A Manual Of Photography

24 niSTOBY OF PHOTOGBAPHY. each of them absorbs certain portions of the total incidental light, and reflects especially towards your eye the complementary por- tions, wherein predominate the rays proper to form the tint of which it would give you the sensation. But the chemically active reagent which the same parts of the picture receive and reflect is distinct from the light which affects your retina. In order that the chemical effect which it produces on the sensible paper, or on M. Daguerre’s layer of iodine, may present, in light or in shade, the equivalent of the coloured parts, it is requisite —1st, that this reflected radiation be chemically active ; 2d, that the energy of its action be proportional to the intensity of illu- mination operated in the eye by the portion of luminous radiation reflected from the same point of the picture. Now this latter concordance certainly should not be fulfilled in an equal degree, by the various colouring matters, which affect the eye in the same manner, and which the painter may substitute for one another in his work. Substances of the same tint may present, in the quantity, or the nature of the invisible radiations which they reflect, as many diversities, or diversities of the same order, as substances of a different tint present relative to light: inversely they may be similar in their property of reflecting chemical radiations, when they are dissimilar to the eye : so that the differences of tint which they presented in the picture made for the eye, will disappear in the chemical picture, and will be confused in it in a shade, or of an uniform whiteness. These are the difficulties generally inherent in the formation of chemical pictures ; and they show, I think, evidently, the illu- sion of the experimenters who hope to reconcile, not only the intensity, but the tints of the chemical impressions produced by radiations, with the colours of the objects from which these radiations emanate. However, the distant or near relations of these two species of phenomena are very curious to study, not only as regards the photogenic art, since that name has, very improperly, been given it, but likewise as regards experimental physics. I doubt not that examples of these peculiarities may be remarked in the images of natural objects and coloured pictures executed by the Daguerreotype ; but very apparent ones may be seen among Mr. Talbot’s present impressions. Thus, some of them represent white porcelain vases, coloured shells, a candlestick (of metal) with its taper, a stand of white hyacinths: The whole of these objects are felt and perceived very well in their chemical image; but the parts which reflect the purely white light, probably also the radiations of every kind, are, relatively to the others, in an exaggerated proportion of illumination, which, it seems to me, must result, partially, from the capillary communication during