328 On the Mechanifm of tht Eye. 



relaxed ftatc of the eye, are found to be in all parts ftraight ; and, that neither the form 

 nor the relative fituation of the cornea is concerned, appears from the application of water 

 already mentioned. 



The truth of this explanation is fully confirmed by the optometer. When I look through 

 four narrow flits, without exertion, the lines always appear to meet in one point : but, 

 when I make the interfe£tion approach me, the two outer lines meet confiderably beyond 

 the intier ones, and the two lines of the fame fide crofs each other at a ftill greater diftancc. 

 (Plate XV. Fig. 24.) 



The experiment will not fucceed with every eye j nor can it be expefted that fuch an 

 iniperfeftion fliould be univerfal : but one cafe is fufficient to eftablilh the argument, even 

 if no other were found. I do not however doubt, that in thofe who have a large pupil, 

 the aberration may be very frequently obfervable. In Dr. Wollafton's eye, the diverfity of 

 appearance is imperceptible ; but Mr. Konig defctibed the interfe(Jiion exaftly as they ap- 

 pear to me, although he had received no hint of what I had obferved. The lateral refrac- 

 tion is the moft eafily afcertained, by fubftituting for the flits a tapering piece of card, fo 

 as to cover all the central parts of the pupil, and thus determining the nearcfl; crofllng of 

 the fhadows tranfmitted through the marginal parts only. When the furtheft interfedion 

 was at 38, I could bring it to 22 parts with two narrow flits ; but with the tapered card only 

 to 29. From thefe data we may determine pretty nearly, into what form the lens muft be 

 changed, fuppofing both the furfaces to undergo proportional alterations of curvature, and 

 taking for granted the dimenfions already laid down ; for, from the lateral aberration thus 

 given, we may find (by Prop. III.) the fubtangents at about one-tenth of an inch from 

 the axis ; and the radius of curvature at each vertex, is already determined to be about 2 1 

 and 15 hundredths of an inch. Hence the anterior furface muft be a portion of a hyper- 

 boloid, of which the greater axis is about 50; and the pofterior furface will be nearly 

 parabolical. In this manner the change will be efFe£l:ed, without any diminution of the 

 tranfverfe diameter of the lens. The elongation of its axis will not exceed the fiftieth of an 

 inch; and, on the fuppofition with which we fet out, the protrufion will be chiefly at the 

 pofterior vertex. The form of the lens thus will be nearly that of Plate XV. Fig. 26 ; the 

 relaxed ftate being nearly as reprefented in Fig. 25. Should, however, the rigidity of the 

 internal parts, or any other confiderations, render it convenient to fuppofe the anterior 

 furface more changed, it would ftill have room, without interfering with the uvea ; or it 

 might even force the uvea a little forwards, without any vifible alteration of the external 

 appearance of the eye. 



From this inveftigation of the change of the figure of the lens, it appears that the adlion 

 which I formerly attributed to the external coats, cannot aftbrd an explanation of the phe- 

 nomenon. The necefl"ary efi^eft of fuch an action would be, to produce a figure approach- 

 ing to that of an oblate fpheroid; and, to fay nothing of the inconvenience attending a 

 diminution of the diameter of the lens, the lateral rcfraftion would be much more increafed 

 than the centr^al ; nor would the flight change of denfity, at an equal diftance from the 



axis 



i 



