Page:Popular Science Monthly Volume 83.djvu/456

452 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 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 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 impressions 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. 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