Color Sensibility Of The Retina. - This aspect was touched upon in Chapter III, but the differences in the sensibility of various areas of the retina to various colors are of sufficient importance to be discussed further. The ability to distinguish light and color gradually fades or decreases at the periphery of the visual field, but the actual areas of the fields of perception vary considerably, depending upon the hue or spectral character of the light reaching the retina. The extreme peripheral region of the visual field is "color-blind"; that is, color ceases to be perceived before brightness-perception vanishes in the outskirts of the visual field. These fields for various colors depend in size and contour not only upon the hue or spectral character of the light-stimuli but also upon the intensity and perhaps upon the size of the stimuli. Some disagree about the relative sizes of these fields, however, it appears like they will increase in size in this order: green, red, blue, white (colorless). The performances of after-images, and the rates of growth and decay of sensation vary for different colors and for different areas of the retina, but it would be tedious to peruse the many details of these aspects of vision. They are mentioned in order that the reader may take them into account in any specific case.
As already stated, the central part of the visual field - the fovea upon which we depend for acute vision - contains a yellowish pigmentation, which is responsible for the term "yellow spot." This operates as a yellow filter for this central area and modifies the appearance of visual fields quite the same as if a similar yellow filter was placed in the central position of the field of vision. The effect of the selectivity of the "yellow spot" is noticeable in viewing certain colors.
