PROPORTIONATE RENDERING OF CONTRAST.
The foregoing experiment will show that the contrasts obtained with the same plate and filter may be totally different when the exposure is unduly prolonged. Among the purposes for which the screen may be used, first and foremost comes that of toning down or reducing the ac tivity of the violet and blue, and of elimi nating entirely the ultra violet rays. These latter, as has already been shown, are exceedingly actinic, whilst being in visible to the eye ; and the deposits pro duced by their aid must, under ordinary circumstances, lead to a. false impression. In each case, however, the aim and ob ject should be to render the proportionate ratues rather than the actual colour inten sities. Photography can only register differences of intensity or luminosity, and when two different colours are of the same luminosity, they should photograph (upon a plate correctly orthochromatised and properly screened) exactly alike. The
fact that the rays producing the different colour sensations are of different wave length, and may possibly produce a vary ing physical effect, need not be taken into account. The novice is apt to look upon certain colours as bright under all conch tions, yellow being said to come out dark and blue light. Still, practical ex perience proves that such is not always the case, for it is possible so to shade a blue that it photographs considerably darker than a yellow which is receiving more light. In the same way a screen which reduces the actinism of the blue may be made so deep as to practically obliter ate it, the result being that the blue is rendered as black. This will mean, under ordinary circumstances, that the subject is falsely rendered as regards its propor tionate values or contrast, but it is a great power in the hands of the photographer who is capable of using it discriminately.