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THE POPULAR SCIENCE MONTHLY. 



Move the two carefully until they are at a distance apart equal to 

 the sum of the focal lengths of the lenses, and you will see a 

 magnified image of the distant object. In other words, you have 

 constructed a simple refracting telescope. Then take the mag- 

 nifying mirror, and, turning your back to the object to be looked 

 at, use the small lens as before that is to say, hold it between 

 your eye and the mirror, so that its distance from the latter is 

 equal to the sum of the focal lengths of the mirror and the lens, 



FIG. 2. IMAGE AT THE Focus OF A CONCAVE MIREOE. 



and you will see again a magnified image of the distant object. 

 This time it is a reflecting telescope that you hold in your hands. 

 The magnification of the image reminds us of the second pur- 

 pose which is subserved by a telescope. A telescope, whether re- 

 fracting or reflecting, consists of two essential parts, the first 

 being a lens, or a mirror, to form an image, and the second a 

 microscope, called an eyepiece, to magnify the image. The 

 same eyepieces will serve for either the reflector or the refractor. 

 But in order that the magnification may be done with effect, and 



