0« the Mechanifm of the Eye. 3 03 



«ny exertion, bring the two images nearer together, ahhough the exertion makes thenv 

 more diftindV, no doubt by contradling the pupil. The experiment with the optometer 

 was condu£ted, in the prefence of Mr. Ware, with patience and perfeverance ; nor was 

 any opinion given to make her report partial. 



Confideriiig the difiieulty of finding an eye perfefliy fuitable for the experiments, thefe 

 proofs may be deemed tolerably fatisfaftory. But, fince one pofitive argument will coun- 

 terbalance many negative ones, provided it be equally grounded on fact, it becomes- 

 ncceflary to inquire into the competency of the evidence employed to afccrtain the power of 

 accommodation attributed, in the Croonian Le£ture for 1794, to the eye of Benjamin 

 Clerk. And it appears, that the diftinftion long iincc very properly made by Dr. Jurin, 

 between diftinft vifion aad perfeiSl vifiou, will readily explain iway the whole of that 

 evidence. 



It is obvious that vifion may be made diftinc^ to any given extent, by means of an. 

 aperture fufficiently fmall, provided at the fame time, that a fufficient quantity of light be. 

 left, while the refratlive powers of the eye remain unchanged. And it is remarkable, that 

 in thofe experiments, when the comparifon witli the perfeft eye was made, the aperture 

 of the imperfeft eye only was very confiderably reduced. Benjamin Clerk, with an aper- 

 ture of ;f?y of an inch, could read with the fame glafs at i^ inch, and at 7 inches.* With- 

 an equal aperture, I can read at li inch and at 30 inches i and I can retain the ftate o£ 

 perfedt relaxation, and read with the fame aperture at a^ inches ; and this is as great a^ 

 difference as was obferved in Benjamin's Clerk's eye. It is alfo a fa£t of no fmall im- 

 portance, that Sir Henry Englefield was much aftoniflied, as well as the other obfervers, at 

 the accuracy with which the man's eye was adjuftedto the fame diftance, in the repeated^ 

 trials that were made with it. f This circumftance alone makes it highly probable,, that 

 its perfect vifion was confined within very narrow limits- 



(To be continued.) 



IV. 



I)efcriptlon of a Support applicable to Balances of all Dimetiftons., and proper to render the 

 Operations performed -with thefe Injlruments more expeditious and convenient without affi^ing^ 

 their Precifwn. % By R, pROSY, of the National Inftitute of France. 



i^OME experiments which I was engaged in during the coutfb of lafl winter obliged me 

 to contrive a fupport, to which the beam of any balance whatever might be applied without 

 diftinftloD, wJiether provided with its frame of fupport or not, and which, without de- 



* Phi), Tranf. for 1795, p. 9, f. P^il. Tranf. for 1795, p. 8, 



J Annales de Chlmie, XXXVI. 50. 



prived! 



