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.)

    4/16
    is perfectly transparent to light, but opaque or impermeable to heat. Glass which has been stained yellow with oxide or chloride of silver allows a flood of light to pass through it, but permits no permeation of an Actinic ray. And on the contrary, if we use a glass colored deeply blue, with the oxide of cobalt, though but very little light can pass through it, experiment proves that it offers no obstruction to the chemical rays, that is, it permits the permeation of the Actinic ray. This fact of yellow glass interfering with and intercept- ing the actinism of the sunbeam, has lately been taken advantage of in photography. The photographer no longer shuts himself and his prepared plate in a dark dungeon, for now the old dark chamber, camera oscura, is beautifully illumined by the sun's rays passing through yellow glass which effectually excludes the actinic, the chemical rays which alone the photographer dreads in this part of his pro- cess, but which allows all illuminating rays to be transmitted. The strength of evidence appears to be in favor of con- sidering light, heat and actinism as three distinct principles or powers, active in regulating the great phenomena of nature. These agents are unceasingly at work. It is im- possible to expose any body, however solid and persistent it may appear, to the influence of sunshine, without its undergoing a molecular or chemical change. In darkness, all bodies appear to possess the power of restoring them- selves to their normal state. Should the sun shine uninter- ruptedly upon a granite monolith or a bronze statue, it would perish independently of any other destructive in- fluences. Night seems as necessary to secure the permanence of the inorganic world as darkness and sleep are essential to maintain in healthful life the organized creations. At the enormous distance of 95,000,000 miles from us is the sun, a great orb having a diameter of 882,000 miles,