VISUAL ACUITY 713 



individual will confuse colored skeins, most usually the reds, greens, and 

 grays. 



25. Color Mixing. Use Bradley' s color wheel and test the effect of 

 simultaneous stimulation of the retina with two or more colors, by placing 

 on the wheel two or more colored discs, rotating the wheel at a speed 

 sufficient to cause complete fusion. The sensation produced by two colors 

 applied simultaneously will be entirely different from that produced by 

 either alone. Red and green (or greenish-blue), when mixed in the proper 

 proportion, produce a sensation of gray. The same effect may be had 

 from yellow and blue, orange and violet, or any of the complementary 

 colors chosen according to the geometrical color table, figure 473. By 

 mixing three colors, red, green, and violet, in the proper proportion one 

 can produce a sensation almost the same as that produced by white light. 



26. Color After-images. Color after-images can be demonstrated 

 by looking continuously at the center of one of the primary colors of Bradley' s 

 color charts against a white or gray w r all until there is apparent fatigue, 

 then suddenly removing the chart. An after-image of approximately the 

 complementary color will appear in the course of a few seconds. Occasion- 

 ally these images are very vivid. The experiments are brilliant if performed 

 in the dark room, using colored gelatin screens through which an intense 

 light shines. When the light is turned off, a brilliant after-image of the 

 complementary color appears. 



27. Retinoscopy. Use the ordinary small ophthalmoscope and ex- 

 amine the retina of the eye of a cat or rabbit. Dilate the pupil by the use of 

 atropine. Place the animal whose eye is to be examined on a support in 

 front of a bright but uniform light (an Argand burner). Reflect the light 

 from the mirror of the ophthalmoscope through the pupil into the retinal cup 

 of the animal. Usually the ophthalmoscope has to be focused for a cat's 

 retina. When a good light is secured, the retinal cup will appear as a bril- 

 liantly colored disc, with the branching blood vessels, and usually with 

 some brilliant bluish-green pigment in the lower portions of the retinal 

 disc. 



After some practice on the cat or rabbit, the student should examine 

 the retina of one of his mates, preferably an eye that has an unusually wide 

 pupil. In some cases a light dosage of homatropine may be used on one eye. 

 This will dilate the pupil and the examination will be much easier. 



Students are not recommended to use atropine unless under conditions 

 which permit the eye to rest for two or three days following. 



28. Visual Acuity. The visual acuity of the eye should be tested first 

 for the right eye, then for the left. Hang a test chart at a distance of twenty 

 feet, so that its disc is well illuminated, and allow the individual tested 

 to read off the letters on the chart, beginning with the larger ones at the top. 

 The letters on this chart are constructed on the basis of a visual angle of 



