PLEASURES OF THE TELESCOPE, 215 



illustrates the essential difference between refracting and reflect- 

 ing telescopes, may be performed by any one who possesses a 

 reading glass and a magnifying hand mirror. In a room that is 

 not too brightly illuminated pin a sheet of white paper on the 

 wall opposite to a window that, by preference, should face the 

 north, or away from the position of the sun. Taking first the 

 reading glass, hold it between the window and the wall parallel 

 to the sheet of paper, and a foot or more distant from the latter. 

 By moving it to and fro a little you will be able to find a distance, 

 corresponding to the focal length of the lens, at which a picture 

 of the window is formed on the paper. This picture, or image, 

 will be upside down, because the rays of light cross at the focus. 

 By moving the glass a little closer to the wall you will cause the 

 picture of the window to become indistinct, while a beautiful 

 image of the houses, trees, or other objects of the outdoor world 

 beyond, will be formed upon the paper. We thus learn that the 

 distance of the image from the lens varies with the distance of 

 the object whose image is formed. In precisely a similar manner 

 an image is formed at the focus of the object glass of a refracting 

 telescope. 



Take next your magnifying or concave mirror, and detaching 

 the sheet of paper from the wall, hold it nearly in front of the 

 mirror between the latter and the window. When you have 

 adjusted the distance to the focal length of the mirror, you will 

 see an image of the window projected upon the paper, and by 

 varying the distance, as before, you will be able to produce, at 

 will, pictures of nearer or more remote objects. It is in this way 

 that images are formed at the focus of the mirror of a reflecting 

 telescope. ' 



Now, 5^ou will have observed that the chief ajDparent difference 

 between these two methods of forming an image of distant ob- 

 jects is that in the first case the rays of light, passing through 

 the transparent lens, are brought to a focus on the side opposite 

 to that where the real object is, while in the second case the rays, 

 being reflected from the brilliant surface of the opaque mirror, 

 come to a focus on the same side as that on which the object itself 

 is. From this follows the most striking difference in the method 

 of using refracting and reflecting telescopes. In the refractor the 

 observer looks toward the object; in the reflector he looks away 

 from it. Sir William Herschel made his great discoveries with 

 his back to the sky. He used reflecting telescojDes. This prin- 

 ciple, again, can be readily illustrated by means of our simple 

 experiment with a reading glass and a magnifying mirror. Hold 

 the reading glass between the eye and a distant object with one 

 hand, and with the other hand place a smaller lens such as a 

 pocket magnifier, near the eye, and in line with the reading glass. 



