LIGHT AND ITS ARTIFICIAL PRODUCTION. 293 



If we gradually beat this platiuum strip [experiment] you will see 

 that it first shines with a reddish light. This first stage of luminosity 

 is called red heat. All solid substances which are incombustible at high 

 temperatures behave in the same manner as platinum. The tempera- 

 ture at which solids begin to glow was determined by Draper some fifty 

 years ago and was found by him to be about 525° G. Most solid 

 bodies, therefore, would have to be heated to an absolute temperature 

 of 273° + 525° or 800° before becoming self-luminous. In order that 

 the other wave lengths may affect the eye the temperature must be 

 increased very much above 525° 0. The observations of Draper, that 

 solids first emit a red glow, were not contested for a long time, since 

 they agreed with ordinary daily experiences. I need only instance a 

 red hot poker, a very hot stove, etc. It was all the more interesting, 

 therefore, when W. F. Weber showed, some ten years ago, that the red 

 heat is not at all the first stage of luminosity. 



On repeating the exj)eriments of Draper in 1886, Weber carefully 

 excluded light from all external sources and observed that solid bodies 

 emit, at much lower temperatures than that corresponding to redness, a 

 foggy, grayish light. The first trace of the grayish light appears to the 

 eye as an unsteady gleaming, flitting hither and thither. Its intensity 

 increases very rapidly with increasing temperature, while its appear- 

 ance changes from dim gray successively to ashen gray, a yellowish 

 gray, and finally to red. To use Weber's words, "with the first appear- 

 ance of the redness the last trace of grayness disappeared, as also did 

 the unsteadiness, which has been prominent in all stages of gray heat.'' 



As might be deduced from Kirchhoft's law, the redness as well as the 

 preliminary stage of luminosity begins at a temperature depending 

 upon the nature of the substance. In fact, investigations by E. Emden 

 show that gold begins to glow at 403° while platinum does not begin 

 until its temi^erature is 423° 0. 



Weber's observations are specially interesting to me because they 

 seem to give us some explanation of the structure of the eye, and espe- 

 cially of the properties of the retinal elements (the rods and cones) and 

 of their functions in color sensation. Stenger first pointed out that 

 Weber's experiments did not give any evidence concerning the physical 

 nature of the spectrum emitted by the body. I will show that these 

 queer and ghost-like phenomena of the "gray" light can only be 

 explained by attributing entirely different functions to the rods on the 

 one hand and to the cones on the other, considering them as two differ- 

 ent organs of sight, just as is done in modern physiology. 



Based on the latest physiological researches of Hering, Hillebrand, 

 Ebbinghaus, Preyer, Brodhun, Tonn, and others, and based on the 

 theory of A. Konig^ of the function of "visual purj)le" in sight, J. v. 

 Kries^ of Freiburg, suggested a theory of vision which easily explains 



'A. Konig: "Ueber den meuschlichen Sehpurpur," etc. Sitzsber. d. Akad. zu Berlin, 

 1894, S. 577. 



-J. vonKries: "Ueber die Funktion der Netzbautstabclien." Z. S. f. Psychologie 

 u. Physiol, d. Sinnesorgane, Bd. IX, S. 81-123. 



