520 THE MICROSCOPE. 



a lens of 6 cm focal length produces an image which is 

 25 



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= 5 '166 times as large as the object. In order 



to obtain a greater magnifying power, compound micro- 

 scopes are used, which consist of several lenses arranged 

 within a tube. We must confine ourselves here more 

 to an explanation of the principles of such instruments 

 than to an actual description of their details. Their 

 construction is therefore assumed to be somewhat more 

 simple than it actually is in instruments of that kind. 

 The lenses used in such instruments are not simple, as 

 assumed in our description, but mostly systems of 

 lenses, that is, combinations of several lenses which 

 together produce the same effect as a simple lens, 

 but render the image much more distinct and precise 

 than could be done by a simple lens. Moreover, an 

 apparently single lens in such an instrument consists 

 often of a combination of two lenses, each of a different 

 kind of glass, differing in dispersive power, and so 

 combined as to free the images as much as possible 

 from the coloured fringes which surround objects when 

 seen through simple lenses. Such compound lenses are 

 called achromatic. 



The Microscope (fig. 283) has a foot /, upon which 

 is fixed perpendicularly the pillar s. This carries the 

 'stage' #, a horizontal plate perforated in the middle, 

 upon which the objects to be observed are placed be- 

 tween the glass plates 0, the object being over the 

 aperture in the stage, so that diffused daylight or the 

 light of a lamp or candle may be reflected to it by the 

 moveable mirror b. When, owing to the opacity of 

 the objects, they cannot be lighted in this manner from 



