In the present state of the screen colour process, it is hardly advisable for us to give practical directions. Improvements are introduced almost daily, and the directions issued by the manufacturers of the various plates are so complete, that any supplementary advice on our part would be unnecessary. The method of interference colour photography has more than a passing interest to the student, and it has very recently been brought within the reach of the ordinary worker, who would otherwise have been barred by the difficulties of making grainless plates.
The following emulsion is suggested by Mr. Edgar Senior:
A. Gelatine........ 75 gr.
Potassium Bromide . . . . . . 32 ,,
Distilled Water....... 8 oz.
B. Gelatine........ 75 gr.
Silver Nitrate....... 45 ,,
Distilled Water....... 8 oz.
Soak the gelatine and dissolve in water, raise the whole, when the salts are added, to a temperature of 950 Fahr., mix the two at this temperature, filter through finest silk or muslin, and coat the plates immediately; after cooling they must be washed for about half an hour. Clean plate glass of good quality must be used, and the emulsion must not be left after filtration, or it will ripen and acquire a grain. The plates must then be panchromatised with the usual dyes. Such plates require a somewhat long exposure, unless they are flooded with an alcoholic solution of aceto-nitrate of silver just before use. However, Messrs. Carl Zeiss, who have been mainly instrumental in reviving the Lippmann process, have arranged for a ready-prepared plate to be placed on the market.
The plate may be handled quite safely in red light. It is transparent, and some difficulty may be experienced in discovering which is the coated side, especially as it must not be touched by the hand, finger marks, and even the minute scratches of a cloth or brush, showing on the finished positive. A special dark slide is employed, into which the plate is introduced, with its coated side away from the lens, screwed into position with a rubber marginal fitting, and the mercury backing is then poured in through a patent filler. After exposure, the mercury is extracted and poured back into the bottle, by the aid of the same apparatus which served to fill the space at the back of the slide. Exposure is necessarily long - at least, one to two minutes in bright sunshine at f/4.5; especially as the yellow correction filter is essential.
Before developing, brush the plate well with a soft-camel hair brush, to remove any minute globules of mercury. Any good standard developer, diluted, will serve the purpose, but amidol is generally preferred.
Amidol ........ 2 gr.
Sodium Sulphite ....... 20 ,,
Water ........ 1 oz.
Messrs. Zeiss recommend a very much weaker solution - about 1/4 grain amidol to the ounce of water - above all if intense contrasts are present. It is very important to guard against any pronounced density, and development ought to be complete for the purpose within two or three minutes, when the plate may be rinsed and fixed for 1 1/2 minutes in a 15 per cent. solution of hypo. If left long in this the delicate silver deposit would be attacked. Wash well in running water to remove all traces of hypo, and bleach the image in:
Mercury Chloride . . . . . . . 6 gr.
Water....... 7 oz.
After which, redevelop in the amidol developer as before. This completes the preparation of the plate, which may now be washed for five minutes and set up to dry.
Pyro is not an unsuitable developer, provided that a sufficiently intense illumination has been given for the mixed colours, and is specially recommended for photographic spectra.
When the plate is dry, its appearance will be somewhat disappointing to those who expect to get vivid colouring. As a transparency the pictures show merely a uniform dark-brown colour, but a certain amount of colour is visible by reflected light, albeit far from being correct. Prof. C. Wiener has explained this defect as due to the light, reflected from the outer surface of the film, coming into conflict with the coloured rays from the internal strata. These effects may be minimised by placing the film in intimate contact with a wedge-shaped layer of some substance possessing approximately the same refractive properties. Prof. Wiener found the simplest means to be a narrow glass trough filled with benzole. The picture to be viewed is placed in this trough in a slanting position, the bottom of the trough being sloped to one side so that each plate set in it will automatically take up the proper angle. Such an apparatus is, no doubt, most suitable for testing the colour qualities of a plate, but rather troublesome for the amateur. A method long in favour is to cement a prism of low angle, or wedge-shaped glass, upon the film by means of thick Canada balsam, and then blacking the whole rim and back of the plate with dead black varnish, afterwards protected with paper. The prisms need not be optically worked, and therefore are not expensive.
The pictures are most easily seen in a room lighted by one window rather high above the ground, and the particular angle most suitable for observing the colours must be found by practice. Several instruments, after the nature of the closed stereoscope, have been recently devised, as well as a contrivance for fitting on the ordinary magic lantern for the optical projection of Lippmann slides.
No doubt, many of the earlier so-called solutions of the colour problem, particularly the exhibits of Poitevin and Vidal, were really due to interference phenomena. About twenty years ago, Messrs. Townson & Mercer introduced an albumen paper with a blue ground, which gave most suggestive and pleasing tints under orthochromatic negatives. Unfortunately, before we had time to submit it to any exhaustive tests, the paper was withdrawn from sale. A few days ago, a friend showed us a delightful print of an apple tree in bloom. The tints were not very pronounced, but were soft and delicate, as became the subject. We were informed that it was printed from an ordinary negative on an emulsion coated on silvered paper. Apparently, a little white matter, either starch or barium sulphate, had been added to give the required degree of opacity in the high lights.