It will, of course, be gathered that our object in making the screens is to get the colour correction with as little loss of actinic light in the camera as may be. The visual intensity of the light is of no importance so long as it does its work on the plate. In lighting the dark room the conditions are to some extent reversed. We want the brightest light visually that we can get with the least possible action on the plate. Bearing in mind the way in which the complementary colours cut each other out, and that the Barnet plate is sensitive to yellow but not to red, it is obvious that we must for our greatest convenience use the brightest red we can get, and that will be the red just beyond the orange. Ruby glass passes very little of this red, but does pass a considerable amount of dangerous violet light, so that it is necessary to supplement it with a sheet of yellow. Those who do not mind taking a little trouble can make for themselves a much better combination. Reference to Fig. 6 will show that naphthol yellow is a good base for the purpose, particularly as it is a very bright yellow visually. In conjunction with this we must have some dye that will cut out the green. A second sheet of glass coated with gelatine, dyed with rosaniline, will do this effectually, and we are left with a plenitude of bright orange that will make everything in the dark room clearly visible. With ordinary care, plates of the nature of that we have been considering may be worked in such a light, since it will not be necessary to go closer than within a couple of feet from the lamp. With plates that are sensitive to red, and for workers who are not speedy in manipulation, it may be advisable to add a little methyl blue to the yellow. This will cut out the orange and still give a good working light.
The lamp should be glazed with a piece of ground glass, and the compound screen fitted half an inch or so in front of it to lessen the risk of the gelatine being scorched.
Since writing the above I have made a number of screens dyed with a combination of naphthol yellow and naphthol green with excellent results. Various proportions have been used, but a good working formula is two grains of naphthol green and one grain of naphthol yellow in six ounces of water. The depth of tint to which the gelatine is to be dyed will depend largely upon the class of work for which the screens are being made. For landscape work in which there are strong contrasts of colour, and in which even the faintest traces of clouds present in the sky are to be preserved in printing strength, screens requiring six times the normal exposure will be found useful. In making such screens the gelatine may be immersed in a solution of dye of the concentration given, and allowed to take up all it will absorb. It should then be rinsed several times in cold water till the tint is sufficiently reduced. Better and more regular results are obtained in this way than by dyeing to the strength required and then rinsing briefly.
A six times screen necessitates inconveniently long exposures for many subjects, and is unsuitable for most flower studies, so it is as well to make a series ranging from one and a half times up to six. The cost and trouble of making such a series is little more whilst one is engaged in the matter than in making one, and a good plan is to dye, say, three pieces as described, then to dilute the solution to half strength, to dye three more, and so on. The pieces of gelatine may be washed out to different degrees of tint, and all may be mounted, or a selection only may be so treated and the remainder stored away till wanted.
For the benefit of those who wish to make one or two screens only, it may be pointed out that a piece of roll film fixed, washed, dried, and dyed is practically as good, and is somewhat easier to manipulate, than plain sheet gelatine. It must, of course, be cemented between two pieces of clear glass.
Naphthol green stops but little of the red, and with the screen described correct colour values are given well into the orange.
In conclusion, it may be said that those who have once mastered the orthochromatic plate will be loth to revert to the ordinary type for subjects in which colour plays a conspicuous part.