THE MICROSCOPE. 



239 



7. A Compound Microscope consists of two por- 

 tions : the optical portion, or the lenses, and the mechanical 

 portion, or the instrument which bears 



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the lenses. The glasses of a compound 

 microscope are two : the object-glass (D), 

 Fig. 60, and the lower lens of Fig. 59, 

 and the ocular or eye-piece (A), Fig. 60, 

 and the upper piece of Fig. 59. Both 

 the object-glass and the eye-piece may, 

 and usually do, consist of more than 

 one lens, for, as previously mentioned, 

 better results are obtained by a com- 

 bination of lenses of moderate power 

 than by single lenses of high power 

 and great curvature. 



8. How to choose and use a 

 Microscope. No attractiveness in 

 the mechanical part of a microscope 

 can compensate for inferior lenses; 

 and the very first consideration in the 

 choice of an instrument should be the 

 excellence of the optical part of the 

 instrument. In the use of the instru- 

 ment, care should be exercised to keep 

 the lenses clean, free from dust, not to 

 press the object-glass upon the object 

 under observation, and not to wet it 

 in the water in which most objects are 

 examined. A good microscope re- 

 quires its own table ; and when not 

 in use, should be covered by a bell 

 glass or a clean linen cloth. 



9. The mechanical portion of the instrument varies 



7, Portions, in a compound microscope ? The glasses ? 



8. How to choose a microscope ? How to use it? 



il. The characteristics of the best instrument? What special requisites should 

 be insisted upon ? Why, as to a horizontal stage ? 



FIG. 59. 

 COMPOUND MICROSCOPE. 



