OPTICS. "275 



reduced to a perfect spherule, or very small 

 sphere. 



If tiie radius of the spherule be one-tenth of an 

 inch, the eye will have distinct vision of an object 

 by means thereof, at the distance of lj radius; 

 i. e. three-twentieths of an inch, which, as it is but 

 the fortieth part of six inches, shows that the length 

 of an object will be magnified forty times, the sur- 

 face 1600 times, and the solidity 64,000 times, by 

 such a small sphere. 



If the radius of a spherule be but one-twentieth 

 of an inch, then will the eye have distinct vision of 

 an object at the distance of three-fortieths of an 

 inch, which, as it is but the eightieth part of six 

 inches, shows the length of objects will appear 

 80 times greater, the surface 6400 times, and the 

 bulk 512,000 times greater to the naked eye at six 

 inches distance. 



In using these spherule microscopes, the objects 

 are to be placed in one focus, and the eye in the 

 other ; and since the focus is so exceeding near the 

 glass, it is impossible to view any but pellucid 

 bodies ; for if any opaque object were to be ap- 

 plied, the eye being, as it were, just on the sphe- 

 rule, would entirely prevent any light falling on it, 

 and it would be too obscure to be viewed. 



It was with this sort of microscopes that the 

 famous Dutch philosopher, Leuwhenhoek, made 

 wonderful discoveries. But the great difficulty of 

 making very small and at the same time very good 

 ones ; their prejudice to the eyes, in poring very 

 hard and near ; the trouble of placing objects at a 

 due distance, and the very small part which can be 

 seen of any, make this sort of microscopes very 

 little known or used. 



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