188 THfi PHILQSOPHY 



and the refra^lion of the different parts of the eye, which act 

 as a lens, necCiTarily makes them crofs each other Jn their 

 pafTage to the retina. After croffing, they diverge till they 

 are flopped by the retina, where they form an inverted pic- 

 ture. The upper part of the object is painted on the lower 

 part of the retina, and the right fide upon the left, &c. The 

 celebrated Kepler firft difcovered, that diftindl, but inverted 

 pictures of every object we behold are painted on the retina 

 by the rays of light proceeding from vifible obje(5ts. This 

 difcovery naturally led Kepler, as well as many other philo-* 

 fophers lince his time, to inquire how \re fliould fee objects 

 erect from inverted images on the retina. 



Many ingenious theories have been invented, and many 

 volumes have been written, in order to explain this feeming- 

 ly difficult queftion. To give even a curfory view of thefe 

 theories would not only be tedious, but in a great meafure 

 tifelefs. V/e fhall therefore only remark, that their ail* 

 thors uniformly alTumed it as a principle, that, becaufe the 

 pictures are inverted on the retina, the mind ought alfo to 

 perceive them in the fame pofjtion. It is certain, that, unlefs 

 di(tin(5t images are painted on the retina, objects cannot be 

 clearly perceived. If, from too little light, remotenefs, or 

 any other caufe, a picture is indiftin6tly painted on the reti- 

 na, an obfcure or indiitin£t idea of the objedt is conveyed to 

 the mind. The picture on the retina, therefore, is fo far 

 tJie caufe of vifion, that, unlefs this pi6ture be clear and well 

 defined, our ideas of the figure, colour, and other qualities 

 of any object prefented to the eye, will be obfcure and im- 

 perfect. The retina of the eye refembles a canvas on which 

 objects are painted. The colours of thefe pi^ures are bright 

 or obfcure, in proportion to the diftances of the objects re- 

 prefented. When objects are very remote, their pictures on 

 the retina, are fo faint, that they are entirely, obliterated by 

 the vigorous and lively impreffions of nearer obje^s, with 



