domingo, 20 de junho de 2010



28  de Setembro



No. 1184


London, Saturday, september, 28, 1839

Pag. 621

Section A.Mathematical and Physical Sciences.

Theory of Light and the Photogenic Process.

 —A letter from Sir J. Herschel was read, in which he described a remarkable property in the extreme red rays of the prismatic spectrum, which had occurred to him in his experiments on Mr. Talbot's photogenic paper. It thence appeared, “that the extreme red rays (such as are insulated from the rest of the spectrum by a dark blue glass coloured by cobalt, and which are not seen in the spectrum, unless the eye be defended by such a glass from the glare of the other colours,) not only have no tendency to darken the prepared paper, but actually exert a contrary influence, and preserve the whiteness of paper on which they are received, when exposed, at the same time, to the action of a dispersed light sufficient of itself to produce a considerable impression. When a slip of sensitive paper is exposed to a highly concentrated spectrum, a picture of it is rapidly impressed on the paper—not merely in black, but in colours, of which the red is tolerably vivid, but is rather of a brick colour than a pure prismatic red. What is remarkable is, that its termination falls materially short of the visible termination of the spectrum. The green is of a sombre, metallic hue; the blue still more so, and rapidly passing into blackness. The yellow is deficient. The whole length of the chemical spectrum is not far short of double that of the luminous one, and at its more refrangible end a slight ruddy or pinkish hue begins to appear. The place of the extreme red, however, is marked by no colour."

[The letter alluded to M. Arago's rather equivocal allusion to this fact in his account of M. Daguerre's process, and concluded as follows :—]

“It is impossible in this climate to form a brilliant and condensed spectrum without a good deal of dispersed light in its confines; and this light, if the exposure of the paper be prolonged, acts, of course, upon every part of its surface. The coloured picture is formed, therefore, on a ground not purely white, but rendered dusky over its whole extent, with one remarkable exception,—viz. in that spot where the extreme red rays fall, the whiteness of which is preserved, and becomes gradually more and more strikingly apparent, the longer the exposure and the greater the consequent general darkening of the paper. The above is not the only singular property possessed by the extreme red rays. Their action on paper, already discoloured by the other rays, is still more curious and extraordinary. When the spectrum is received on paper already discoloured slightly by the violet and blue rays only, they produce, not a white, but a red impression, which, however, I am disposed to regard as only the commencement of a process of discoloration, which would be complete if prolonged sufficiently. For I have found that if, instead of using a prism, a strong sunshine is transmitted through a combination of glasses, carefully prepared, so as to transmit absolutely no ray but that definite red at the extreme of refrangibility, a paper previously darkened by exposure under a green glass has its colour heightened from a sombre neutral tint to a bright red ; and a specimen of paper, rendered almost completely black by exposure to daylight, when exposed for some time under the same glass, assumed a rich purple hue: the rationale of which effect, I am disposed to believe, consists in a very slow and gradual destruction, or stripping off as it were, of layers of colour, deposited or generated by the other rays, the action being quicker on the tints produced by the more refrangible rays, in proportion to their refrangibilities. It seems to me evident that a vast field is thus opened to further inquiries. A deoxydizing power has been attributed to the red rays of the spectrum, on the strength of the curious experiments of Wollaston, on the discoloration of tincture of guaiacum, which ought to be repeated; but in the sensitive papers, and still more in Daguerre's marvellous ioduretted silver, we have re-agents so delicate and manageable, that every thing may be expected from their application."

A picture of the spectrum thus formed, by lamp or candle light, not being fixed, was exhibited.

The President remarked on the importance of the discovery, that the actions of certain rays interfered with the action of others, so that instead of one series of discoloration, for all the rays, as hitherto supposed, each was liable to be affected and modified by the rest.

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