OV TME MECHANISM OF THE EYE. 



585 



ferent parts of a prismatic spectrum; and as- 

 certaining the utmost distances, at wliich the 

 eye can collect the rays of different colours 

 to a focus. B}' these means I find, that the 

 red rays, from a point at 12 inches distance, 

 are as much refracted as white or yellow 

 light at 11. 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 ef- 

 fect of a lens 44 inches in focus. ]f, there- 

 fore, we can depend upon this calculation, 

 the dispersive power of the eye, collectively, 

 is one third of the dispersive power of crown 

 glass, at an equal angle of deviation. 1 can- 

 not observe much aberration in the violet 

 rays. This may be, in part, owing to their 

 faintness ; but yet I think their aberration 

 must be less than that of the red rays. I be- 

 lieve it was Mr. ilamsdeii'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 should be obliged to 

 attribute the whole dispersion to the aqueous 

 humour ; and its dispersive power would be 

 half that of crown glass, at the same devia- 

 tion. But we have an instance, in the at- 

 mos[>here, of a very gradual change of den- 

 sity ; and yet Mr. Gilpin informs me, that 

 the stars, when near the horizon, appear very 

 evidently coloured ; and Dr.Herschel has even 

 given us the dimensions of a spectrum thus 

 formed. 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 

 VOL. u. 



greater accuracy than by the e.vperiment 

 here related. Had the dispersive 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 

 ra3s, in the same state of the mean refrac- 

 tive powers. 



VI. The faculty of accommodating the eye 

 to various distances appears to exist in very 

 different degrees in different individuals. The 

 shortest distance of perfect vision, in my eye, 

 is 26 tenths of an inch for horizontal, and 29 

 for vertical rays. This power is equivalent to 

 the addition of a lens of 4 inches focus. Dr. 

 WoUaston can see at seven inches, and with 

 rays slightly converging; the difference an- 

 swering to 6 inches focal length. Mr. Aberne- 

 thy has perfect vision from 3 inches to 30, or a 

 power equal to that of a lens 3^ mches in fo- 

 cus. A young lady of my acquaintance can 

 see at 2 inches and at 4 ; the difference being 

 equivalent to 4 inches focus : a middle aged 

 lady at 3 and at 4 ; the power of accommo- 

 dation being only equal to the effect of a 

 lens of 12 inches focus. In general, I have 

 reason to think, that the faculty diminishes, 

 in some measure, as persons advance in life; 

 but some also of a middle age appear to pos- 

 sess it in a very small degree. I shall take 

 the range of my own eye, as being probably 

 about the medium, and inquire what changes 

 will be necessary, in order to produce it ; 

 whether we suppose the radius of the cornea 

 to be diminished, or the distance of the lens 

 from the retina to be increased, or these two 

 causes to act conjointly, or the figure of the 

 lens itself to undergo an alteration. 



1. We have calculated, that when the 

 eye is in a state of relaxation, the refraction 

 of the cornea is such as to collect rays di- 

 verging from a point ten inches distant, to 

 4f 



