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

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PHOTOGRAPHY IN "NATURAL COLOURS 249 it can be seen that the plate must be placed in the dark slide so that the film side is away from the lens. To better understand the action of this screen, let us imagine that in one small spot there are three grains adjacent — one red, one green, the other violet. Now suppose light of any one of these colours to fall upon such a spot, then it will only be transmitted by the coloured starch which is of its own colour, and behind that grain the film will be affected, so that on developing we shall find in that position a microscopic black dot. The film will not be affected at all behind the other two grains. Hence, when viewed by transmitted light, the colours of these two grains will mingle and the result will be a colour complementary to that to which the film was exposed. Were it not possible to alter this by some simple means the whole process would of necessity be a failure ; fortunately it is quite possible and easy to transform the negative into a positive, so that no such false rendering of colours takes place. Since the autochrome plates are sensitive to all rays, great care must be taken to have a safe lamp for the dark room. This can be obtained by special Virida papers, supplied by Lumiere. Even then precautions must be taken so that the plate shall not be exposed directly to the light until it is immersed in the developer. As the plate must be exposed with its film side away from the lens, it is necessary to see before placing the plate in the dark slide that its glass surface is perfectly clean. A special piece of black cardboard is supplied with the plates, and this should be put in contact with the film, and the plate and cardboard then placed together in the slide, so that the film may be kept free from scratches. The focussing screen of the stand cameras must also be arranged so that its rough side is outwards. A special colour filter is necessary when using these plates, which may be placed either in front of or behind