On tfit Mefhati^m of the Eye. fe53 



HCwce the rsdius of the cornea is 31 Imndredths. It may be thought that t aflign too great 

 axonvcxity to the cornea ; but I have corrected it by a number of concurrent obfervations, 

 which wiil be enumerated hereafter. 



■ The eye being direfted towards its im^ge, the proje£llon of the margin of the fclerotica 

 'is 22 hundredths from-thertargin of the cornea, towards the external angle, and 27 towards 

 the internal angle of the eye : fo that the cornea has an eccentricity of orrc-foTticth of an inch, 

 ■with refpedt to the feiSion of the eye perpendicular to the vifual axis. 



The aperture of the pupil varies from 27 to 13 hundredths ; at leaft this is its apparent 

 fize, which muft be fomewhat diminiflied, on. account of the magnifying power of the 

 cornea, perhaps to 2; and 12. When dilated, it is nearly as eccentric as the cornea; but, 

 when moft contra(£led, its centre coincides with the refieftion of an image from an obje£l 

 held immediately before the eye; and this image very nearly with the centre of the whole 

 apparent margia of the fclerotica : fo that the cornea is perpendicularly interfe£led by the 

 •vifual axis. 



My eye, in a ftate of relaxation, colie£is to a focus on the retina, thofe rays which 

 diverge vertically from an objedi at the diftance of ten inches from the Cornea, and the rays 

 which diverge horizontally from an obje£l of feven inches diftance. For, if I hold the 

 plane of the optometer vertically, the images of the line appear to crofs at ten inches : if 

 horizontally, at feven. The difference is expreffed by a focal length of 23 inches. I have 

 never experienced any inconvenience from this imperfeciion, nor did I ever difcover it fill 

 I made thefe experiments ; and I believe I can examine minute objedls with as much ac- 

 curacy as mofl of thofe whofe eyes are differently formed. On mentioning it to Mr. Gary, 

 he informed me, that he had frequently taken notice of a fimilar circumftance ; that many 

 perfons were obliged to hold a concave glafs obliquely, in order to fee with diftindincfs, 

 counterbalancing, by tlie inclination of the glafs, the too great refra61:ive power of the 

 eye in the diredlion of that inclination, (Cor. 10. Prop. IV.) and finding but littl6 afhfl- 

 ance from fpeftacles of the fame focal length. The difference is not in the cornea, for it 

 exifts when the effeft of the cornea is removed by a method to be defcribed heteaftw. 

 The caufe is, without doubt, the obliquity of the uvea, and of the cryftalline lens, which 

 is nearly parallel to it, with rcfpeft to the vifual axis : this obliquity will appear, from the 

 dimenfions already given, to be about 10 degrees. Without entering into a very accurate, 

 calculation, the difference obferved is found (by the fame corollary) to require an inclination 

 of about 13 degrees ; and the remaining three degrees may eafily be added, by the greater 

 obliquity of the pofterior furface-of the cryftalline oppofite the pupil. There would be no 

 difficulty in fixing the glaffes of fpeitaclesj or the concave eye-glafs of a ceiefcope, in fuoh 

 a pofition as to remedy the defeft. 



In order to afcertain the focal diftance of the lens, we muft affign its probable diftance 

 from the cornea. Now the verfed fine of tlie. cornea being ii hundredths, and the uvea 

 being nearly flat, the anterior furface of the lens muft probably be fomewhat behind the 

 chord of the cornea ; but by a very inconfiderable diftance, for the uvea hits the fubftance 



of 



