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PHOTOGRAPHY IN NATURAL COLOURS 253
other end. These are photographed quite plainly on the negative, so that if the colour screen which was used in the exposure is used with the positive made from the negative, all that is necessary is to place the screen and plate so that the crosses are in superposition. But a little thought will make it quite apparent that even these registration marks are not at all necessary. The screens being mechanically made, so that the circles and spaces are all of a definite fixed size, a little patience will enable one to obtain perfect register with a colour screen not used with the plate, since there must be many hundreds of such positions and a very slight movement will produce the required result. In such cases, however, great care must be taken to note the true value of the colours of the object, so as to be certain when the best position is obtained. Unless the beginner has a good eye for colour the more mechanical method is to be recommended.
One very great advantage of the " separate " method is that by its means it is possible to obtain duplicates of the photograph — in fact, the same negative can be used for obtaining any desired number of positives, each of which when bound in register with a colour screen will give a photograph in natural colours. When it is desired to use the negative for this purpose, it should be developed and fixed just like an ordinary negative. Any plate which gives good density like a lantern plate can be used for obtaining a positive by contact with the negative.
The " separate " method may cause a little inconvenience in those cases in which the usual dark slides will not accommodate the two thicknesses of glass, but this is not an insurmountable difficult}' nor one which it is very expensive to overcome.
The " Combined " plate should be developed and reversed in much the same manner as the autochrome plates. Full instructions for these operations are issued with the plates.