ON THE MECHANISM OF THE EYE. 



579 



ject at seven inches distance. For, if I hold 

 the plane of the optometer vertically, the 

 images of the line appear to cross at ten 

 inciies; if horizontally, at seven. The dif- 

 ference is expressed hy a focal length of 23 

 inches. I have never experienced "any in- 

 convenience from this imperfection, nor did 

 I ever discover it till I made these experi- 

 ments ; and I helieve I can examine minute 

 objects wiih as much accuracy as most of 

 those whose eyes are differently formed. On 

 mentioning it to Mr. Gary, he informed me 

 that he had frequently taken notice of a 

 similar circumstance ; that many persons 

 were obliged to hold a concave glass ob- 

 liquely, in order to see with distinctness, 

 counterbalancing, by the inclination of the 

 glass, the too great refractive power of the eye 

 in the direction of thatinclination, and finding 

 but little assistance frotn common spectacles 

 of the same focal length. The difference is 

 not in the cornea, for it exists when the effect 

 of the cornea is removed, by a method to be 

 described hereafter. The cause is, without 

 doubt, the obliquity of the uvea, and of the 

 -crystalline lens, which is nearly parallel to it, 

 with respect to the visual axis : this obliquity 

 will appear, from the dimensions already 

 given, to be about 10 degrees. Without en- 

 tering into a very accurate calculation, the 

 difference observed is found to require an in- 

 clination of about 13 degrees; and the re- 

 maining three degrees may easily be added, 

 by the greater obliquity of the posterior sur- 

 face of the crystalline opposite the pupil. 

 There would be no difficulty in fixing the 

 glasses of spectacles, or the concave eye 

 glass of a telescope, in such a position as to 

 remedy the defect. 



In order to ascertain the focal distance of 

 the lens, we must assign its probable dis- 



tance from the cornea. Now the versed sine 

 of the cornea being 1 1 hundredths, and the 

 uvea being nearly flat, the anterior surface of 

 the lens must probably be somewhat behind 

 the chord of the cornea ; but by a very in- 

 considerable distance, for l.he uvea has the 

 substance of a thin membrane, and the lens 

 approaches very near to it : we will there- 

 fore call this distance 12 hundredths. The 

 axis and proportions of the lens must be 

 estimated by comparison with anatomical 

 observations ; since they affect, in a small 

 degree, the determination of its focal dis- 

 tance. M. Petit found the axis almost al- 

 ways about two lines, or 18 hundredths of an 

 inch. The radius of the anterior surface was 

 in the greatest number 3 lines, but oftener 

 more than less. We will suppose mine to be 

 3^, or nearly -rV of an inch. The radius of 

 the posterior surface was most frequently 2i 

 lines, or 1^ of an inch*. The optical centre 



will be therefore ( ^^x^° —\ about one tenth 



\30+'22 / 



of an inch from the anterior surface : hence 

 we have 22 hundredths, for the distance of 

 the centre from the cornea. Now, taking 10 

 inches as the distance of the radiant point, 

 the focus of the cornea will be llo^hun- 

 diedths behind the centre of the lens. But 

 the actual joint focus is (gi — 22 = ) 69 be- 

 hind the centre : hence, disregarding the 

 thickness of the lens, its principal focal dis- 

 tance is 173 hundredths. For the index of 

 its refractive power in the eye, we have 

 4 j4- Calculating upon this refractive power, 

 with the consideration of the thickness also, 

 wc find that it requires a correction, and 

 comes near to the ratio of 14 to 13 lor the 

 sines. It is well known that the refractive 

 powers of the humours are equal to that of 



* Mam. cle I'Acad. de Paris. 1730. 6. Ed. Amst. 



