LIGHT AND ITS ARTIFICIAL PRODUCTION. 295 



with rods even at the point of most distinct vision, where we have only 

 cones, and that there are even prowlers of the night which have only 

 rods on the retina and.no cones whatever. Like the latter class of 

 animals, we see in the dark by the same retinal elements, and indeed 

 we are color-blind if the intensity of the light is too weak to excite the 

 cones. 



The theory of Kries receives a new confirmation if applied to the 

 explanation of Weber's experiments on the initial stages of luminosity 

 of bodies, and at the same time it throws a new light on these little- 

 understood phenomena. 



Let us first deduce from the theory of Kries what phenomena should 

 be observed if a normal eye intently views the surface of a piece of 

 sheet platinum gradually heated by an electric current. Let us assume, 

 moreover, that the eye is directed to the center of the sheet, so that 

 not only the fovea centralis, but also the surrounding parts of the 

 retina, may be affected by the rays emitted. If the eye has been suffi- 

 ciently rested so that the rods have attained their maximum sensibility, 

 the latter are excited as soon as the platinum reaches a certain defi- 

 nite temperature (400° C, according to Weber), producing in the brain 

 the sensation of light without color, which increases in intensity rapidly 

 with increasing temperature. So long as the cones are not sensibly 

 affected, the fo veal parts of the retina transmit no sensation to the 

 brain. This is attended to by those parts of the retina which are not 

 ordinarily involved in direct vision. The curious condition then arises 

 that we see something which we are not looking at, and therefore the 

 object thus seen escapes as soon as the eye is directed to it. 



The sensation of gray is therefore produced by the rods, and can 

 only be produced by indirect vision. This easily explains the " unsteady 

 flitting about of the light" observed in all stages of the phenomenon by 

 Weber. It ceases as soon as the temperature, and therefore the inten- 

 sity, is increased to a sufficient amount to excite the cones. Then we 

 begin to see the glowing by direct vision, exactly as we are accus- 

 tomed to do, and the unsteady gray changes gradually into the reddish 

 glow observed. This transition occurs, according to Draper, at about 

 5l'5o 0. 



The observed color depends partly on the size of the portion of the 

 retina producing the sensation. Each set of elements acts independ- 

 ently of the other in transmitting its intelligence to the brain, and 

 therefore the effect i^roduced is a composite one. 



If we assume that the cones first respond to yellowish-red waves, the 

 first color sensation would likely be a foggy red, changing gradually to 

 a reddish-yellow, just as Weber describes. 



On account of the greater number of ^the cones, the rods, only 

 endowed with their greatest sensibility in the dark, lose their suprem- 

 acy with the increase of luminosity, and thus the color becomes more 

 and more fiery — 600° C. corresponding to bright redness. 



