. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 288 LIGHT AND ITS ARTIFICIAL PRODUCTION. ultravioJet rays is very small compared with that of the red and infra- red rays. The heat emitted by a candle at a distance of 1 meter would raise the temperature of 1 gram of water, hardly a thimbleful, by only 1° 0. in one and one-fourth years, provided it could be stored for that length of time. This will give you some notiou of the extreme sensi- tiveness of the eye for light rays. But neverthel
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 288 LIGHT AND ITS ARTIFICIAL PRODUCTION. ultravioJet rays is very small compared with that of the red and infra- red rays. The heat emitted by a candle at a distance of 1 meter would raise the temperature of 1 gram of water, hardly a thimbleful, by only 1° 0. in one and one-fourth years, provided it could be stored for that length of time. This will give you some notiou of the extreme sensi- tiveness of the eye for light rays. But nevertheless it takes a definite quantity of energy, a very small quantity, to produce the sensation of light. Hence a body radiating light waves of this intensity j ust begins to become visible. This incipient incandescent state begins at a rela- tively high temperature in the sources of light commonly used. It is not that these waves are not present at lower temperatures, but their energy is too small to affect the optic nerve. Just as soon as the crit- ical temperature is passed, the heated body becomes luminous and its brightness increases rapidly with increasing temperature. The amount of energy producing the sensation of light is however a minute fraction of the total energy radiated by the body. This total radiation obeys definite laws iu respect to its increase with the temperature, and so does that x)ortion of it which affects the eye. Before considering these laws, let us first see how these two kinds of radiation can be separated. Separation of heat radiation from light radiation.—I have here a strip of sheet platinum tlirough which an electric current of any strength up to 100 amperes can be transmitted. [Experiment.] The current heats the strip, and as it increases the temperature of the metal increases until it finally begins to glow, and by a still further increase it is heated to intense whiteness and then melts. Before it begins to glow it nevertheless emits energy,
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