THE MICROSCOPE. 23 



rays after passing through the lens, sees the object apparently 

 magnified. This principle is the basis upon which all micro- 

 scopes are constructed. 



The concave lens produces a precisely contrary effect to that 

 described above. The rays of light diverge on passing through 

 it, and the object appears diminished in size. 



SIMPLE MICROSCOPES. 



A piano or double convex lens, especially when mounted, or 

 arranged with conveniences for viewing objects, is called a 

 simple microscope. 



The magnifying power of a simple microscope is in propor- 

 tion to the shortness of its focal length. Thus, a lens of 2 

 inches focal distance, magnifies 5 diameters (or the superficies 

 25 times) of 1 inch focus, 10 diameters |ths of an inch, 15 

 diameters inch, 20 diameters i inch, 40 diameters gth 

 inch, 80 diameters J^th inch, 100 diameters. 



This table of magnifying powers is not invariably correct, 

 owing to the difference of vision in different individuals, but it 

 is sufficient for all practical purposes. 



Simple microscopes are mounted in a variety of ways, ac- 

 cording to the purposes for which they are intended. Some 

 are made to turn upon a hinge into a case, so as to carry in the 

 pocket; and others are fixed on a handle, with a pin or small 

 pair of forceps in the focus, on which a small object, as an 

 insect, &c., may be placed. 



The cut, Fig. 2, exhibits the arrangement of Dr. Withering's 

 Botanical Microscope, which is valuable from its simplicity. 

 It consists of three brass plates, a, I, c, parallel with each other, 



