20 



CONSTRUCTION OF 



tance be an inch, the object will have its apparent magnitude six times 

 greater than when it is seen at the distance of six inches without the 

 lens. It is therefore said to be magnified six times by the lens. 

 This lens therefore is a single microscope, and the magnifying 

 power of such microscopes may be always found by dividing six inches 

 by the focal distance of the lens. A lens, for example the tenth of an 

 inch in focal length, will magnify 60 times ; and one the hundredth of 

 an inch, 600 times. 



11. SIN OLE MICROSCOPES. Single microscopes may consist, as we 

 have already said, of one or more lenses, but in all cases it is the object 

 itself which is magnified. The simplest form of single microscope is 

 that which consists of a single lens or spherical globule, fitted up so 

 that it may be conveniently held to the eye, the object being at the 

 focus of the lens. A common hollow globe of glass an inch in diameter, 

 and filled with water, is of itself a single microscope ; so would be a 

 drop of water, placed upon a hole drilled in a thin piece of brass. A 

 lens is, of course, far preferable to either of these contrivances. For a 

 variety of purposes, a single lens is an extremely useful instrument, 

 and they are usually mounted by opticians in a number of ways for por- 

 tability and convenience. Figs. 1 and 2 represent forms of single 

 microscopic lenses, mounted in such a manner as to be folded up and 

 carried in the pocket. 



Fig. 1. 



Fig. 2. 



Fig. 3. 



