Microscopical Essays. 



33 



but becaufe at the diftance from the eye which would be neceffary 

 for this purpofe, it would appear exceedingly confufed ; for 

 which reafon, we never bring our eye fo near to it, and confe- 

 quently, as we have not been accuftomed to fee the object of this 

 fize, it appears an extraordinary one. 



On account of the extreme minutenefs of the atoms of light, 

 it is clear, a fmgle ray, or even a fmall number of rays, cannot 

 make a fenfible impremon on the organ of fight, whofe fibres are 

 very grofs, when compared to thefe atoms ; it is neceffary, there- 

 fore, that a great number mould proceed from the furface of an 

 objecl, to render it vifible. But as the rays of light, which 

 proceed from an objecl, are continually diverging, different 

 methods have been contrived, either of uniting them in a given 

 point, or of feparating them' at pleafure : the manner of doing 

 this, is the fubjecl of dioptrics and catoptrics. 



By the help of glaffes, we unite in the fame fenfible point a 

 great number of rays, proceeding from one point of an object ; 

 and as each ray carries with it the image of the point from whence 

 it proceeded, all the rays united muff; form an image of the 

 object from whence they were emitted. This image is brighter, 

 in proportion as there are more rays united ; and more diftincl;, 

 in proportion as the order, in which they proceeded, is better 

 preferved in their union. This may be rendered evident ; for, if 

 a white and poliffied plane is placed where the union is formed, 

 we mail fee the image of the object painted in all it's colours on 

 this plane ; which image will be brighter, if all adventitious light 

 is excluded from the plane on which it is received. 



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