252 



Oil the MicMmfm of the Eye. 



Table II. For placing the numbers indicating the focal length of convex glajjes. 



jTab/e IIL For concave glares. 



Number. 





 1 



2 

 3 

 4 



5 

 6 



Neareft 

 place. 



1.71 

 1.63 

 1.54 

 1.44 

 1.35 

 1.22 

 1.02 



VI. Being convinced of the advantage of making every obfervation with as little" aflliiance 

 9B poflible, I have endeavoured to confine moft of my experiments to my own eyes ; and'I 

 (hall, in general, ground my calculations on the fuppofition of an eye nearly fimilar to my 

 own. I fliall therefore firft endeavour to afcertain all its dimenfionsj and all its faculties. 



For meafuring the diameters, I fix a fmall key on each point of a pair of compafles ; and 

 I can venture to bring the rings into immediate contadi with the fclerotica. The tranfverfe 

 diameter is externally 98 hundredths of an- inch. 



To find the axis, I turn the eye as much inwards as poflible, and prefs one of the keys 

 cloft to the fclerotica, at the external angle, till it arrives at the fpot where the fpeftrum 

 formed by its preflure coincides with the dire£lion of the vifual axis, a«d, looking in a glafs, 



I bring the other key to the cornea. The optical axis of the eye, making allowance of three 

 hundredths for the coats, is thus found to be 91 hundredths of an inich, from the external 

 furface of the cornea to the retina. With an eye lefs prominent,j thi-s method might not 

 have fucceeded. ' 



The vertical diameter, or rather chord, of the cornea, is 45 hundredths : its verfed fine 



I I hundredths. To afcertain the verfed fine, I looked with the right eye at the image of the 

 left, in a fmall fpeculum held clofe to the nofe, while the left eye was fo averted that the 

 margin of the cornea appeared as a ftraight line, and compared the projedion of the cornea 

 with the image of a cancellated fcale held in a.proper direflion behind the left eye, and clofe 

 to the left temple. The horizontal chord of the cornea is nearly 49 hundredths. 



Hence 



