294 LIGHT AND ITS ARTIFICIAL PRODUCTION. 



many contradictions in the observations hitherto made, and which 

 unfolds many new relations. According to this theory the cones pro- 

 duce the sensation of sight when the stimulus is very intense, and their 

 excitation produces in the brain a color sensation, while the rods repre- 

 sent the organs of a color-blind eye and make no distinction of color 

 whatever. If the intensity is very small the rods are at first only 

 affected and evoke in the brain an impression of light without color, 

 and thus increase the sensitiveness of the organ very greatly in the 

 dark. But as soon as the stimulus is sufficiently increased, the cones 

 are affected and the sensation of color is added to that of lighjb. 



Even Konig had assigned a special role to the rods in his color theory. 

 Based on the absorptive properties of "visual purple" for waves of all 

 wave lengths, Konig drew the conclusion that the absence of color to 

 the normal eye in the spectrum, if its intensity is very small, and the 

 sense of sight of persons who are totally color blind, is essentially a 

 consequence of the decomposition of "visual purple."^ Since only the 

 rods are charged with " visual purple," and since the cones do not 

 contain any, the rods are the only x)arts of the eye affected in colorless 

 vision of the normal eye when the intensity is very small, and are the 

 only elements that can be affected in total color blindness by any 

 luminous intensity. But while Konig draws the conclusion from his 

 experiments that the cones do not respond to blue light, since " visual 

 yellow" or decomposed "visual purple" produces the sensation of blue, 

 Kries was led to the hypothesis that the cones are excited by light of 

 all colors without exception. Now there is on the retina a central por- 

 tion called the retinal depression or fovea centralis, on which there are 

 only cones and no rods, while on the rest of the retina both elements 

 are found; so that as we pass from the center outward the number of 

 rods increases in comparison with the number of cones, until near the 

 periphery the former outnumber the latter. Moreover, the fovea cen- 

 tralis is tbat portion of the retina corresponding to most distinct vision, 

 the part on which the image of an object which we wish to closely 

 scrutinize is focused. 



In direct or foveal vision, therefore, the rods are not concerned at all, 

 while in indirect or peripheral vision both the rods and cones are 

 excited. Therefore if the luminous intensity is small enough the rods 

 only are affected, producing the sensation of grayness, of light without 

 color. Long before physiologists had reached these conclusions com- 

 parative anatomists were led to the recognition that the rods of a 

 retina make it possible for an eye to see in the dark. 



Zoologists, Max Schultze for example, as early as 1866 were aware 

 that animals such as the owl, which preys in the darkness, or the 

 mole, condemned to spend its life beneath the ground, were provided 



1 This kind of total color blindness for small intensities in normal eyes, discovered 

 and thoroughly investigated by Hering and Hille brand in 1889, bad already been 

 observed in 1873 by W. v. Bezold in connection with his investigations on the law of 

 color mixtures and the theory of color vision. 



