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THE PHOTOGRAPHIC NEWS. Vol. XXXV. No. 1696.—March 6, 1891. CONTENTS. PAGE The New Discoveries in Heliochromy 177 Dark Room Ventilation 179 The History of Photography. By Dr. J. M. Eder 180 Testing the Sensitiveness of Colour-Sensitive Plates. By Dr. H. W. Vogel 181 The Federation of London Photographic Societies 182 Photography in Germany. By Hermann E. Gunther 183 Photography at Harrogate 185 The Science of Colour 185 Notes ‘ ’86 Photographic Shutters. By Francis Blake 188 PAG M. Lippmann’s Discoveries in Heliochromy. By the Right Hon. Lord Rayleigh 189 The Portraiture of the Moon. By James Mew 189 The Mica Industry 190 Collotype Printing Without Machinery 190 A New Hand-Camera 191 Patent Intelligence 192 Correspondence.—The London and Provincial Photographic Association 194 Proceedings of Societies 194 Answers to Correspondents 196 THE NEW DISCOVERIES IN HELIOCHROMY. Among the chief points in which experienced practical photographers might attempt to help to adapt the recent great discovery of Professor Lippmann to utili tarian purposes are: — (1) The production of an absolutely colourless and tr insparent dry plate highly । sensitive to light; (2) to find a developer for the said | dry plate which will give a shining silvery-white image by reflected light, and a pure black or grey one by transmitted light; (3) the application of suitable orthochromatic methods of rendering the plates more sensitive to red and yellow light. By the wet-plate process there was no difliculty in getting silvery whites, as in the collodion positive pro cess, in which an iron developer was used in conjunc tion with a suitable proportion of glacial acetic or nitric acid; a trace of nitric acid had a strong influence in the yielding of a brilliant white deposit. The wet process is, however, in all probability, not applicable to these heliochromic pictures, because of the film being expanded with liquid at the time of making the exposure ; consequently, any interference bands regis tered by means of the film will not be of the same distance apart when the film shrinks in drying. The Photo-Gaiette, of Paris, in its number of Feb ruary 25th last, gives the accompanying engraving of the apparatus used by Professor Lippmann in photo graphing what the Daily Neios correspondent called his “stained glass window.” This window is represented at D, and it consists of three pieces of coloured glass, mounted in a circular frame. A is a loose cap over the chimney of the electric lamp, to exclude stray light from the room ; B is a parallel beam of light from the electric lamp ; C is a vessel of water to cut off some of the heat rays; E is a vessel containing elianthine to cut off the blue rays while giving a longer exposure to the red; F is the trough carrying the sensitive plate and its mercurial backing. A communication from Lord Rayleigh, in another column, shows that he long ago published the idea that the colours in Becquerel’s photographs were those of thin plates. He speaks of Wiener’s experiments; Wiener seems to have been the first to photograph interference bands in thin, trans parent sensitive films, and he just missed discovering the value of the method in heliochromy. A week before M. Lippmann announced his discovery at the Academy of Sciences, M.Cornu drew the attention of that august body to a paper by Herr Weiner, contained in Wiedemann’s Annalen, vol. xl., page 203, 1890, giving the experi mental solution of the problem of determining the direction of vibration in polarised light. “ The method consists in letting a wide beam of polarised light fall upon a reflecting surface at an angle of 45°. As the