On the Mechamfm of the Ey?. apj 



PTence, the fame portion of the retina that fubtended an angle of'feven parts dt the centre 

 of motion of the eye, fubtended an angle of five at the fuppofed intcrfeftion of the prin- 

 cipal rays; (Plate XIV. Fig. ii.) and the'diftance of this intefrfei^ion from the retina waj 

 637 thoufandths. This nearly correfponds with the foi'mer cSiculatiorl ; nor can the 

 dift^Hci of the centre of the optic- nerv6 from thi point of moft'perfeft vifion be, on any 

 fuppofition, much lefs than that which is here affigiisd. Artd, in'the eyes of quadrupeds* 

 the mod ftrongly painted part of the choroid is further from the nerve than the real axis of 

 the eye. 



'Thave endfeavoured to exprefs in four figures, the'form of every part of my eye, asnearl| 

 as 1 have been able to afcertain it; the firft (PI. XV. Fig. 17.) is a vertical feftion ; the 

 fecoi!d(Fig. 18.) a horizontal feclion ; the third and fourth are front viewsj in different 

 {fates of the pupil. (Fig'- 19 and 20;) 



' Confidering how little inconvenience is experienceid from fo material an inequality in the 

 refra£tion of the lens as Ihave defcribed, we have no reafon to expeft a very accurate pro* 

 vifion for correfting the aberration of the lateral rays. But, as far as can be afcertained by 

 tiie optometer, the aberration arifing from figure is completely corrected j fince four of 

 nibir imaged of the fame line' appear to meet exadll'y in the fame point, which they wbhld 

 not do if the lateral rays were materially more refra£led than the rays near the axis. The 

 figure of the furfaces is fometimes, and perhaps always, more or lefs hyperbolical * or ellip- 

 tical : in the interior laminx indeed, the folid angle of the margin is fomewhat rounded 

 ofif; but the weaker refraflive power of the external parts-, muft greatly tend to correft the 

 aberration arifing from the too great curvature towards- the margin of the difc. Had th© 

 refra£tive power been uniform, it might have collected the lateral rays of a dire£l pencil 

 nearly as well ; but it would have be«n lefs adapted to oblique pencils of rays ; and the eye 

 muft alfo have been encumbered with a mafs of much greater denfity than is now required* 

 even for the central parts : and, if the whole lens had been fmaller, it would alfo have ad-i- 

 rnitted too little light. It is poffible too, that Mr. Ramfden's obfervation, f- on the advan- 

 tage of having no reflefting furfaee, may be well-founded : but it has'notbcen derrtonftrased, 

 that lefs light' ia loft in pafiing through a medium of variable denfity, than in a fuddenr 

 tranfition from one part of that medium to another ; nor are we yet fufliciently acquainted 

 with the caufe of this refieftion, to be enabled to reafon fatlsfadlorily on the fubjeft. 

 But, neither this gradation, nor any other provifion, has the efFedl of rendering; the eye 

 perfedly achromatic. Dr. Jurin had remarked this, long ago, i from obferving the colour 

 bordering che image of an object feen Indrftinftly. Dr. Wollafton pointed out to me on 

 the optometer, the red and blue appearance of the oppofitc iiUernal angles of the crofling. 

 lines ; and mentioned, at the fame time, a very elegant experiment for proving the dif- 

 perfive power of the eye. He looks through a prifm ata fmall lucid point, which of courfe- 



*- Petit M£n)^.de I'Acad. 17;^?<.p, 20. 

 t. PJiil. Tranf. for 1W5; p. i," '" X Slnith, e. SG. . 



b'iecomes 



