Ofi the Mechan'ifm of the Eyv. 3« i 



the oflvce of helping to fupport the eye on that fide where the bones are fnoft tleficiifnt : 

 they feem alfo well calculated to prevent its being drawn too much backwards by "ihe adioti 

 of the ftraight mufcles. And, even if there were no difficulty in fuppefing the mufcles to 

 elongate the eye in every pofition, yet at leaft fome fmall difference would be expeded in 

 the extent of the change, virhert the eye is in different fituations, at an interval of mqiS 

 than a right angle from each other; but the optometer fhews that there is none. 



Dr. Hofack alleges that he was able, by making a preffure on the eye, to accommodate 

 it to a nearer object : * it does not appear that he made ufe of very accurate means of 

 afcertaining the fa£t ; but, if fuch atx effedt took plkce, the caufe mud have been ah in- 

 flexion of the cornea. 



It is unneceffary to dwell on the opinion which fuppofes a joint operation, of changes in 

 the curvature of the cornea and in the length of the axis. This opinion had derived very 

 great refpedlability, from the moft ingenious and elegant manner in which Dr. Olbers had 

 treated it, and from being the laft refult of the invedigation of Mr, Home and Mr. 

 Ramfden. But either of the feries of experiments which have been related, appears to be 

 fulBcient to confute it. 



X. It now remains to inquire Into the pretenfions of the cryftalline lens to the power 

 of altering the focal length of the eye. The grand objeilioa to the efficacy of a change of 

 figure in the lens, was derived from the experiments in which thofe who have been de- 

 prived of it have appeared to poffefs the faculty of accommodation. 



My friend Mr. "Ware, convinced as he was' of the neatnefs and accuracy of the ex- 

 periments related in the Croonian Le£lure for 1795, yet could not ftill help imagining, 

 from the obvious advantage all his patients found, aftef the extradllon of the lens, in ufing 

 two kinds of fpedtacles, that there muft, in fuch cafes, be a deficiency in that faculty. 

 This circumftance, combined with a confideration of the direclions very judicioully given 

 by Dr. Porterfield, for afcertaining the point in queftion, fivft made me wifli to repeat, 

 the experiments upon various individuals, and With' ttie inftrument which I have aboVd des- 

 fcribed as an improvement of Dr. Porterfield's optortietefr: and I muft here acknowledge 

 my great obligation to Mr. Ware, for the readinefs arid liberality with which he ih^ 

 troduced me to fuch of his numerous patients as he thought moft likely to furnirfi 

 a fatisfa£tory determination. It is unneceffary to enumerate every particular experih^eriitf}. 

 but the univerfal refult is, contrary to the expedlation with which L entered on the inquiry, 

 that in an eye deprived of the cryftalline lens, the adlual focal diftauce is totally unchange- 

 able. This will appear from a feleftion of the moft decifive obfervatioi:s. 



I. Mr. R. can read at four inches and at fix only,- \vith the fame glafs; He favr the 

 double lines meeting at three inches, and always at the fame point; but the cornea was 

 fomewhat irregularly prominent, and his vifion not very diftlnciti not had I, at the tinie I 

 faw him, a convenient apparatus. 



Phil. Tranf. for 1794, p. 213. 



l; after-* 



