TRICHROMATIC PHOTOGRAPHY IN MEDICINE.

By W. J. S. BYTHELL, B.A., M.D.

EVERY photographer has probably expressed the wish, at some time or other, that it were possible to reproduce the colours of nature that he has focussed upon the ground glass of his camera. The problem of photography in natural colours has always seemed to me a most interesting and attractive one, and I am glad to be able to state from personal experience that a satisfactory solution has at last been found. This is true at any rate with regard to the making of transparencies or lantern slides, although the printing of colour photographs upon paper is still beset with considerable difficulty.

The further development of the process, namely, the successful reproduction of the colour photographs for illustration purposes by means of half-tone blocks, has already proved itself of great scientific and commercial value. For just as the "process block" has almost entirely replaced wood engraving, so the three-colour process threatens to oust lithography from the field. Like its parent, the black and white half-tone process, it is a purely photographic method; it has also this further advantage over lithography, that it reduces the impressions to three-the same colours being used in every case, whereas in lithography the number of colours and their precise tones are left to the individual judgment and taste of the printer.

Several methods of photography in natural colours have been introduced during the last few years. These have all been based upon the well-known fact that by making use of the three primary colours, blue, red and yellow, we can obtain all the colours visible to our eyes in nature. It thus becomes necessary to take three separate photographs of the primary colours, and this can be accomplished by placing "light-filters" in front of the lens of the camera. These filters, consisting either of coloured glasses or solutions of dyes, have the power of absorbing or filtering out certain colours. In order therefore to obtain photographic records of the three primary colours use is made of the following system of filters:

1st negative, taken through red filter, gives blue print.

2nd

purple,,

red yellow

The three negatives of the same object having been exposed and developed, prints are made from them upon transparent celluloid films coated with gelatine and sensitized in a solution of potassium. bichromate; these are developed in the same manner as ordinary carbons, and are then stained respectively blue, red and yellow by means of certain aniline dyes. Finally the films are superimposed in correct register, the combined picture reproducing all the colours of the original object.

Transparencies obtained in this way can, with little more skill or trouble than is required for ordinary photography, be made to give very beautiful results. The colouring is both brilliant and true to nature. The range of subjects available is, moreover, practically unlimited-landscape, portraiture, 'still life" and scientific objects all come out equally well. In its application to medicine trichromatic photography will probably be of very great service, not only for the illustrations in journals and textbooks, but also in the form of lantern slides for clinical and pathological demonstrations.

The two photographs given here were taken by the SangerShepherd process, and the half-tone blocks were made directly from my negatives of the original objects. The adaptation of the halftone process to trichromatic work is not yet without its faults, chiefly owing to the imperfect blending of pigments when superimposed upon paper. The technical difficulties, however, are being steadily overcome, and there is every reason to believe that both in richness and gradation of tones, as well as in correctness of colouring, the three-colour process will shortly surpass anything that can be done by lithography.

FIG. 1. Case of acute psoriasis of leg. Taken at the Manchester Skin Hospital. Exposure, in very poor light, about four minutes, at f/11.

FIG. 2. Micro-photograph, x 100, showing inflammatory infiltration around a broken-down lymphatic gland; section stained with hæmatein, rubin and orange. Exposure, with " Sol" lamp, about 20 minutes.