. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine. Physiology, Comparative. 542 COMPAEATIVB PHYSIOLOGY. retina being stimulated, or the original impression having faded, etc. In the case of a top or (better seen) color-disk, painted into black and white sectors, it may be observed that with a faint light the different colors cease to appear distinct with a slower rotation than when a bright light is used. The variation is between about ^ and -jV of a second, according to the intensity of the light used. Fusion is also readier with some
. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine. Physiology, Comparative. 542 COMPAEATIVB PHYSIOLOGY. retina being stimulated, or the original impression having faded, etc. In the case of a top or (better seen) color-disk, painted into black and white sectors, it may be observed that with a faint light the different colors cease to appear distinct with a slower rotation than when a bright light is used. The variation is between about ^ and -jV of a second, according to the intensity of the light used. Fusion is also readier with some colors than others. It is a remarkable fact that one can distinguish as readily between the quantity of light emanating from 10 and 11 candles as between 100 and 110. The Visual Angle.—If two points be marked out with ink on a sheet of white paper, so close together that they can be just distinguished as two at the distance of 13 to 20 inches, then on removing them a little farther away they seem to merge into one. The principle involved may be stated thus: When the dis-. tance between two points is such that they subtend a less visual angle than 60 seconds, they cease to be distinguished as two. Fig. 391 illustrates the visual angle. It will be noticed that a larger object at a greater distance subtends the same visual angle as a smaller one much nearer. The size of the retinal. Fig. 391.—The visual angle. The object at A" appears no larger than the one at A (Le Conte). image corresponding to 60 seconds is "004 mm. (4 \£), and this is about the diameter of a single rod or cone. It is" not, how- ever, true that when two cones are stimulated two objects are inferred to exist in every case by the mind; for the retina va- ries in different parts very greatly in general sensibility and in sensibility to color. It is noticeable that visual discriminative power can be greatly improved by culture, a remark which applies especially to colors. It seems altogether pro
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