Lesson xxxvn.] THE EYE. f Si 



through thep*pzV and different humours, will be 

 converged to various points on the retina at the 

 bottom or' the Eye, and will form upon it an in- 

 VTted picture of the object J or, if this inverted 

 image be not formed, the object cannot be seen : 

 when, unfortunately, any of the parts of the Eye 

 are so injured as to lose their transparency, the 

 person becomes blind. 



But it is not sufficient, in order to our seeing 

 objects, that their images should be painted on the 

 retina : some persons, it is said, are blind, though 

 this takes place. Hence it should seem, that 

 images painted on the retina are not the immediate 

 object of vision, and that the perception of the 

 soul is communicated some other way. The small 

 nerves of the retina (it is now generally admitted) 

 are agitated by the rays of light which form the 

 image at the bottom of the Eye j and this agita-. 

 tion is transmitted by the optic nerve to the brain. 

 It is there the mental perception is formed ; but the 

 nut dextrous anatomist is unable to pursue these 

 nerves 10 their source: the union of the soul with 

 the body will probably for ever remain a mystery. 



There is an astonishing apparatus of muscles, 

 with which the Eye is furnished, to produce all the 

 necessary and convenient motions : among these I 

 cannot help alluding to that admirable contrivance 

 by which the pupil is contracted or dilated, ac- 

 cor.-iing as vision requires. This variability of the 

 pupil is essentially necessary to vision : for, if an 

 involuntary contraction were not to take place 

 when the Eye was brought into a very enlight- 

 ened 



