Copy 1

The life of Sir Isaac Newton / By David Brewster.

  • David Brewster
Date:
1831
Catalogue details

Licence: Public Domain Mark

Credit: The life of Sir Isaac Newton / By David Brewster. Source: Wellcome Collection.

    126/392 page 104
    lens, and, instead of receiving the shadow and its fringes upon a smooth white surface, as was done by Newton, he viewed them directly with his eye through a lens placed behind the shadow ; and by means of a microscope he was able to measure the dimensions of the fringes with the greatest exact- ness. By this mode of observation he made the remarkable discovery, that the inflexion of the light depended on the distance of the inflecting body from the aperture or from the focus of divergence, * the fringes being observed to dilate as the body ap- proached that focus, and to contract as it receded from it, their relative distances from each other, and from the margin of the shadow continuing in- variable. In attempting to account for the forma- tion of the exterior fringes, M. Fresnel found it necessary to reject the supposition of Dr Young, that they were owing to light reflected from the edge of the body. He not only ascertained that the real place of the fringe was the ;//,dth of a millime- tre different from what it should be on that suppo- sition, but he found that the fringes preserved the same intensity of light, whether the inflecting body had a round or a sharp edge, and even when the edge was such as not to afford sufficient light for their production. From this difficulty the undu- latory theory speedily released him, and he was led by its indications to consider the exterior fringes, as produced by an infinite number of elementary * This effect is so great, that, at the distance of fowr inches from the point of divergence, the angular inflexion of the red rays of the first fringe is 12’ 6”, while at the distance of about twenty feet, it is only 3’ 55”,
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