VISION. 



age base line being 64 mm., the average Ma may be 

 thus expressed: Cot Ma=^9; Ma=l 50'. It is not 

 customary to use in practice the absolute values for the 

 angles, but a convenient series of approximate values 

 suggested by Nagel. 



If d = 500mm. (^m.), ang. c = 2 Ma; if d ^ m., 



FIG. 34. 



FIG. 34. For description 

 see LI-C. 



FIG. 35. 



FIG. 35. For description 

 see Ll-C-(5). 



ang. c = 3 Ma. If d ^ m., the accommodation = 4 D 

 and angle of convergence = 4 Ma. 



Besides the convenience of this system, it indicates 

 at once the direct relation between accommodation and 

 convergence. 



The amplitude of convergence is the total number of 



