OF NATURAL HISTORY. tSi 



Kepler, as well as many other philofophers fince his time, to inquire 

 how we fhould fee objeds ere£l from inverted images on the retina. 



Many Ingenious theories have been invented, and many volumes 

 have been written, in order to explain this fecmingly difficult quef- 

 tion. To give even a curfory view of thefe theories would not only 

 be tedious, but in a great meafure ufelefs. We fhall therefore only 

 remark, that their authors uniformly aflumed it as a principle, that, 

 becaufe the pidures are inverted on the retina, the mind ought alfo 

 to perceive them in the fame pofition. It is certain, that, unlefs di- 

 ftinft Images are painted on the retina, objeds cannot be clearly per- 

 ceived. If, from too little light, remotenefs, or any other caufe, a 

 pidture is indiflindly painted on the retina, an obfcure or indiftinft 

 idea of the objed is conveyed to the mind. The piclure on the re- 

 tina, therefore, is fo far the caufe of vifion, that, unlefs this pldture 

 be clear and well defined, our Ideas of the figure, colour, and other 

 qualities of any objed prefented to the eye, will be obfcure and ira- 

 perfedt. The retina of the eye refembles a canvas on which ob- 

 je£ts are painted. The colours of thefe pictures are bright or ob- 

 fcure, in proportion to the dillances of the objeds reprefented. 

 When objeds are very remote, their pidures on the retina are fo 

 faint, that they are entirely obliterated by the vigorous and lively 

 impreflions of nearer objeds, with which we are every way fur- 

 rounded. On the other hand, when near objeds emit a feeble light- 

 only, compared with that which proceeds fron^ a remote objed, as, 

 for example, when we view luminous bodies in the night, then very 

 diftant objeds make diftind pidures on the retina, and become per- 

 fedly vifible. Hence a man, by placing himfelf in a dark fituation, 

 and looking through a long tube, without the intervention of a 

 glafs, may make a kind of telefcope, which will have a confiderable 

 effed even during the day. For the fame reafon, a man at the bot-» 

 torn of a deep pit can fee the ftars at noon* 



Ths- 



