184 THE PERCEPTION OP LIGHT AND COLOR. 



decreased will finally appear as an illuminated colorless band. Repeat- 

 ing this experiment, Von Kries showed that this is only true for indirect 

 vision for the fovea, although less sensitive to the simple luminous 

 sensation, perceives the colors disappear without becoming white. 



In the same year Koenig made similar observations. 



In 1895 Hering, in an important paper on the Purkinje phenomenon, of 

 which we will speak later, established the fact that with dark sur- 

 roundings colors appear less saturated, a result of great importance, 

 according to M. Henri, although well known for a number of years, as 

 we might add. 



In 1896 Von Kries established the fact that in the fovea the ratio of 

 the intensities of the different spectral colors does not vary with the 

 illumination, as is the case in indirect vision. He also showed that red 

 is an exception to the law according to which spectral colors appear as 

 uncolored in indirect vision. 



In 1897 the investigations on visual sensations rapidly increased, 

 especially in the laboratory of Von Kries at Freiburg, in Breisgau. 



Von Kries, in collaboration with Dr. JSTagel, was engaged particularly 

 in experimentally testing Hering' s law in regard to the white value of 

 a mixture of two spectral colors, and they discovered that, even if it 

 be verified by observations made with the yellow spot, it is no longer 

 the same in indirect visiou by means of a retina adapted to obscurity. 1 

 They also studied the variations in the brightness of different colors, 

 according as the eye is adapted to the full daylight or to obscurity. 

 They showed that in central vision colors never give rise to the sensa- 

 tion of gray, as in peripheral vision, and finally they determined the 

 sensitiveness of the different parts of the retina to various chromatic 

 excitations. 



Interesting comparisons might be made between their numerical re- 

 sults and those obtained by MM. Charpentier and Parinaud, but taken 

 as a whole they do not bring to light anything new. Kries and his 

 collaborators have, however, made curious observations on subjects 

 totally or partially color-blind, of which we shall have occasion to speak 

 later. Concerning some of the experiments of Shermann that tend to 

 prove, in contradiction to Kries and Koenig (and let us add in contra- 

 diction to Parinaud, but in agreement with Charpentier), that points 

 very feebly illuminated are seen as uncolored by the yellow spot, M. 

 Henri objects that the fixation of the images is very difficult under 

 these circumstances and consequently these images may very easily 

 wander from the aforesaid spot. 



By this brief analysis of the G-erman researches it may be seen 

 that as a whole they have fully confirmed the results previously 

 obtained by Charpentier and Parinaud. Before taking up a study of 

 the physiological conditions which appear to underlie these phenomena 



1 It is unfortunate; that M. Victor Henri, ordinarily so exact, lias given in his 

 analysis a tahle of figures difficult to interpret. 



