298 On ike Mechanifm of the E^e.- 



of the lens, which is reduced by the water to a focal length of about 16 tenths, (Cor. 5^ 

 Prop. IV.) is not fufficient to fupply the place of the cornea, rendered inefficacious by the 

 intervention of the water ; but the addition of another lens, of five inches and a half focus, 

 reftores my eye to its natural ftate, and fomewhat more. I then apply the optometer, 

 and I find the fame inequality in the horizontal and vertical refradlions as without the 

 water ; and I have, in both dlredlions, a power of accommodation equivalent to a focal 

 length of four inches, as before. As firft light indeed, the accommodation appears to be 

 fomewhat lefs, and only able to bring the eye from the ftate fitted for parallel rays to a 

 focus at five inches diftance ; and this made me once imagine, that the cornea might have 

 fome flight efl^e£t in the natural ftate ; but, confidering that the artificial cornea was about 

 a tenth of an inch before the place of the natural cornea, I calculated the efFefl of this 

 diiFerencc, and found it exactly fufficient to account for the diminution of the range of 

 vifion. I cannot afcertain the diftance of the glafs lens from the cornea to the hundredth 

 of an inch ; but the error cannot be much greater, and it may be on either fide. 



After this, it is almoft necefl'ary to apologize for having ftated the former experiments j 

 but, in fo delicate a fubjcft, we cannot have too great a variety of concurring evidence. 



IX. Having fatisfied myfelf that the cornea is not concerned in the accommodation of 

 the eye, my next objeft was to inquire if any alteration in the length of its axis could be 

 difcovered ; for this appeared to be the only pofilble alternative : and, confidering that fuch 

 a change muft amount to one-feventh of the diameter of the eye, I flattered myfelf with the 

 expeftation of fubmitting it to meafurement. Now, if the axis of the eye were elongated 

 one-feventh, its tranfverfe diameter muft be diminiflied one-fourteenth, and the femt- 

 diameter would be fhortened a thirtieth of an inch. 



I therefore placed two candles fo that when the eye was turned inwards, and dire£te(l 

 towards its own image in a glafs, the light refle£led from one of the candles by the fclerotica 

 appeared upon its external margin, fo as to define it diftin£lly by a bright line ; and the 

 image of the other candle was feen in the centre of the cornea. I then applied the double 

 eye-glafs, and the fcale of the looking-glafs, in the manner already defcribed ; but neither 

 of them indicated any diminution of the diftance, when the focal length of the eye was 

 changed. 



Another teft, and a much more delicate one, was the application of the ring of a key at 

 the external angle, when the eye was turned as much inwards as poffible, and confined at 

 the fame time by a ftrong oval iron ring, prefled againft it at the internal angle. The key 

 was forced in as far as the fenfibility of the integuments would admit, and was wedged, 

 by a moderate preflTure, between the eye and the bone. In this fituation, the phantom 

 caufed by the preflure extended within the field of perfe£l vifion, and was very accurately 

 defined ; nor did it, as I formerly imagined, by any means prevent a diftinft perception of 

 the objects a£tually feen in that direftion ; and a ftraight line coming within the field of 

 this oval phantom, appeared fomewhat inflc^ed towards its centre j (Plate XIV. Fig. 14,) 



a dif* 



