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

  • Draper, John William, 1811-1882.
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.

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    mirror there are therefore three foci. At a distance of eleven inches thei-e is one, e, Fig. 88, giving a spectrum image of the sun. Still fiirther there is a second,/, which is a spectrum image of the slit, a, in which, if the prism be at its angle of minimum deviation, and the other ad- justments be correctly made, will be seen the Fraunhofer lines. Again, still farther off, at (/, is a focal image of the rectangular opening of the black paper, c c, on the front face of the prism. This im- age, arising from the recombination of all the dispersed rays, is consequent- ly white. The sec- ond and third foci are at distances from the mirror de- pending on the dis- tance of the slit, <2, and the black pa- per, c G, respectively. With the intention of being certain that the light com- ing through the slit, a, is falling properly on the rectangu- lar opening in the prism screen, c c, a small looking-glass IS placed at p. The experimenter, sitting near the mul- tiplier, w, can then see distinctly the reflected image of that opening. At the place where the second focal image, with its Fraunhofer lines, forms, two screens of white pasteboard, h, i, are ari-anged. By suitably placing the former of these, 7i, the more refrangible rays may be intercepted, and m like manner by the other, ^, the less refrangible. In usmg these screens, and particularly h, care must be taken that no rays passing from the prism to the mirror Fig. 87.