THE COMPOUND MICROSCOPE 



mirrors will reflect parallel rays of light to a point, 

 just as a double convex lens will so direct them 

 from their course that they meet at a point. The 

 concave mirror gives the more powerful illumina- 

 tion, because it reflects more light rays than a flat 

 mirror of the same diameter. 



We have mentioned that, to obtain full advantage 

 from the mirror it should be capable of movement 

 to and from the stage. When we desire strong 

 illumination we arrange the mirror so that its re- 

 flected rays meet at a point coinciding with our 

 object. Should less intense illumination be required, 



>^ 



we slide the mirror nearer to the stage, and of course 

 nearer to our object, so that the reflected rays meet 

 at a point above the object. 



The two diagrams, given below, show the path 

 of the rays of light, where O is the object, and 

 a trial with our microscope will soon show which 

 position gives the more powerful illumination. 



For high-power work, such as bacteriology or even 



the examination of sections of plants, etc., even the 



,^best concave mirror will not give a sufficiently 



. powerful illumination; accordingly an instrument, 



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