67 i 



ox THE MECPANISM OF THE EYE. 



finally conclusive in favour of my former 

 opinion, as far as tiiat opinion attributed to 

 the lens a pojver of changing its figure. At 

 the same time, I must remark, thatever^^ per- 

 son, who has been engaged in experiments 

 of this nrMne, will be siwinre of ^he extreme 

 delicacy and precaution requisite, both in 

 conducting tliem, and in drawing inferences 

 from them ; and will also readily allow, that 

 no apology is necessary for the fallacies 

 vvhiah have misled man}' others, as well as 

 myself, in the application of 'those experi- 

 ments to optical and physiological determi- 

 nations. 



II. Besides the inquiry, respecting the ac- 

 commodation of tlie eye to different dis- 

 .tanees, I shall have occasion to notice some 

 other particulars relative to its fu«ctiujis;and 

 1 shall begin with a general consideration of 

 the sense of vision. I shall then describe an 

 instrument for readily ascertaining the focal 

 dist.%>ce of the eye ; and with the assistance 

 ef this instr«me-nt, I shall investigate the 

 'dimensions and refractive powers of the 

 human eye in its quiescent state; and the 

 form and magnitude of the picture, which is 

 delineated on the retina. I shall next inquire, 

 how great are' the changes which the eye ad- 

 mits, and what degree of alteration in itspro- 

 fKjrtions will be necessary for these changes, 

 on the varioussuppositions that are principal- 

 ly deserving of comparison. • Ishall proceed 

 to relate a variety of exi^eriments, which ap- 

 ,pear to be the most proper to decide on the 

 truth ofeach of these suppositions, and toexa- 

 mine such arguments> as have been brought 

 forwards, against the opinion which 1 shall 

 endeavour to maintain ; and I shall conclude 

 with some apatomical illustrations of the ca- 

 pacity of the organs of various classes of ani- 

 mals, for the functions attributed tothem. 



III. Of all the external senses, the eye is 

 generally supposed to be by far the best uir- • 

 derstood ; yet so complicated and so diversi- 

 fied are its powers, that many of them have 

 been hitherto uninvestigated : and on others, 

 much, laborious rese^sirch has been spent in 

 vain. It cannot indeed be denied, that we 

 are capable of cvplaining the use and opera- 

 tion of its different parts, in a far more satis- 

 factory and interesting manner thata those of 

 the ear, which is the only organ that can be 

 strictly com{)ared with it; since, in smell- 

 ing, tasting, and feeling, the objects to be 

 examined come, almost unprepared, into im- 

 mediate contact with the extremities of the 

 nerves; and the only difficulty is, in conceiv- 

 ing the nature of the effect produced by them, 

 andof its communication to the sensorium. But 

 the eye and the ear are merely preparatory 

 organs, calculated for transmitting the im- 

 pressions of light and sound, to the retina, and 

 to the termination of the soft auditory nerve. 

 In the eye, light is conveyed to the retina, 

 without any change of the nature of its pro- 

 pagation : in the ear, it is 'very probable, 

 that instead of the successive motion of dif- 

 ferent parts of the same elastic medium, the 

 small bones transmit the vibrations of sound, 

 as passive hard bodies, obeying the motions 

 of the air nearly in their whole extent at the 

 same instant. In the eye, we judge very pre- 

 cisely of the direction of light, from the 

 part of the retina on which it impinges; in 

 the ear, we have no other criterion than t^e 

 slight ' difference of motion in the small 

 bones, according to' the part of the tyiupa- 

 num on which the sound, concentrated by 

 different reflectionsy first strikes ; hence, the 

 idea of direction is necessarily very indistinct; 

 and there is no reason to suppose, tljat dif- 

 ferent parts of the auditory nerve axe ejcclu- 



