Experhntnts on the Conies, A^i 



flat furfaces made rough and blacked, to prevent an image being feen from both furfaces. 

 One of thcfe was ftuck upon a piece of wood, diretlly oppofite a window, at 12 feet diftance 

 from it. A board, four feet long, and fix inches broad, was placed perpendicularly agalnft 

 the fafli of the window, and its image reflected from the mirror upon the objed glafsof an 

 achromatic microfcope, with a divided eye-glafs micrometer. 



The two images were feparated by means of the divided eye-glafs till their furface of 

 contact, which appears like a black line, was rendered as fmall as pollible. When this 

 eflPecl was produced on the images fr.-m the mirror of -/^ihs of an inch focus, that mirror 

 was removed and the other put in its place. The conta£l of the two images, which before 

 appeared like a line, had now acquired confiderable breadth, correfponding exa£lly to the 

 difference between the convexities of the mirrors. 



When the fame experiment was repeated upon the eye, there was a perceptible, though 

 extremely fmall, cliange in the micrometer at firfl when the eye was frefli. This was not 

 however feen afterwards, and the eye very fjon became fo mush fatigued, that it was ne- 

 ceflary to defill. It was found that every time the eye adapted itfelf to different diftances 

 it became neceflary to alter the diftance of the objeft-glafs of the microfcope from the 

 cornea. 



This experiment was repeated on four different days with the fame refult ; namely, a 

 change in the micrometer at firll, which in the fubfequent trials could not be detected. Two 

 fuppolitlons offered themfelves as likely to account for this effeft, namely, a motion of the 

 head forwards, or an action of the mufcles of the head itfelf ; but, as the author remarks, this 

 effect ought in the firft cafe to have been more frequent, the greater the fatigue of the eye ; 

 and the latter circumftance would have produced a contrary refult. It appeared, therefore, 

 that the effe£t on the micrometer did really arife from a change in the cornea, though too 

 fmall to be detected with certainty in this way. 



With two other mirrors, of which the focal diftances were y^ths and ^'sV^ths of an inch, the 

 difference between the fize of the images was juft vifiblc in the micrometer ; but it did not 

 appear probable that the fame difference would have been vifible had the mirror not been 

 perfeftly at reft. From the unfteadinefs of the eye, it might therefore be reafonably fup- 

 pofed that a change of this magnitude might take place in the cornea without being dif- 

 tin£lly feen. 



To give an idea of the fliort time that a 'part can remain nicely adjufted by mufcular ac- 

 tion, the author points out an experiment which any one may make upon himfelf. Let him 

 take a glafs fpirit level, and reft one end of it on a table, fupporting the other with his hand, 

 and endeavour to keep the air bubble in the middle. If the hand is very fleady, the bub- 

 ble may be kept nearly in its place, but not exactly fo ; for it will undulate in correfpon- 

 cncc with the a£tion of the mufcles, making up for want of (teadincfs, by fliort mc- 

 tions, in contrary direiSlions. 



From thefc experiments, the change in the curvature of the cornea could not be more 

 than -i\,iCn part of an inch, as any greater quantity would probably have been ditliniftly feen 

 in the micrometer. This, however, is (till more than was afccrtained by the former expe- 

 riments, which made it to exceed ^\,J\\ part of an inch. 



This change in tlie cornea, at firit view of the fubje6t, appeared fufficii'nt to account 



3 I' 2 for 



