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THE POPULAR SCIENCE MONTHLY. 



manded at will, yet they are turned out with increasing fre- 

 quency, and the best artists are generally able, at all times, to 

 approximate so closely to perfection that any shortcoming may be 

 disregarded. 



In % what is said above I refer, of course, to the refracting tele- 

 scope, which is the form of instrument that I should recommend 

 to all amateurs in preference to the reflector. But, before pro- 

 ceeding further, it may be well to recall briefly the principal 



FIG. 1. IMAGE AT THE Focus OF A LENS. 



points of difference between these two kinds of telescopes. The 

 purpose of a telescope of either description is, first, to form an 

 image of the object looked at by concentrating the rays of light 

 proceeding from that object at a focus. The refractor achieves 

 this by means of a carefully shaped lens, called the object glass, 

 or objective. The reflector, on the other hand, forms the image 

 at the focus of a concave mirror. 



A very pretty little experiment, which illustrates these two 

 methods of forming an optical image, and, by way of corollary, 



