OV THE MECHANISM OF THE EYE. 



583 



manner; uml t'le three following, the result 

 of all the successive refractions. The tenth 

 curve is a repetition of the ninth, with a 

 slight correction near the axis, at F, where, 

 from the breadth of the pupil, some perpen- 

 dicular rays must fall. By comparing this 

 with the eleventh, which is the form of the 

 retina, it will aj)pear that notliing more is 

 wanting for their perfect coincidence, than 

 a moderate diminution of density in the late- 

 ral parts of the lens. If the law, by which 

 this density varies, were more accurately as- 

 certained, its effect on the image might 

 easily be estimated ; 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 

 distance, retire sixteen feet, and direct my 

 eye to a point four feet to the right or left of 

 the middle of the space between them : .they 

 are then lost in a confused spot of light; but 

 any inclination of the eye brings one or the 

 other of them into the field of view. In Ber- 

 noulli's eye, a greater deviation was required 

 for the direction of the axis* ; and the ob- 

 scured part appeared to be of greater extent. 

 From the experiment here related, the dis- 

 tance of the centre of the optic nerve from 

 the visual axis is found to be \6 hundredths 

 of an inch ; and the diameter of the most 

 insensible part of the retina, one thirtieth of 

 an inch. In order to ascertain the distance 

 of the optic nerve from the point opposite to 

 the pupil, I took the sclerotica of the human 

 eye, divided it into segments, from thie centre 

 of the cornea towards the optic nerve, and 

 extendedjt on a plane. I then measured 

 the longest and shortest distances from the 

 cornea to the perforation made by the nerve, 

 * Cprora. Petrop. 1, 314, 



and their difference was exactly one fifth of 

 an inch. To this we must add a fiftieth, on 

 account of the eccentricity of the pupil in 

 the uvea, which in the eye that I measured 

 was not great, and the distance of the cen- 

 tre of the nerve from the point opposite the 

 pupil will be 11 hundredths. Hence it ap- 

 pears, that the visual axis is five hundredths, 

 or one twentieth of an inch, further from the 

 optic nerve than the point opposite the pu- 

 pil. It is possible, that this distance may be 

 different in different eyes : in mine, the obli- 

 quity of the lens, and the eccentricity of the 

 pupil with respect to ir, will tend to throw a 

 direct ray upon it, without much inclination 

 of the whole eye ; and it is not improbable, 

 that the eye is also turned slightly outwards, 

 when looking at any object before it, although 

 the inclination is too small to be subjected to 

 measurement. 



It must also be observed, that it is very dif- 

 ficult to ascertain the proportions of the eye 

 so exactly, as to determine, with certainty, 

 the size of an image on the retina ; the situ- 

 ation, curvature, and constitution of the lens, 

 make so material a difference in the result, 

 that there may possibly be an error of al- 

 most one tenth of the whole. In order, there- 

 fore, to obtain some confirmation from ex- 

 periment, t placed two candles at a small 

 distance from each othei-, turned the eye. in- 

 wards, and applied the ring of a key so as to 

 produce a spectrum, of which the edge 

 coincided with the inner candle; then, fixinn- 

 my eye on the outward one, I found that the 

 spectrum advanced over two sevenths of the 

 distance between them. Hence, the same 

 portion of the retina that subtended an angle 

 of seven parts at the centre of motion of the 

 eye, subtended an angle of five at the sup- 

 posed intersection of the principal rays ; 



