The sunbeam and the spectrascope / by Howard Townsend.

  • Townsend, Howard, 1823-1867.
Date:
1863
Catalogue details

Licence: Public Domain Mark

Credit: The sunbeam and the spectrascope / by Howard Townsend. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

    6/16
    ordinary circumstances. If though the rays of light be intercepted by a solution of sulphate of quinine or of horse chestnut bark, or by a crystal of fluor spar, these extra spectral rays are rendered apparent. These rays, which were unknown to Newton, have been investigated by Prof. Stokes, who has named them the fluorescent rays. They are luminous probably under all circumstances to those animals whose eyes are adjusted, as are the eyes of most of the night roaming creatures, to admit the rays of the highest refrangibility and to vibrate in unison with their vibrations ; but unless peculiar conditions be established, the fluorescent rays are not sensible to the human eye. Such, then, is the amount of our knowledge respecting the luminous principle of the sunbeam. It must be remembered that these rays vary considerably in the intensity of their illuminating power. The maxi- mum exists in the yellow ray, and it diminishes as we recede from it towards either end of the spectrum. The least refrangible, or the red rays, give a modified amount of light, but the maximum of heat exists in them. The most refrangible or the blue end of the spectrum is less luminous, but the maximum of chemical, action is fixed at this extremity. The fluorescent rays beyond the spectrum of Newton, being only visible under the peculiar circumstances already mentioned. If now we examine these beautifully colored bands of light when well defined upon a screen, with a small tele- scope, a new set of phenomena will become apparent. The spectrum is then seen crossed by a number of black lines. Every ray, even the most brilliant, will be found to have spaces in which there is an entire absence of light. It was Dr. Wollaston who first observed these non-lumin- ous spaces in the prismatic spectrum. Fraunhofer, how- ever, was the first to make a full investigation of these lines and to publish a map of them, and they have hence generally been called Fraunhofer's lines.