452 THE POPULAR SCIENCE MONTHLY 



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 falling on the central portion which is 

 distinctly seen. "When we regard a stretch of country, for example, it is 

 only 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 part 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 regard with the central por- 

 tion 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 sci- 

 entific 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 degree 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 impres- 

 sions previously received through other senses and stored away as 

 memories, all play a part in determining the effect which a color or a 

 pattern of opposed colors, has upon us. But even granting all this, 

 there are many of the physiological 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 which 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 diffraction grating. 1 

 The spectral colors are red, orange, yellow, green, blue (indigo) and 

 violet, the various shades of purple being entirely absent. When we look 

 at such a spectrum we are at once struck with the fact that the colors 

 differ from one another not only in their hue but in their brightness or 

 luminosity, the yellow and the immediately adjacent portions being 

 much brighter than the others. At once then we recognize two 



i In the light 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 fine equidistant lines) the spaces occupied 

 by the various hues are proportional to their differences in wave-lengths. 



