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138 FHE PHOTOGRAPHIC NEWS. LFEBRUARY 27, 1891. of a finely cut Golconda diamond ; why “ Golconda ” we know not, for the Brazilian diamonds are of as “ clear water,” and can be cut in the same way. The working of the Indian mines was entirely or for the most part stopped years ago because the yield was not remunerative; we have not heard of any resump tion there of the search for the gem. The lustre of the wings of some green and other beetles, also of the wings of some of the more gorgeous tropical butterflies, is supposed to be due to the interference of light set up by numerous transparent, horny layers! The conclusion, therefore, is, that in these helio chromes we have brilliant colours not to be seen elsewhere at present in connection with any branch of pictorial art. Such pictures, somewhat less perfect in colour, were to be seen once, for Niepce de St. Victor, one of the imitators of Becquerel, appropriated the best of discoveries in heliochromy without acknowledgment, and succeeded in turning out some remarkable pictures by means of long exposures in the camera. If Niepce de St. Victor were allowed to dress a doll in such materials and of such colours as he chose, he could somewhat faithfully produce a photograph in the true colours of the doll, and one of the most remarkable facts was, that gold ornaments did not come out upon the plate of an ordinary yellow colour, but with metallic lustre, resembling gold itself. Before long, in the light which M. Lippmann has thrown upon the subject, varieties of colour will give less trouble in heliochromy, because orthochromatic photography is now a fact, and will aid in the work. By cyanine and other agents the films used in M. Lippmann’s may be made sensitive to the red ; they can also be made sensitive in the usual way to the yellow. In most cases it will probably be necessary that the dye used shall be washed out of the film, and, possibly, the use of a faint yellow screen may be an advantage. The quickening of the films used in M. Lippmann’s process seems to be but a question of time and of ex periment, chiefly by experienced photographers. The production of a dry plate and devising of a developer, which together shall yield an image silvery white by reflected, and of a pure black tint by transmitted light, is also a matter likely to be suitably accomplished with lapse of time. Perhaps the most beautiful stereoscopic portraits ever produced were those upon Daguerreotype plates, with the images afterwards delicately tinted by hand. Some such were on view at and after the opening of the Crystal Palace at Sydenham, and they wereexhibited,we think, by Claudet; they proved a great attraction to the visitors. How somewhat similar portraits, but with the rainbow-like hues produced by M. Lippmann’s process would appear, has yet to be seen ; he might make a beautifully-coloured doll sit for its portrait in the first instance, to reveal what is in store in this direction in the future. With a mixed body of readers such as this journal possesses, many of them highly scientific, some semi- scientific, and others not scientific at all, but purely artistic, it is difficult to meet the mental requirements of all in one article, so the first-mentioned class may excuse us if a little be now said for the information of others about the well-known phenomena of the inter ference of light upon which the new heliochromic process depends. If two stones be dropped vertically, and at a few yards from each other, into a pond of still water, each stone will initiate a circular series of waves, extending ring-like in all directions. If, when the two outer rings meet, the crests of one series of waves tend to coincide with the crests of the other series of waves, in creased wave motion will be the result of their meeting. If, on the other hand, when they meet the crests of one series of waves tend to coincide with the depressions of another series of waves, as represented in figure 1, the Fig. 1. two series will neutralise each other ; thus, two motions produce no motion. “Beats” in music are due to the same cause ; at intervals two sound-waves of air occa sionally meet in such a manner as to produce momentary silence. So is it with light. Waves of monochromatic light can be experimentally so mixed as to produce darkness in bands upon the screen of a lecture room, and it is often done in public. D— — C E F Fig. 2. Let A B C D be a section of a sensitive film, and D C a mirror backing ; red light failing in the direction G E will be reflected at E and sent back, producing inter ference bands, in the luminous spaces between which silver will afterwards be deposited by the developer. Let blue waves fall in the direction H E; the same phenomenon will be repeated, but the blue waves being shorter than the red, the interference bands will be closer together. Suppose, now, the plate to be deve loped and fixed, then exposed to daylight; the blue rays alone in white light will pass freely backwards and forwards through the area H F, and the red rays freely through the space G E; hence the difference in the colours of these spaces to the eye, although the silver deposits themselves are practically colourless. All this suggests the use of a developer which will produce images more resembling stains than coarse deposits, so as to produce as perfect and clean a series of bands as possible ; in fact, M. Lippmann has laid down a useful, practical working hypothesis, which seems at present a true theory, and may afterwards be proved to be such. A few months or even a few years may elapse before photography in colours becomes an every-day, familiar fact, but such a result now appears to be within measurable distance.