on the Mechanism of the Eye. 5 1 
12 inches distance, are as much refracted as white or yellow 
light at 1 1. The difference is equal to the refraction of a lens 
132 inches in focus. But the aberration of the red rays in a 
lens of crown glass, of equal mean refractive power with the 
eye, would be equivalent to the effect of a lens 4,4, inches in 
focus. If, therefore, we can depend upon this calculation, the 
dispersive power of the eye collectively, is one-third of the dis- 
persive power of crown glass, at an equal angle of deviation. I 
cannot observe much aberration in the violet rays. This may 
be, in part, owing to their faintness ; but yet I think their aber- 
ration must be less than that of the red rays. I believe it was 
Mr. Ramsden’s opinion, that since the separation of coloured 
rays is only observed where there is a sudden change of density, 
such a body as the lens, of a density gradually varying, would 
have no effect whatever in separating the rays of different colours. 
If this hypothesis should appear to be well-founded, we must 
attribute the whole dispersion to the aqueous humour; and its 
dispersive power will be half that of crown glass, at the same 
deviation. But we have an instance, in the atmosphere, of a 
very gradual change of density ; and yet Mr. Gilpin informs 
me, that the stars, when near the horizon, appear very evidently 
coloured. At a more favourable season of the year, it would 
not be difficult to ascertain, by means of the optometer, the 
dispersive power of the eye, and of its different parts, with greater 
accuracy than by the experiment here related. Had the dis- 
persive power of the whole eye been equal to that of flint glass, 
the distances of perfect vision would have varied from 12 inches 
to 7 for different rays, in the same state of the mean refractive 
powers. 
VII. The faculty of accommodating the eye to various 
H 2 
