In regard to the actual development of isochromatic plates, their treatment does not essentially differ from that applicable to ordinary plates, only that with those of Edwards' make it is very easy to secure density, so that with subjects showing much contrast less pyro. is necessary.
The makers, in their instructions, advise the use of the fixed alkalis, either the carbonates of potash or soda, rather than liquor ammonia, inasmuch as if an excess of the latter be used fog would probably result. The same, however, applies, though perhaps in a less degree, to ordinary plates, and I invariably employ ammonia without meeting with any ill effects. Hydroquinone is recommended by some workers, but, owing to its tendency to give negatives of a chalky nature, I seldom use There is, however, less chance of fog when employing it than when the pyro. and ammonia developer is used. There is one precaution which, after what has been said, will probably be obvious, and that is, that when examining the negative for detail and density during the progress of development it should not be exposed for long, or held too near the dark-room window or lamp, as the plates are, to some extent, sensitive to even the ruby light of the dark-room.
In order to obtain the full advantages of colour-sensitive plates it is necessary to use a "screen." This is a disc of yellow glass, varying in depth of tint from a very pale lemon colour to a much darker yellow. It may be obtained to fit inside the lens mounts, or behind the lens itself. The object of using a "screen " is to cut off some of the blue and violet rays, which have too great an effect upon even an isochromatic plate when used without a screen.
The chief advantage of the screen is found when copying paintings. Its use is then invaluable, and it would be both interesting and instructive to the reader if he were to make three copies of a brilliantly coloured painting or chromo lithograph ; the one on an ordinary plate, the next two on isochromatic pla,tes with and without a screen. If full justice has been done to each negative, a most striking difference in quality will be appa,rent, the isochromatic plate used with a screen giving by far the naost truthful rendering of the subject.
For photographing cloud and sunset effects isochromatic plates give results distinctly better than those obtainable on ordinary ones. The finest effects of this kind are usually
obtainable in the spring and autumn, when, as rule, stormy- and strongly-marked skies prevail. From a,bout an hour before to the period of actual sunset is, perhaps, the best time to get effects of this character. A suitable stand point having been selected (if the negatives are to be used for double printing, a low horizon should be chosen) the camera is directed towards the sun itself. A moment when it is obscured or partially obscured by a cloud should be selected for the exposure, which should be given by means of a, shutter of the " blind" type, or with one of the Kershaw pattern. Supposing the lens to have been stopped down to about f/22, a medium isochromatic plate used without a, screen would require about quarter of a second exposure. This is assuming the sun to be obscured and very near the horizon, and the sunset to be a yellow one. Very beautiful effects are to be obtained in this way. For such work a, screen is generally unnecessary, the yellowness of the sunset compensating for its absence.
For general landscape work it is not necessary to use a screen, though very often better results will be obtainable if one be used. In such case it should be very light in colour—a pale lemon yellow. The effect of the screen is to rendei the plate still less sensitive to violet and blue; there fore it will be seen that if too deep a. colour is chosen the photograph will be untrue, owing to too much of those colours having been cut off. For copying coloured pictures, diagrams, or paintings, or for photogra,phing flowers, a deeper screen should be employed, which will, of course, necessitate a much more prolonged exposure. For copying purposes, or for landscape work where the full effect of the process is desired, the medium rapid isochromatic plates should always be used in preference to the instantaneous. The use of a light screen will double or treble the exposure, and with a dark screen four or five times the ordinary exposure may be required, but when copying length of exposure is a matter of minor importance. No definite rule, however, can be given, and these times are merely 9pproximate.