44 



Microscopical Essays* 



Of the Single Microscope.. 



The fingle microfcope renders minute objects vifible, by mean® 

 of a fmall glafs globule, or convex tens, of a ffiort focus. Let; 

 E Y, Fig. li, Plate I. reprefent the eye; and OB, a fmall ob- 

 ject, fituated very near to it, confequently the angle of it's, 

 apparent magnitude very large. Let the convex lens R S be in- 

 terpofed between the eye and the object, fo that the diflance' 

 between it and the objecl; may be equal to the focal length; and, 

 the rays which diverge from the objecl;, and pafs through the 

 lens, will afterwards proceed, and confequently enter the eye, 

 parallel : after which,, they will be converged, and form an in- 

 verted picture on the retina, and the objecl; will be clearly feen ;. 

 though, if removed to the diflance of fix inches, it's fmallnefs 

 would render it invifible, 



When the lens is not held clofe to the eye, the objeft is fome- 

 what more magnified; becaufe the pencils, which pafs at a? 

 diflance from the center of the lens, are refracted inward toward, 

 the axis, and confequently feem to come from points more- 

 remote from the center of the objecl;, as may be feen in Fig. 12, 

 Plate I. where the pencils which proceed from O and B are re- 

 fracted inwards, and feem to come from the point i and m.. 



Fig. 8, Plate I. may, perhaps, give the reader a ftuT clearer 

 view, why a convex lens increafes the angle of virion. Without 

 alens, as FG, the eye at A would fee the dart BC under the 

 angle b Ae> but the rays B F and C G from, the extremities of 

 the dart in patting through the lens,, are refracted to t^e eye im 

 the direaions f A and Ga, which caufes the dart to be feen under 



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