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THE PHOTOGRAPHIC NEWS Vol. XXXV. No. 1698.—March 20, 1891. CONTENTS. PAGE Heliochromy at Home 217 Film Pictures for the Lantern 218 Electro-Chemical Reversals with Thio-Carbamides. By Colonel J. Waterhouse 219 The Photographic Chart of the Heavens. By William E. Plummer 221 A Rapid Sensitive Salt Free from Silver 222 Photographic Shutters. By Francis Blake 223 The International Microscopic Exhibition at Antwerp 224 The Federation of London Photographic Societies 224 American International Copyright Granted for Photographs 225 PAGE Notes 226 Photographic Chemistry : Prof. Meldola at the Society of Arts 228 The Science of Colour 229 Pictorial Composition. By P. H. Newman 229 Photography at the Royal Meteorological Society 231 Hospital Photography 231 Patent Intelligence 233 Correspondence.—The Liverpool Inte national Photographic Exhibition—The New York May Exhibition 234 Proceedings of Societies 234 Answers to Correspondents 236 HELIOCHROMY AT HOME. A popular idea is current, to some extent, that, in order to try experiments with Professor Lippmann’s process, costly apparatus is necessary, including an electric lamp, and an optical system for the casting of a pure spectrum, assuming pure colours to be necessary. The pure blue colour of the spectrum, however, is one of those which can be obtained by transmitting day light through suitably coloured glasses. Mr. H. G. Madan has discovered that if a piece of Chance’s “ signal green ” glass be laid upon a piece of rich cobalt blue glass, the two together transmit only the pure blue of the spectrum. In all such devices, the depth of colour of the glasses should bear some propor tion to the intensity of the incident light, for, when sufficiently thin, all coloured films are practically colourless and transparent. A tolerably pure red is transmitted by oxide of copper ruby glass. As the photographic films used are most sensitive to the blue rays, the novice in the process may well confine his attention at first to the taking of blue prints, and obtain a pure blue light to work with by means of the two sheets of glass just described. He will require, in addition, but M. Lippmann’s mercurial trough, already pictured in these pages, and one or two other simple appliances. The blues thus photographed will not resemble pigmentary blues, but possess a metallic iridescent lustre, in some cases, perhaps, like the blue in the tail of a peacock, in others like the blues some times seen upon steel, and so on ; in fact, by discovering how the shade of colour varies with different emulsions, colloid vehicles, developers, and modes of the treatment, the experienced photographer is likely to aid consider ably in the practical development of the new discovery. An idea is current that when light strikes the reflect ing mirror at an angle, it is reflected in another direc tion, so that the waves do not come straight back upon themselves to produce interference phenomena. There is some force in this, but its utterers forget the exces sive thinness of the dry sensitive film with which they are dealing, and that that thickness can only be ad vantageously increased up to a certain maximum. Theoretically, no doubt, the more the rays fall at a right-angle to the plane of the plate, the better. In printing by blue light, it might be well to put the blue screen in an aperture of one end of a long diaphragmed box blackened inside, and the mercury trough and sen sitive plate at the other, so as to work with rays as nearly parallel as can be conveniently obtained. In trying to print from a negative in this way, a disad vantage of Professor Lippmann’s process would come in, for the printing would have to be done through the glass of the sensitive plate, so there would be blurring, but less than many would expect if the glass be excessively thin. Perhaps, at first, the experimenter might simply gain experience in photographing differ ent shades of blue, and when he becomes expert he may abolish the mercury trough, and use plates of polished silver bearing sensitive films; he can then print on a surface in close contact with the negative, and, in addition, get more brilliant colours, because silver reflects light more powerfully than does mercury. A popular error in relation to the process has reached our notice from several different directions. The colours seen in the prints are those of thin plates, so some persons think that there ought to be a tangible thin plate, which they bite if they like, somewhere in the apparatus; others think that a layer of air between the mercury and the film forms a thin plate, and that no colours at all can be obtained if the emulsion be in optical contact with a plate of polished silver. The real hypothesis is, that when light strikes the reflecting surface at or near a right-angle, there is interference close in front of that surface, but we cannot see it in the surrounding general flood of light. Hence the ingenuity of Herr Wiener, who employed nearly transparent sensitive films to search for these interference phenomena, and to make them register themselves inside the film for subsequent examination ; hence, also, the ingenuity of M. Lippmann, who has registered colours by interference in the same way ; and hence, likewise, the ingenuity of Lord Rayleigh, who published two or three years ago the principles of producing photographs in colours, which M. Lippmann has just proved to be practically true.