On the A'lechamfm of the Eye. "" 297 



rhuft remark that, by a little habit, I have acquired a very ready command over the accom- 

 modation of my eye, fo as to be able to vievir anobjedl with attention, without adjufting my 

 eye to its diftance. 



I alfo ftrctched two threads, a little inclined to each other, acrofs a ring, and divldect 

 them by fpots of ink into equal fpaces. I then fixed a ring, applied my eye clofe behind it, 

 and placed two candles in proper fituations before me, and a third on one fide, to illuminatft 

 the threads. Then, fetting a fmall looking-glafs, firft at four inches diilance, and next at 

 two, I looked at the images reflefted in it, and obferved at what part of the threads they 

 cxa£tly reached acrofs in each cafe ; and with the fame refult as before. 



I next fixed the cancellated micrometer at a proper diftance, illuminated it ftrongly, and 

 viewed it through a pin-hole, by which means it became diftinft in every ftafe of the eye i 

 and, looking with the other eye into a fmall glafs, I compared the image with the micro- 

 meter in the manner already defcribed. I then changed the focal diftance of the eye, fo that 

 the lucid points appeared to fpread into furfaces, from being too remote for perfect vifion^ 

 and I noted on the fcale, the diftance of their centres ; but that diftance was invariable. 



Laftly, I drew a diagonal fcale, with a diamond, on a looking-glafs, (Plate XIV. Fig. 12.) 

 and brought the images into conta£l with the lines of the fcale. Then, fince the image 

 of the eye occupies on the furface of a glafs half its real dimenfions, at whatever diftance it 

 is viewed, its true fize is always double the meafure thus obtained. I illuminated the glafs 

 ftrongly, and made a perforation in a narrow flip of black card, which I held betweeil 

 the images; and was thus enabled to compare" them with the fcale, although their apparent 

 diftance was double that of the fcale. I viewed them in all ftates of the eye; but I could 

 perceive no variation in the interval between them. 



The fufficiency of thefe methods may be thus demonftrated. Make a preflure along the 

 edge of the upper eyelid with any fmall cylinder, for inftance a pencil, and the optometer 

 will {how that the focus of horizontal rays is a little elongated, while that of vertical rays is 

 (hortened ; an efFe£t which can only be owing to a change of curvature in the cornea. Not 

 only the apparatus here defcribed, but even the eye unaffifted, will be capable of difcoVering 

 a confiderable change in the images reflefted from the cornea, although the change be much 

 fmaller than that which is requifite for the accommodation of the eye to different diftances. 

 On the whole, I cannot hefitate to conclude, that if the radius of the cornea were diminifhed 

 but one-twentieth, the change would be very readily perceptible by fome of the experiments 

 related ; and the whole alteration of the eye requires one-fifth. 



But a much more accurate and decifive experiment remains. I take out of a fmall 

 botanical microfcope, a double convex lens, of eight-tenths radius and focal diftance, fixed 

 in a focket one-fifth of an inch in depth ; fecuring its edges with wax, I drop into it a little 

 water, nearly cold, till it is three-fourths full, and then apply it to my eye, fo that the 

 cornea enters half way into the focket, and is every where in contaft with the water. 

 (Plate XIV. Fig. 13.) My eye immediately becomes prefbyopic, and the rcfi:a'6live power 



Vol. V. — ^November 1801. Q,q of 



