MICROSCOPES. 



21 



Fig. 3 is a useful combination of three lenses, mounted in a case of 

 a convenient size for the pocket. By varying the arrangement of the 

 lenses, or by taking each separately, it affords many degrees of mag- 

 nifying power. 



Fig. 4 and 5 represent other forms of microscopic lens, much in 

 use for the examination of linen cloth, and for ascertaining the number 

 of threads in a given space. 



Fig. 4. Fig. 5. 



12. STANHOPE LENS. But, perhaps, the most powerful of these 

 simple lenses is that invented by Lord Stanhope. It is a double con- 

 vex lens, the end however which is nearest to the eye being ground 

 more convex than the other, in the proportion of three to five. It is 

 set .in a metal cylinder, the length of which is the exact focus of the 

 lens, and thus it possesses great advantages over every other form of 

 common lens. By it all difficulty of retaining the instrument to the 

 exact focus, and the loss of light and small field are obviated. The 

 object to be examined has only to be placed on the less convex end of 

 the lens, or to be brought in contact with it, when the advantage of 

 great magnifying power and a field of nearly five inches are obtained. 

 Its general form is represented in the annexed figure. 



Fig. 6. 

 So powerful are these lenses that many interesting objects may be 



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