VISION. 235 



line readily, let him try the line above. If he 

 reads that readily his visual acuteness would be: 

 V = g=:-; two-thirds normal. It is usual, however, 

 not to reduce the fraction but to use 6 for the nume- 

 rator always. 



(3) How shall one express visual acuteness for an in- 

 dividual who reads at 6 m. what he should read at 

 21m.? At 24 m ? At 30 m.? At 4.5 m.? At 3 m.? 



(4) How many members of the class have a visual 

 acuteness greater than unity? May a visual acute- 

 ness above the normal be attributed in any degree 

 to cultivation of the vision, or is it to be interpreted 

 solely as a natural endowment? 



(5) Make upon a white card with india ink a series of 

 vertical lines 1 cm. apart, beginning with a line of 

 1 mm. breadth, and decreasing gradually to a hair 

 line; place the card upon a blackboard 6 m. di-stant; 

 let a subject with high visual acuteness say how 

 many of these lines he can see. 



With dividers and rule measure the breadth of 

 the finest of the lines seen. What is the visual 

 angle of that breadth? What is the breadth of the 

 retinal image of the line? Can the subject see the 

 same number of lines when they are horizontal? If 

 not, how may the fact be accounted for? 



(6) If it be found that the subject cannot see clearly 

 the largest letters upon the test chart let him move 

 to a shorter distance. 



Suppose that he sees clearly the 30 m. type at 2 

 meters, what is the value of V? How far would he 

 be able to read the 6 m. type? At what distance 

 would he probably have to hold a book whose type 

 has a height of 1.8 mm.? 



(7) (a) Let a subject take the seat, 6 m. distant 



