Scientific memoirs : being experimental contributions to a knowledge of radiant energy / by John William Draper.

  • John William Draper
Date:
1878
Catalogue details

Licence: Public Domain Mark

Credit: Scientific memoirs : being experimental contributions to a knowledge of radiant energy / by John William Draper. Source: Wellcome Collection.

    412/488 (page 408)
    action of diffused light on this iodide is mainly due to the more refrangible rays it contains, we are brought by these experiments to the following conclusions: 1st. Every ray in the spectrum acts on silver iodide. 2d. The more refrangible rays apparently promote the action of daylight on that substance; the less refrangible apparently arrest it. 3d. For the display of this arresting or antagonizing effect, it is not necessary that the less and more refran- gible rays should be acting simultaneously. An interval may elapse, and they may act successively. Hence the effect is not due to the contemporaneous interference of waves of different periods of vibration with one another —the material particles of the changing substance of the silver iodide are involved. I abstain for the moment from giving further details of these spectrum impressions. That has been very completely done by Herschel in the case of one I sent him many years ago. His examination of it, illustrated by a lithograph, may be found in the Pliiloso])liical Mag- azine (Feb., 1843). I shall have to return to the subject of the behavior of silver iodide in presence of radiations on a subsequent page of this Memoir. The main point at present established is this, that the iodide under proper treatment is affected by every ray that a flint-glass prism can transmit, and therefore it is altoo-ether erroneous to suppose that chemical force is restricted to the more refrangible portions of the spec- trum. ^ ^, 7 7-> . 2(f.—Case of Bitumens and liesms. These substances are of special interest in the history of photography, since in the hands of Niepce they prob- ably were the first on which impressions in the camera were obtained and fixed. Their use has been abandoned in consequence, as it seems to me, of an incorrect opinion