VISION. . 245 



() Let the subject look in turn at the needle when 

 the post is set at 50 cm.; at 40 cm.; at 30 cm.; at 

 25 cm.; at 20 cm.; what is the ang. c in each case, 

 expressed in meter angles ? 



(<:) From this point if the needle appears perfectly 

 clearly defined move the post up toward the eyes 

 1 cm. at a time as long as the vision is binocular 

 and the image single. 



As soon as the image is double one may be cer- 

 tain that the eyes are no longer able to converge 

 sufficiently to bring the images upon corresponding 

 points of the retina and that the punctum proxi- 

 mum of convergence (p c ) has been passed. Find 

 the nearest point at which the image is single the 

 nearest point at which the fine printing on the card 

 is perfectly clear; this is the punctum proximum of 

 convergence (p c ). 

 (//) Determine the punctum proximum of conver 



gence for each individual in the class. 

 (4) To determine the punctum remotum of convergence (r c ). 

 If the eyes can be directed parallel but cannot 

 diverge, the punctum remotum may be expressed as 

 follows: t c = ~ = ~0. Landoldt says, however, 

 that "the majority of normal eyes can diverge more 

 or less," i. e., there is a negative convergence ( r c ). 



The formula (2) . . . a c = p c r c becomes 



a' = p c _( r c ); or 

 (2'). . .a c = p c +r c 



Let it be noted that in this case the value of r 

 cannot be determined by carrying the object to a 

 greater distance, but recourse must be had to abduct- 

 ing prisms, i. e., prisms whose apices are turned away 

 from the median line. The negative convergence may 



