294 On tie Mechanijm of tie Eye. 



becomes a V'^ear fpe£lrujn. But the eye cannot fo adapt Itfflf as to make the whole 

 fpedlrum appear a line ; for, if the focus be adapted to colle£l; the red rays to a point, the 

 '^)lue will be too much refrafted, and expand into a furface ; and the reverfe will happen 

 ^ the eye be adapted to the blue rays; fo that, in either cafe, the line will be feen as a 

 iyiangular fpace. The obfervation Is confirmed, by placing a fmall concave fpeculum in 

 ■different parts of a prifmatic fpedlrum, and afcertaining the utmoft diftances at which the 

 ^e can colIe6l the rays of different colours to a focus. By thefe means I find, that the red 

 rays, from a point at 12 inches diftance, are as much refra£ted as white or yellow light at 

 II. The difference is equal to the refradlion of a lens 132 inches in focus. But the aber- 

 ration of the red rays In a lens of crown glafs, of equal mean refra£live power with the eye, 

 ■would be equivalent to the eftedi of a lens 44 inches in focus. If, therefore, we can de* 

 pend upon this calculation, the difperfive power of the eye colle£lively. Is one-third of the 

 difperfive power of crown glafs, at ati equal angle of deviation. I cannot obfervc much 

 aberration in the violet rays. This may be, in part, owing to their faintnefs ; but yet I 

 think their aberration mud be lefs than that of the red rays. I believe it was Mr. Ramfden's 

 opinion, that fince the feparation of coloured rays is only obferved where there is a fudden 

 change of denfity, fuch a body as the lens> of a denfity gradually varying, would have no 

 <fte£l: whatever in feparating the rays of different colours. If this hypothefis fliould appear 

 to be well-foHnded, we mufl: attribute the whole difperfion to the aqueous humour; and its 

 difperfive power will be half that of crown glafs, at the fame deviation. But we have an 

 inftance, in the atmofphere, of a very gradual change of denfity ; and yet Mr. Gilpin in- 

 forms me, that the ftars, when near the horizon, appear very evidently coloured. At a 

 more favourable feafon of the year, it would not be difficult to afcertain, by means of the 

 optometer, the difperfive power of the eye, and of Its different pans, with greater accuracy 

 than by the experiment here related. Had the difperfive power of the whole eye been 

 equal to that of flint glafs, the diftances of perfefl vifion would have varied from 12 Inches 

 to 7 for different rays, in the fame ftate of the mean refraftive powers. 



VII. The faculty of accommodating the eye to various diftances, appears to exift In 

 Tcry different degrees In different Individuals. The (horteft diftance of perfed vifion 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. Wollafton can fee at 7 inches, 

 and with converging rays ; the difference anfwerlng to 6 Inches focal length. Mr. Aber- 

 nethy has perfe£l vifion from 3 inches to 30, or a power equal to that of a lens 34 inches in 

 focus. A young lady of my acquaintance can fee 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 pf accommo* 

 dation being only equal to the effeft of a lens of 12 inches focus. In general, I have reafon 

 to think, that the faculty diminifties in fome degree, as perfons advance in life; but fqme 

 alfo of a middle age appear to poffefs It in a very fmall degree. I fiiall take the range of 

 my own eye, as being probably about the medium, and inquire what changes will be 



neceffary 



