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228 THE ADVANCE OF PHOTOGRAPHY
of silver of any thickness, according to the duration of the current. The brownish subchloride of silver is thus produced, and this is chiefly sensitive to coloured light. This sensitiveness is, however, not great : it suffices to fix a powerful spectrum, but it requires a very long exposure to obtain pictures in the camera obscura ; and all suchpictures, unfortunately, darken in the light. Becquerel found that the sensitiveness was increased by heating the plates. This observation was turned to account b}T his successor, Niepce de St Victor (the nephew of Nicophore Niepce, see p. 21), who made numerous attempts from 1851 to 1867 to produce coloured photographs, accounts of which he communicated to the Paris Academy.
He worked, like Becquerel, with silver plates, which he chlorinated by immersion in a solution of the chlorides of iron and copper, and then heated them strongly. He thus obtained plates which appeared ten times more sensitive than Becquerel's, and allowed him to copy in the camera obscura, engravings, flowers, church windows, etc. He relates that he not only obtained the colours of objects in his pictures, but that gold and silver retained their metallic splendour, and the picture of a peacock's feather the lustre of nature.
Niepce de St Victor introduced a further improvement,, by covering the plate of chloride of silver with a peculiar varnish, consisting of dextrine and a solution of chloride of lead. This coating made the plate still more sensitive and durable. At the Paris Exhibition of 1867, Niepce exhibited various coloured photographs, which lasted about a week in a subdued daylight (they were shown in half -closed boxes). Other persons who paid attention to coloured photographs at this time were Poitevin at Paris, Dr Zenker * at Berlin, and Simpson and Sir W. de Abney in London. The two former investigators reverted to
1 Those who take a special interest in the historical part of matter arereferred to Dr Zenker's " Lehrbuch der Photochromie." Berlin, 1868.