Hastings — Optical Constants of the Human Eye. 207 



The results given above must be regarded as very accurate 

 for the myopic eye studied, but it is not quite obvious how 

 they should be employed as a basis for estimating the dis- 

 persive power of the average human eye, which may be sup- 

 posed to be accurately equivalent to the schematic eye described 

 in Helmholtz.* It seems probable, however, that the assump- 

 tion that such an eye would possess the same differential power 

 for different wave-lengths of light will prove not far from the 

 truth. Under these conditions, since it is known that the dis- 

 persive powers of the aqueous and vitreous humors are the 

 same as that of water, it is possible to calculate the virtual dis- 

 persive power of the lens of the eye, and it is found that we 

 must assume a dispersive power in the lens of the schematic 

 eye about half way between that possessed by ordinary crown 

 glass and by dense flint glass, or, quite accurately, that of tur- 

 pentine. If it were supposed that the eye examined differed 

 from the schematic eye only in having a thicker lens, this value 

 would be somewhat smaller. A discussion of known facts 

 bearing on this point will appear at the end of the article. 



We are now in a position to calculate all the cardinal points 

 of the schematic eye for all accommodations and for all wave- 

 lengths, but as these values cannot be given as linear functions 

 of the variables, it is more useful to arrange a sufficient num- 

 ber in tables so that any required value can be found by proper 

 interpolation. As a basis of the computations I adopt, after 

 Helmholtz, as the mean refractive index of the aqueous and 

 vitreous humors, n = 1*3365, and as that of the lens, n\— 1*4371. 

 Also, the radii of the cornea, the front surface of the lens, 

 and the back surface, are, respectively, 0*728, 1*000, and 0*600 

 centimeters. Finally, the dispersive power of the two humors 

 being equal to that of water, we have : 



ABC D 



n—?i — -00558 — -00446 — '00310 — -00183 



E F G H 



+ -00045 -f -00241 + -00590 +'00900 



while, to meet the measured chromatic error of the eye as 

 determined above, 



Sn'/Sn = 1-57. 



With these data I have computed the following tables. The 

 values of c' in both tables are the reciprocals of the distance 



*See Helmholtz, Physiologische Optik, 2te Auflage s. 140. 



