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THE PHOTOGRAPHIC NEWS Vol. XXXV. No. 1695.—February 27, 1891. CONTENTS. PAGE The New discoveries in Heliochrom 1.... 15? The Magic Lantern Mission 159 Plates for Helic chromy. By Gabriel Lippmann 160 The Camera and its Various Motions. By Prof. W. K. Burton 160 Lantern Slides. By F. Goldby 161 The Portraiture of the Moon. By James Mew 163 Photographiog in Colours 164 A Zinc Sulphide Compound Sensitive to Light. By John Cawley 164 Photographic Shutters. By Francis Blake 165 Notes 166 PAGE Photographic Tourists in Iceland. By J. Reynolds 168 Heliochromic Researches 169 Photography in the Ordnance Office 169 The History of Photography. By Dr. J. M. Eder 170 Aluminium. By G. L. Addenbrooke 171 The Science of Colour 171 Patent Intelligence 171 Correspondence 172 Proceedings of Societies 174 Answers to Correspondents 176 THE NEW DISCOVERIES IN HELIOCHROMY. This month of February, 1891, will long be celebrated in tho annals of photography. At the beginning of ■the month, photography in natural colours as a prac tical working, every-day process seemed a problem unlikely to be solved during the present generation ; at the end of the month the solution of the problem seems both possible and probable; moreover, in a scientific but uncommercial sense it has already been achieved. Becquerel seems to think that his process is different in principle from the new one, but that is apparently a mistake. His heatings of the sensitive film on a silver plate probably gradually altered the “ cheesy ” physical structure of the film to one of a somewhat more transparent nature, so that the bright surface at the back brought interference phenomena into play, and yielded coloured images, which dissolved with the rest of the film in fixing solutions. In some what similar processes upon paper, the preliminary exposure to diffused light probably produced a number ■of minute particles or spangles of reduced silver, which ■acted as imperfect reflectors, and produced imperfect interference phenomena. The film having been ren dered an objectionable colour by the first exposure, the heliochromic photographs on paper never had the same brilliancy of colour or the more suitable background of those on plates of polished silver. Becquerel worked most industriously, yet empirically, with no lamp to guide his footsteps, and, under the circum stances, the results were remarkably good, but would not long resist the action of daylight. Professor Alexander Herschel once published that he had seen a photograph of the spectrum by Becquerel good enough to figure as the coloured frontispiece of a scientific book. We also have seen one answering that descrip tion in the possession of the late Warren De la Rue, who first exhibited it to us more than twenty years ago, and again did so quite recently, but a few months before his death. It did not seem to have deteriorated meanwhile, but its owner kept it excluded from light in a leather case, except when momentarily showing it to visitors. He had another of these photographs of the spectrum, which once belonged to Faraday, but this was a poor one, and required a certain amount of faith to recognise the colours. Probably it had not always been carefully protected from light before it reached Mr. Warren De la Rue. The theory of Becquerel’s photographs was not understood, but it was speculated that the colours might be those of thin plates, or that the molecular condition of the film was so modified by light that the particles threw off the particular colours by which they had been impressed. Professor Lippmann has scientifically solved the problem by declaring them to be the colours of thin plates, and by using continuous and almost transparent films resting upon a brilliant reflecting surface, so as to get nearly perfect interference bands in the film, he has succeeded in photographing colours. He has also revealed that the layers of silver deposit in the film should be colourless, and that incident daylight would cause different colours to be reflected, according to the distances apart of the bands at any particular part of the film. This makes future investigation easy, for the practical photographer who seeks to perfect the process now knows what result he has to try to obtain, and how to set about the work. The secret also has been revealed how and why the photographs can be fixed. Without having seen them, it is easy to realise the nature of the colours obtained, and to describe their pictorial character. They are those of the diffraction grating, and, like Daguerreotypes, can be seen best with a particular incidence,of white light. They re semble the interference colours of mother-of-pearl, but are brighter, and M. Lippmann says that they are purer than those of the soap-bubble. Those who want to see the colours of the soap-bubble in perfection should blow bubbles from a solution like that used by Lord Rayleigh, and described in these pages last week ; the bubble should be illuminated on one side only, and by a strong light, such as that coming through the condensers of a limelight magic lantern, and should be viewed by the light the bubble reflects. The corres pondent of the Daily News describes the colours ob tained by M. Lippmann as equalling in brilliancy those