678 THE SENSES 



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

 color charts against a white or gray wall 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. Occasionally 

 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 comple- 

 mentary color appears. 



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

 amine the retina of the eye of a cat or rabbit. Dilate the eye 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 focussed 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 the 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 five 

 degrees. When the letters marked "twenty feet" or "six meters " represent 

 the limit of accurate identification, the visual acuity is said to be i, or 

 normal. If the line marked "thirty feet" is the limit, then the acuity is one 

 and a half; if "fifteen feet," then the visual acuity is three-fourths, etc. 



If the eyes tested are astigmatic, or have other optical defects, these must 

 first be corrected before testing for visual acuity. 



29. The Test Set. The student is recommended to close the ex- 

 periments on the eye by fitting glasses for himself and at least two others. 

 He should correct for the defects that have been revealed in the preceding 

 experiments, especially for astigmatism; myopia, or hypermetropia ; and 

 presbyopia. Of course each eye must be tested and fitted separately. 



