234 LABORATORY G UWE IN PH YSIOL OGY. 



to 3^ above the average or so that the minimum visual 

 angle for acute vision equals 4'. What is the size of 

 the image when it subtends an angle of 4'? The test 

 letters are made with the width of the strokes \ the 

 height of the letter. What is the width of the retinal 

 image of one of the strokes?* 

 ^. Experiments and Observations. 



(I) To test the form sense, In all of the tests here de- 

 scribed it is understood unless otherwise stated that 

 the subject sit directly facing the chart which should 

 be six meters distant, and well illuminated. 



(1) Let the subject put on the test frames with the 

 left eye shaded, and direct the right eye to the let- 

 ters of the line marked 6 m. These letters in their 

 vertical dimension subtend an angle of 5'. The 

 average normal eye will be able to recognize 

 easily every letter in the line. Should there be any 

 hesitation in the differentiation of C from G, of P 

 from D or F, of Kfrom X, etc., make a note of it; its 

 significance will be apparent later. 



Now in recording the acuteness of vision one com- 

 pares the minimum angle of distinct vision in the 

 subject under observation with the normal. If the 

 subject reads readily at 6 m. the type that is normal 

 for 6 m., he is credited with normal vision or with a 

 minimum visual angle normal or unity. This is ex- 

 pressed in the following manner: Let V equal visual 

 acuteness; d, the distance from chart; D, the dis- 

 tance at which the type should be read: V = -~ . In 

 the above case V=g- or 1, i. e., normal vision. 



(2) Suppose that the subject cannot read the 6 m - 



* The size of the cones of the macular region varies from 0.0033 

 to 0.0036 mm. in diameter. 



