704 SPECIAL SENSES 



ception and recognition and for the production of visual memories. 

 This may be called the psychical visual centre. Psychical blindness 

 may exist, indeed, without loss of visual sensation. 



3. The angular convolution is not a visual centre, as was claimed by 

 Ferrier. It is related to visual perception only in so far as it affects 

 " the memories of the appearance of written or printed words" (Hun). 

 In cases of word-blindness lesions have been found in this situation. 



Perception of Colors. By far the most obscure question connected 

 with vision relates to the perception of colors. In treatises on physi- 

 ology, the difficulty of this subject is well illustrated in the indefinite 

 manner in which it is discussed ; and I am tempted to dismiss the 

 question with the remark which appeared in earlier writings on 

 vision that "nothing is known of the mechanism of color-percep- 

 tion." It is well known, however, that white light may be decomposed 

 into the colors of the spectrum and that different colors have different 

 often widely different wave-lengths. If a beam of white light, 

 with a certain wave-length, impinges on the retina, it produces a certain 

 impression that is appreciated by the visual centres. If, now, beams of 

 light, different in color and with different wave-lengths, reach the retina, 

 they must produce different impressions and a different impression for 

 each color and combination of colors. When these impressions have 

 been frequently repeated and the visual centres have, so to speak, 

 become educated, different colors are recognized. It seems a ques- 

 tion, indeed, if there be anything more than this in color-perception. 

 Certain it is, that if the waves that make the impression on the retina 

 which gives the sensation of light and color are perceived in a certain 

 way, differences in wave-lengths must involve differences in the nature 

 of the impressions, corresponding with necessary differences in the na- 

 ture of the stimuli. It almost seems better to take this view, however 

 unsatisfactory it may appear, than to enter into a discussion that is not 

 likely to lead to positive conclusions. In olfaction, of course different 

 odorous impressions are made by different substances composed of dif- 

 ferent numbers of molecules. It has been found that the odor of ethyl 

 mercaptan is recognizable in a dilution of one part in fifty billions of air. 

 If such infinitesimally small particles can make a decided and peculiar 

 impression on the nerves of smell, it is not unreasonable to suppose that 

 the parts concerned in vision may appreciate the difference between red 

 and violet, with 392,000,000,000,000 and 757,000,000,000,000 vibrations 

 in a second respectively. There are, indeed, quite as wide differences, 

 as regards number of molecules, between red and violet as there is 

 between ethyl mercaptan (C 6 H 6 SH) and benzene sulphide (C 6 H 5 ) 2 and 

 between many other distinctively odorous substances. 



