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PHOTOGRAPHY IN NATURAL COLOURS 257
Although the silver which forms the image in the negative is removed when the plate is placed in the second bath, and the remaining silver bromide is reduced during the second development, and so no fixing should be required after reversal of the image, yet it is often better to run the risk of obtaining a slightly weakened photograph in order to be quite certain that all the unreduced silver bromide may have been removed.
The plates may be varnished so as to preserve their films, and it is also important that they should not be exposed to sunlight more than is necessary, on account of the action of the sun's rays upon the colouring matter contained. The same applies, only in a much greater degree, to the colour-filters used. These should be kept in special light-tight cases, and only exposed to light when required for use, as any change in their tint would greatly modify the results obtained.
Joly's Process. — J. Joly, in 1895, brought out a process in which the screen used was built up of red, green, and violet-blue lines alternate with one another. These lines were ruled with aqueous inks upon a glass support which had been previously coated with a thin layer of gelatine. This screen was a difficult one to produce on a commercial basis. A Chicago firm acquired Joly's patents and made an endeavour to produce such screens with 300 lines to the inch, but the cost of production was so high that the firm very quickly gave up the enterprise.
Still, as an example of the attempts which have been made to obtain coloured photographs by the exposure of only one negative, this process deserves mention.
The same procedure as to exposure and development applies to this method as to those already described. In the negative the different tints of the object photographed will be seen to have produced lines which vary in their degrees of opacity. Like the Thames Colour Plate r it was necessary to view the positive obtained by this
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