Plates treated in this way come nearer to the great desideratum of a plate that will give true color values without a color screen than any other that I have tried. Even in copying diffi cult pictures the very lightest screen is sufficient, or it will do to work in gas or lamp-light with out a color screen.
The plates are very difficult to work, however, and I have no idea whether they will keep or not. I have always used them either wet or soon after they were dry.
Picric Acid for Color-Screens.—Picric acid (or picrate of ammonium) would appear to be of exactly the right color for color-screens to be used for orthochromatic photography.
It should always be borne in mind, in this connection, that we want to cut off with the color-screen the greater part of the violet light, a great part of the violet blue, and some of the blue, but none of any other color. Now most color-screens—notably those made with aurine—are really of a very dilute (if the term may be permitted) orange yellow color. This means that they cut off some green and yellow light as well as some blue and violet. This the light lemon color of picric acid does not do.
The following experiment is an instructive one. A plate is coated with a solution of gela tine and picrate of ammonia in the manner described further on, but the coating is made thick at wit end, thin at the other. This is done by slipping a thin wooden wedge under one end of the plate, while the latter is on the leveling slab, till the solution has left one end entirely, or almost entirely.
This plate, when dry, is examined by the spectroscope, placing it between the spectroscope and the light and slowly passing it along, so that the light passes first through the thinnest part of the film, last through the thickest. The appearance is that the light is gradually cut off from the extreme violet end of the spectrum. First the extreme violet goes, then the violet, then the violet blue, and with the thickest film possible to make in the way described, the greater part of the blue, but no other color is perceptibly affected, not even the blue-green. I have an advertise ment printed in a violet-blue ink—a pure violet blue, not a mixture of blue and red; evidently an aniline, or at any rate a coal-tar product—color. This, looked at through a moderately thick picrate film, looks as black as printers' ink.
A comparison of picric acid and aurine was made by the aid of a cell with plate glass sides, solutions of the two substances being used. The picric acid was found to be immeasurably super ior to the aurine. The highest tinge of it had as great an effect as quite a deep colored solution of aurine, and, moreover, the picric acid gave truer color values than could be got by any strength of aurine solution.
A glass cell filled with a solution is an excellent thing for experimental work, but it is most awkward for practical work. This set me to work to try to make picric acid screens. All attempts to get a clear, transparent film with varnish or collodion failed. The films were always muddy. At last the following method was found successful: As for dipping plates, a cold, saturated solution of picric acid is made, and ammonia is added till picrate of ammonia is produced, as indicated by the smell of ammonia;* to each ounce of this solution are added io to is grains of hard gelatine. This is allowed to swell, and is melted
by heat. Four or five per cent. of alcohol is added, the solution is filtered through cambric, and patent plates are coated with the solution, exactly as dry plates are made by coating with gelatine emulsion.
Plates of different intensity can be produced by giving different thickness of coating. A very thin coating only is needed for orthochromatic purposes with any orthochromatic plates I have ever used. With thickly coated plates very fair orthochromatic results can be got on ordin ary dry plates, but of course with a very greatly increased exposure.
When the picric plates are dry they should be varnished with great care.
It may be objected that "patent plate" is not true enough for color screens; that only "optically ground " surfaces are good enough, and it is quite true that screens made as I have described, and used near the lens, either in front of or behind it, perceptibly degrade the defini tion. To overcome this difficulty, the screens are made of the full size of the plates to be used, and are placed in the dark slide in front of the plates to be used, picrate film to gelatino bromide film, an adjustment equal to the thickness of the screen being, of course, made after focusing unless a special dark slide is used. Working in this way there is no perceptible degradation of definition.
It is to be feared that in these degenerate days it is not every photographer who can coat plates with the perfect evenness that is necessary for this kind of work. I believe professional plate makers might find it pay them to coat such plates of the standard sizes and of different intensity.
It may be objected that two plates cannot be inserted in most dark slides in the way described. They cannot in all kinds of dark slides, but they can in most; though in the case of many double dark slides only one picrate screen and one dry plate can be used in each.
Picratc Plates as Dark-room Lamp have made a few experiments with a plate coated with the solution above described, as the transparent medium for a dark-room lamp. The plate was rather thinly coated. The light appeared nearly as white as that of common gas with out any shade; but with the same amount of visual light, it was found to be very nearly as safe as the light filtered through red flashed glass that been picked out of a large quantity by spec troscope tests as being particularly safe, and the light yellow light is much the more pleasant to work in.
I am proceeding with experiments with plates more thickly coated and with several in front of each other. I hope thus to get illumination of a light yellow color that it will be a pleasure to work with, and that will be at least as safe as any other kind of light.