42 



Microscopical Essays. 



If the image of an object, formed after refraction, is greater or 

 lefs than the angle formed at the eye, by the rays proceeding 

 from the extremities of the object itfelf, the object will appear 

 alfo proportionably enlarged or diminilhed ; fo that if the eye 

 approaches to, or removes from, the laft image, the object will- 

 appear to increafe or diminifh, though the eye mould in reality 

 remove from it in one cafe, or approach towards it in the other ; 

 becaufe the image takes place of the object, and is confidered 

 inflead of it. 



The apparent diflance of an object from the eye, is not meafured 

 by the real diftance from the laft image ; for, as the apparent 

 diflance is eftimated principally by the ideas we have of their fize, it 

 follows, that when we fee objects, whofe images are increafed or 

 diminimed by refraction, we naturally judge them to be nearer 

 or further from the eye, in proportion to the fize thereof, when, 

 compared to that with which we are acquainted. The apparent 

 diflance of an object is confiderably affected by the brightnefs, 

 diftin&nefs, and magnitude thereof. Now as thefe circumftances. 

 are, in a certain degree, altered by the refraction of the rays, in 

 their paffing through different mediums, they will alfo, in fome 

 meafure, affect the eflimation of the apparent diflance. 



In the theory of vifion, it is neceffary to be cautious not to 

 confound the organs of vifion with the being that perceives, or 

 with the perceptive faculty. The eye is not that which fees, it 

 is only the organ by which we fee. A man cannot fee the fatek 

 lites of Jupiter but by a telefeope. Does he conclude from this, 

 that it is the telefeope that fees thofe ftars ? By no means; fuch a 

 conclufion would be abfurd. It is no lefs abfurd to conclude,. 

 £ . that 



