THE MICROSCOPE. 19 



mentioned above of crown glass, and the concave lenses of 

 flint glass, as those two kinds of glass have opposite proper- 

 ties with regard to refrangibility. 



THE MICROSCOPE. 



Microscopes are of two kinds : simple and compound : 



The Simple Microscope. 



The ordinary hand magnifying-glass and the dissecting 

 microscope are examples of simple microscopes. 



Their magnifying power depends upon one lens or several 

 lenses acting as one double convex lens. To obtain a clear, 

 enlarged image of the object, the latter must be in its princi- 

 pal focus, and the shorter the focus of the lens the greater its 

 magnifying power. The focal length, or focus, of the lens 

 depends on the degree of curvature of the lens. 



In expressing the magnifying power of lenses, the size of an 

 object as seen by the unaided eye at ten inches distance is 

 taken as unity. A lens having a magnifying power of ten 

 diameters, or linears, is one which enlarges the object ten 

 times in each linear direction. 



The Compound Microscope. 



The compound, or ordinary, microscope consists of the 

 stand and lenses. 



The stand comprises the following parts : 



1 . Base or foot ; 



2. Pillars, or upright, which may be jointed or not ; 



3. Arm connecting the pillars with the 



4. Body, containing the 



5. Draw-tube ; moved up and down rapidly or slowly by 

 means of the 



6. Coarse adjustment used to bring the object into view ; 



7. Fine adjustment used only when the object is already 

 in view, to bring out more clearly its details. 



