246 LABORATORY GUIDE IN PHYSIOLOGY. 



be determined by finding " the strongest prisms which 

 a person can overcome"; while seeing a distant object, 

 without double vision. The deviation of a prism may 

 be taken as half the angle of the prism; a No. 6 prism, 

 produces a deviation of 3. If only one prism be 

 used the 3 is divided equally between the two eyes. 

 Let it be understood that two prisms of equal angle be 

 used. 



(5) To compute the ang. c in meter- angles for any prism and 

 any length of base-line. 



Let n equal the angle of deviation, i. e., one-half the 

 number of the prism. Let b equal one-half the base- 

 line. Let d equal the distance of the punctum re- 

 motum, to be computed. Then, 



(1) b : d : : sin n : cos n (see Fig. 35). 



But the punctum remotum of convergence (r c ) ex- 

 pressed in meter-angles, is found by dividing 1m. by 

 d, therefore 



(Z\ r c - 100 

 bcotn 



If a person whose base-line is 64 mm. is able by 

 divergence to overcome a pair of No. 6 prisms his 

 punctum remotum of convergence would be negative 

 and equal to 1.63 Ma. Determine the punctum re- 

 motum of convergence for each individual in the class. 



(6) Determine the amplitude of convergence for each 

 member of the class using the formula; a c =p c r c . 

 Tabulate the results. 



(7) Compare this table with the one in which the range 

 of accommodation is recorded. Is there any parallel- 

 ism in the variations of accommodation and conver- 

 gence ? Does age seem to have any appreciable influ- 

 ence upon the amplitude of convergence ? 



