THE MICROSCOPE. 237 



property of picturing upon the retina of the eye an 

 image of an object larger than the image produced there 

 without their aid. The glasses used in microscopes are 

 either double convex lenses (a) or plano-convex lenses (b). 



b 



If a double convex lens or a plano-convex one be placed 

 over a hole in the shutter of a darkened room, or over 

 the key-hole of a door, and a piece of paper be held at a 

 proper distance, a picture of all objects in front of the 

 lens will be thrown on the paper, as in the camera-obscura 

 or the magic-lantern. Now, in the same manner, a lens 

 throws a picture of objects to which it is directed on the 

 retina of the eye, and when*that picture is larger than the 

 image made in the eye by the object, without the aid of 

 the lens, it is magnified, or the lens has served as a micro- 

 scope, so called, from its use in seeing small objects, from 

 mikros, small, and skopeo, to see. 



4. Different Kinds of Microscopes. Microscopes 

 are either simple or compound. The glasses of magnifying 

 spectacles, like those commonly used by aged persons, are 

 simple microscopes. Magnifying glasses, mounted in 

 frames such as are for sale by opticians and others, for 

 the detection of counterfeit money, are simple microscopes, 

 and are useful in studying the coarser structure of plants 

 and animals. 



5. The most powerful simple microscopes are made by 

 melting in a flame a thread of spun glass, so as to form a 



4. Kinds of microscope ? What are simple microscopes ? 



5. Construction of the most powerful simple microscopes? In practice? A 

 doublet ': Triplet ? Why are compound microscopes superior to simple ones ? 



