PHYSIOLOGY OF COLOK VISION KELLEK AND MACLEOD. 725 



this reason is. The innermost layer of the eye, onto which images of 

 exterior objects are focused, is specialized to react to sensation of 

 light, thus setting up nerve impulses which are transmitted to the 

 brain where they are interpreted. This layer of the eye is called the 

 retina, and it is very much more sensitive at a small spot in the 

 center than it is over the much larger outer (peripheral) portions; 

 so that, of the image which is focused on it, it is only that part fall- 

 ing on the central portion which is distinctly seen. AVhen we regard 

 a stretch of country, for example, it is onlj^ in one part of it that the 

 objects are seen in any detail — namely, that part which is focused on 

 the central portion of the retina — the remainder, since it falls on the 

 outer portion, causing only a vague, indefinite impression. We may 

 say, indeed, that the function of the greater pqrt of the retina is 

 merely to give us a general impression of the environment of the object 

 which is being looked at, an impression, that is to say, which will 

 enable us to judge of its relationship to other things. It tells what 

 else there is to look at, and subconsciously we shift our gaze so that, 

 piece by piece, the whole landscape comes to be focused on the central 

 portion. We i-egard with the central portion what we know exists 

 to be regarded on account of the duller image thrown on the rest of 

 the retina. 



Coming now to the question of color, any attempt to apply the 

 scientific principles of color vision in making a picture must surely 

 fail if it be not granted at the outset that it is only to a limited de- 

 gree that those principles can apply. Color appreciation is as much 

 a psychical as a physiological process, and, indeed, it is psychical 

 not only with regard to the objective impression itself, but also with 

 regard to the subjective, the associational mental process. Previous 

 knowledge and training, experience, tradition, the association of 

 color impressions with impressions previously received through 

 other senses and stored away as memories, all play a part in deter- 

 mining the effect which a color or a pattern of apposed colors has 

 upon us. But even granting all this, there are many of the physio- 

 logical laws of color vision which must be adhered to before we can 

 expect to produce these effects. 



In attempting to show how these laws may be employed in art it 

 will be necessary for us to explain briefly some of the physical and 

 physiological observations upon M'hich they depend. The first of 

 these is a physical one — it is the dissociation of white light into the 

 spectral colors by means of a prism, or better, by means of a diffrac- 

 tion grating.^ The spectral colors are red, orange, yellow, green. 



1 In the Ught decomposed by a prism some hues, such as those of red and yellow, occupy 

 much less space than others, such as blue, although they do not correspondingly differ in 

 wave length. When light is decomposed by a diffraction grating (a glass plate ruled with 

 very flue equidistant lines) the spaces occupied by the various hues are proportional to 

 rheir differences in wave lengths. 



