MICROSCOPE AND CAMERA ALLIED 



be quite clear what is meant by a short focus lens. 

 There are various ways of measuring the focus of 

 a lens; in the case of a single lens the operation 

 is fairly simple, but single lenses are made of one 

 glass and most camera lenses are built up of several 

 pieces of glass, then it is much more difficult to 

 measure the focus quite accurately. For ordinary 

 purposes and for our purpose, it is only necessary 

 to know the focus in round figures and to do so we 

 open up the camera and focus some distant object 

 on the ground glass, then the distance in inches 

 from the back of the lens to the ground glass will 

 give us very nearly the correct focus, or as it is 

 often called focal length, of our lens. Suppose we 

 focus on a church steeple a mile away and then find 

 that a space of five inches separates the back of 

 our lens from the ground glass, the lens is of five 

 inches focal length. 



j With a lens of such a focal length we shall require 

 a camera with very long bellows to obtain much 

 magnification of our object but, if our camera is 

 one with only short bellows, we can still overcome 

 our difficulties. If the extra expense is no object 

 we can obtain one of the excellent Aldis lenses of 

 only two inches focal length, especially designed 

 for this work; if we are ingenious we can con- 

 ( struct a device which will answer our purpose 

 admirably and cost but a few pence. With the 

 simple apparatus we are about to describe photo- 

 micrographs up to ten diameters magnification can 

 DC obtained in most cameras. The term, ten dia- 



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