VISION. 



which the subject is capable; so that figures constructed on the 

 4' basis are gradually coming into use. 



For recording visual acuity, the formula V = is used, V 



standing for vision; d, for the distance of the subject from the test 

 type; D, the distance from which it should be read. 



In practice, the acuteness of vision is found by determining the 

 smallest type the subject can read at six metres. Normal vision 

 is represented by the symbol f . That is to say, that at six metres 

 the subject reads the test line marked 6. If, at this distance, the 

 subject can only read the line marked 12, his visual record would 

 be T 6 ^, and so on. 



In instances where the vision of the subject is lowered to the 

 extent that he cannot see even the largest test types, vision is 

 tested by the ability to count fingers at varying distances. For 

 example, V fingers, 3 m. If vision is still lower, V = shadows 

 or light. 



Examination of Near Vision. This includes the ability of the 

 subject to read print. That is the condition of accommodation. 

 The test types are those of von Jaeger and Snellen. 



Exercise. Test the visual acuity of each other, recording your 

 findings, both for distant and near. 



Light sense is the power possessed by the retina of appreciating 

 variations in the intensity of light. It is measured by the photom- 

 eter, which consists, essentially, of an apparatus by which the in- 

 tensity of two sources of light may be compared. 



Color Sense. This is the power the retina has of distinguishing 

 or perceiving colors, or the impression resulting from the impact 

 of light rays having different refrangibilities. 



Holmgren's test consists of testing the power of a subject to 

 match various colored yarns. Three large test skeins, namely, (i) 

 light, pure green, (2) rose-purple, (3) red, are given the patient, 

 and also smaller skeins, comprising various shades and tints of 

 each color. He is requested to pick out the colors similar to his 

 original three skeins. If, for example, he is red-blind, he will not 



