Microscopical Essays. 



4 1 



in the general bufmefs of life, we are accuftomed to fee objeds : 

 now, though the meafure of the image of thcfe obje&s changes 

 considerably when they move from, or approach nearer to us, yet 

 we do not perceive that their fize is much altered : but beyond 

 this diftance, we find the objeas appear to be diminifhed, or in- 

 creafed, in proportion as they are more or lefs diftant from us. 



For inftance, if I place my eye fucceffi vely at two, at four, 

 and at fix feet from the fame perfon, the dimenfions of the image 

 on the retina will be nearly in the proportion of 1, of |, of 

 h and confequently they mould appear to be diminifhed in 

 the fame proportion ; but we do not perceive this diminution, 

 becaufe the mind has reaified the impreflion received on the 

 retina. To prove this, we need only confider, that if we fee 

 a perfon at 120 feet diftance, he will not appear fo ftrikingly 

 Imall, as if the fame perfon mould be viewed from the top of a 

 tower, or other building, 120 feet high, a fituation to which we 

 had not been accuftomed. 



From hence, alfo, it is clear, that when we place a glafs 

 between the objea and the eye, which from it's figure changes 

 the direaion of the rays of light from the objea, this objea 

 ought not to be judged as if it were placed at the ordinary 

 reach of the fight, in which cafe we judge of it's fize more by 

 habit than by the dimenfions of the images formed on the retina': 

 but itmuft be eftimated by the fize of the image in the eye, 

 or by the angle formed at the eye, by the two rays which 

 come from the extremity of the objea. 



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