292 LIGHT AND ITS ARTIFICIAL PRODUCTION. 



therefore emit waves of all wave lengths. Consequently they emit at 

 a sufficiently high temperature white light, since white light consists of 

 a mixture of light of all wave lengths. The absorptive power of an 

 incandescent body can therefore be determined by the light it emits, 

 and conversely, the color of the light emitted can be determined from 

 its absorption. The greenish color of the Welsbach light is most prob- 

 ably due to the selective absorption of green light by the material of 

 the mantle at its high temperature. In order to produce white light it 

 would be necessary to combine the substances now used in its con- 

 struction with materials which would be more partial to the red light. 



Bodies which completely absorb all wave lengths at all tempera- 

 tures and therefore emit rays of all wave length are called "absolutely 

 black" bodies. In nature these ideal conditions are never realized, 

 and, moreover, the laws of Stefan and Wien apply only to them. 

 Nevertheless these properties are almost realized if the interior of a 

 hollow sphere is permitted to radiate through a small opening in its 

 wall.i 



Experiments on the radiation of an absolutely black body are in 

 contemplation at the Eeichs-anstalt, by which it is to be determined 

 how nearly carbon, the metals, and their oxides correspond to an 

 absolutely black body. It is, however, certain that the absolutely 

 black body is the only one which emits the maximum amount of energy 

 theoretically possible. It emits, therefore, other things being equal, 

 more light than any other substance. The absolutely black body 

 would be the first to evoke the sensation of light with gradually 

 increasing temperature and would emit light when metals, e. g., plati- 

 num, at the same temperature would still be nonluminous. The lumi- 

 nosity of a body, especially its first visible manifestation, depends also 

 to a great extent on the organ of sensation; therefore we must in 

 addition study the structure of the eye. But we will first take up the 

 study of the conditions under which heated bodies just begin to emit 

 light. 



THE FIRST SIGNS OF LUMINOSITY OF HEATED BODIES (GRAY HEAT 



AND RED HEAT).^ 



According to the modern theory of heat discussed above, we must 

 assume that a solid body, certainly a black body, emits waves of all 

 possible wave lengths, at all possible temperatures, and therefore even 

 at the ordinary temperature. At every temperature ether waves, of a 

 wave length corresponding to light waves, reach the retina of the eye. 

 These waves are, however, not recognized as light waves until the 

 energy accompanying them is great enough to afiect the optic nerve. 

 Under these circumstances a body is said to emit both heat and light. 



' W. Wien and O. Ijiiramer : A method of investigating the law of radiation of an 

 absolutely black body. Wied. Ann. d. Pliysik u. Chemie, 1895, Bd. 56, S. 451-456. 



2 The statements concerning gi'ay heat and red heat mentioned in the lecture are 

 partly the results of some personal observations as yet unpublished. 



