334 THE MICROSCOPE 



3. Convex Lenses, or magnifying glasses, are disks of glass 

 or other transparent substances, which have the property of 

 picturing itpon 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 (5). If either of these lenses 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 dis- 

 tance, 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 



a b 



* 



FIG. 76 



picture is larger than the image made in the eye l?y the object 

 without the aid of the lens, it is magnified, or the lens has 

 served as a microscope, so called from its use in seeing small 

 objects, from mikros, small, and sJcopeo, 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 micro- 

 scopes. Magnifying glasses, mounted in frames, such as are 

 for sale by opticians and others for the detection of counter- 

 feit 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 melt- 

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



8. What are convex lenses ? Kind of lenses used in microscopes ? Experiment ? 

 Picture thrown upon the eyes ? Derivation of the word microscope ? 



4. Kinds of microscopes ? 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 ? 



