302 OPTICS. 



This is no more than two equal telescopes set in a 

 frame, parallel to one another; and these may be 

 set at a proper distance from one another, by the 

 help of screws ; and that distance is to be the same 

 as the distance of the two pupils of the eyes. 

 When that is adjusted, a person is to look through 

 them both at once; through one with each eye, to 

 any object, which will then be seen by both eyes, 

 and appear far brighter than through a single 

 telescope. 



Telescopes, in general, represent terrestrial ob- 

 jects as if nearer; and this nearness, or seeming 

 approach of the object, is as the magnifying power 

 of the telescope. Thus, looking at a man one 

 hundred yards off, with a telescope that magnifies 

 one hundred times, the man will appear to be only 

 a yard off. 



The magnifying power of a telescope will be 

 found, if you make two equal circles of paper of an 

 inch diameter or more, and fix one of them upon 

 a wall one hundred or two hundred yards distant; 

 and the other at a small distance, in a line with the 

 first. Then look at the farther circle through the 

 telescope with one eye, and at the near circle with 

 the other eye naked. Move the near circle (or else 

 the telescope) backward and forward, until the 

 two circles appear equal, or coincide. Then mea- 

 sure the two distances, from the eye-glass of the 

 telescope to the two circles; divide the greater 

 distance by the less, and you have the magnifying 

 power of the telescope. 



There is no better way for trying the goodness of 

 an object-glass than putting it in a tube, and trying 

 it with several small eye-glasses, by looking at 

 several distant objects, and particularly at the 

 title-page of a book; for that glass which repre- 



