198 SPECIAL PHYSIOLOGY 



(1) Let the subject put on the test frames with the left eye shaded, 

 and direct the right eye to the letters 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 K from X, etc., make a note of it; 

 its significance will be apparent later. 



In recording the acuteness of vision one compares the minimum 

 angle of distinct vision in the subject under observation with the 

 normal. If the subject reads readily at 6 m. he is credited with 

 normal vision or with a minimum visual angle normal or unity. 

 This is expressed in the following manner: Let V equal visual 

 acuteness; d, the distance from the chart; D, the distance at which 



the type should be read: V= =r. In the above case F=-or 1 i.e., 



normal vision. 



(2) Suppose that the subject cannot read the 6 m. line readily, 

 let him try the line above. If he reads that readily his visual acute- 

 ness would be V=jr=~, two-thirds normal. It is usual, however, 



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not to reduce the fraction, but to use the 6 as the numerator always. 



(3) How shall one express visual acuteness for an individual who 

 reads at 6 m. what he should read at 21 m. ? 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 acuteness 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) Let a subject take a seat 6 m. distant from the chart. Hold 

 before his eye a +0.75 D. lens, it will probably make indistinct and 

 blurred distant objects which were, without the lens, clear, (a) If 

 such be the case it is likely that refraction of the eye is normal, and 

 for our purpose it may be recorded as an emmetropic eye. 



(b) If, however, the vision remains perfectly clear for distant 

 objects, with +0.75 D. or the +1 D. lens before the eye it is evident 

 that the refraction of the eye is not normal. 



(c) Suppose, on the other hand, that distant objects cannot be 

 clearly seen with the unaided eye, but with the help of concave 

 lenses clearly seen, it is evident again that the refraction of the 

 eye is abnormal. 



(6) In case (5, c) where were the parallel rays focused when the 

 concave lens was used? Where were the parallel rays focused in the 

 unaided eye ? Would it be possible for the condition to be corrected 

 by an exercise of the accommodation ? If the punctum remotum is 

 2 m., and if the refractive indices and curvatures of the refracting 



