293 Oil the Mechamfm of the Eye, 



refra£Hons. The tenth curve is a repetition of the ninth, with a flight correftlon near th« 

 axis, at F, where, from the breadth of the pupil, fome perpendicular rays muft fall. By 

 comparing this with the eleventh, which is the form of the retina, it will appear that 

 nothing more is wanting for their perfedl coincidence, than a moderate diminution of dea- 

 fity in the lateral parts of the lens. If the law, by which this denficy varies, were mor©- 

 accurately afcertaincd, its efFe6l on the image might be eftimated by means of the eighth 

 propofitionj and probably the image, thus corrected, would approach very .nearly to the 

 form of the twelfth curve. 



To find the place of the entrance of the optic nerve, I fix two candles at ten inches 

 diftancCj retire fixteen feet, and direft my eye to a point four feet to the right or left of 

 the middle of the fpace between them : they are then loft in a confufed fpot of light ; but 

 any inclination of the eye brings one or the other of them into the field of view. In Ber* 

 nouUi's eye, a greater deviation was required for the dircftion of the axis ;* and the ob- 

 fcured part appeared to be of greater extent. From the experiment here related, the 

 diftance of the centre of the optic nerve from the vifual axis is found (by Prop..V.) to be 

 i6 hundredths of aa inch ; and the diameter of the mofl: infenfible part of tlie retina,, one- 

 thirtieth of an inch. In order to afcertain the dillance of the optic nerve from the point 

 oppofite to the pupil, I took the fclerotica of the human eye, divided it into fegments, from 

 the centre of the cornea towards the optic nerve^ and extended it on a plane. I then 

 aieafured the longeft and fliorteft diftances from the cornea to the perforation made by tha 

 nerve, and their difference was exaftly one-fifth o£ an inch. To this we muft add a 

 fiftieth, oh account of the eccentricity of the pupil in the uvea, which in tlie eye that I 

 ineafured was not great, and the diftance of the centre of the nerve from the point 

 oppofite the pupil w^ll be 1 1 hundredths. Hence it appears, that the vifual axis is fiva 

 hundredths, or one-twentieth of an inch, further from the optic nerve than the point 

 oppofite the pupil. It is pofiible that this diftance may be different in different eyes : in 

 mine,, the obliquity of the lens, and the eccentricity of the pupil with refpedlto it, will 

 tend to throw a direft ray upon it, without much inclination' of. the whole eye •, and it is not 

 improbable, that the eye is alfo turned {lightly outwards,, if looking at any-.objeft before itj 

 although the inclination is too fmall to be fub].e£tad to meafurement* 

 ■ -It rauft.alfa he obfervcd, that it is very difficult to afcertain the proportions of the eye fo 

 exactly as to determine, with-certainty, the fize of. an image on the.retina; the fituation, 

 curvature, and conftitution of thelens, make fo material a difference in the refult, that there 

 may.poffiiily be an error of almoft one-tenth of the whole. In order, therefore, to obtain 

 fonie confirmation from experiment, I placed two candles at a fmall diftance from each 

 other,, turned the eye inwards, and applied the ring of a. key fo.as to produce a fpeftrum» . 

 of, which the edge coincided with the inner candle ;, then, fixing my eye on the outward 

 *i,e, I found that the fpeclrum advanced over two-fevenths of the diftance between them.. 



• Comm. Pctrop. I. i>. 314.., 



' HSncCj . 



