a6 Oh the Mechanifnt of the Eyi. 



II. Bcfides the inquiry refpcQing the accommodation of the eye to different diftances, Ifliall 

 have occafion to notice fotr.e other particulars relative to its fun£lions ; and I (hall begin 

 with a general confideration of the fenfe of vifion. I fhall then enumerate fome dioptrical 

 propofuions fubfcrvient to my purpofes, and defcribe aninftrument for readily afccrtaining 

 the focal diftance of the eye. On thefe foundations, I Diall.invedigate the dimenfions and 

 refraclive powers of the human eye in its quiefcent date ; and the form and magnitude of 

 the pifture which is delineated on the retina. I (hall next inquire, how great are the 

 changes which the eye admits, and what degree, of alteration in its proportions will be 

 neceflary for thefe changes, on the various fuppofitions that are principally deferving of 

 conyiarifon. I fliall proceed to relate a variety of experiments xyhich appear to be the moft 

 proper to decide on the truth of each of thefe fuppofitions, and to examine fuch arguments 

 as have been brought forwards, againft the opinion which I fhall endeavour to maintain ; 

 and I (hall conclude with fome anatomical illudrations of the capacity of the organs of 

 various clafles of animals, for the funftions attributed to them. 



III. Of all the external fenfes, the eye is generally fuppofed to be by far the beft under- 

 ftooi; yet fo complicated and fo diverfified are its powers, that many of them have been 

 hitherto uninvefligated.; and on others, much laborious refearch has been fpent in vain. 

 It cannot indeed be denied, that we are capable of explaining the ufe and operation of its 

 different parts, in a far more fatisfa£l:ory and interefling manner than thofe of the ear, 

 which is the only organ that can be ftridtly compared with it ; fince, in fmelling, tafting, 

 and feeling, the objects to be examined come almoft unprepared into immediate contafl 

 with the extremities of the nerves.; and the only difficulty is, in conceiving the nature of 

 the effedl produced by them, and its communication to the fenforium. But the eye and the 

 ear are merely preparatory organs, calculated for tranfmitting the imprefTions of light and 

 found to the retina, and to the termination of the foft auditory nerve. In the eye, light is 

 conveyed to the retina, without any change of the nature of its propagation : in the ear, it 

 Is very probable, that inflead of the fucceffive motion of different parts of the fame elaftic 

 medium, the fmali bones tranfmit the vibrations of found, as paffive iiielaftic hard bodies, 

 obeying the motions of the air in their whole extent at the fiime inflant. In the eye, we 

 judge very precifely of the direftion of light, from the part of the retina on which it im- 

 pinges : in the ear, we have no other criterion than the flight difference of motion in the 

 fmall bones, according to the part of the tympanum on which the found, concentrated by 

 different reflexions, firft flrikes; hence, the idea of dire£lion is neceifarily very indiitind, 

 and there is no reafon to fuppofe, that different parts of the auditory nerve are exclufively 

 affefted by founds in different directions. Each fjnlitive point of ^the retina is capable of 

 receiving diftin£t impreffions, as well of the colour as of the ftrength of light ; but it is 

 not abfolutely certain, that every part of the auditory nerve is capable of receiving the im- 

 preflion of each of the much greater diverfity of tones that we can diftinguifli ; although it 

 is extremely probable, that all the different parts of the furface expofed to the fluid of tfae 

 veftibule, are more or lefs affedted by every found, but in different degrees and fucceflion, 



according 



