical operations have been performed upon the plate as to render the light harmless. Thus the makers recommend that the following operations should be accomplished in