402 Hastings — Optical Errors of the Human Eye. 



of symmetry and approximately midway between these axes 

 and the axes of vision. These values are for wave-lengths of 

 mean refrangibility, the variations in the values for different 

 wave-lengths not being of moment for our present purposes ; 

 the small difference in relative size of the two pupils is also 

 ignored as immaterial. 



The elements of the schematic eye, so far as they are neces- 

 sary to our investigation, are : 



Dist. 



tt 2 to n x — 0-3264 cm 

 n m to retina = 1-55 CI 



Dist. 



" F, 



to tt 2 = 0-0659 cm 

 to n- = 0-035l cm 

 to F" = 0-U566 cm 



These elements are represented in Hg. 1, in which, for the sake 

 of perspicuity, the scale is greatly distorted by increasing those 



Rlaht Eye 



i~~~ — "»«Z ~ 



6 



r 



r 



^r^-^^ 



6 



k 



Left Eye JJs^^^L. b 



n, 



'*' 



h 



of the first column in the ratio of 2'5 to 1 and those of the second 

 column in the ratio of 25 to 1. With this qualification fig. 1 

 represents accurately the necessary elements of a pair of normal 

 eyes, the lines nji^ being the axes of the eyes, the lines a n x the 

 external, and n^r the internal lines of vision, and the points ir 1 

 and 7r 2 the positions of the centers of the external and internal 

 pupils, respectively. The points o and r are the positions of 

 the images of a distant object on the line of vision for two 

 wave-lengths of light corresponding to a difference of refrangi- 

 bility in water of *0100, which corresponds to an interval in 

 spectral colors nearly equivalent to that separating and G. 



