264 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



one may say, of the invention of this telescope. As will be seen 

 further on, in his form of instrument the image of the object magnified 

 by the aid of the eye-piece is formed after double reflection, first, by a 

 large and then by a small concave mirror, whence considerable loss 

 of light results. Newton proposed a different arrangement, in which 

 the reflection took place on two mirrors, one of them being a plane 

 one; and lastly, Sir W. Herschel completely did away with the 

 second reflection in the telescopes of large apertures which are 

 named after him. We will begin with this last system, the most 

 simple of all. 



A concave mirror M, arranged at the bottom end of the tube, receives 

 the rays coming from the object AB, and by reflection gives rise to the 



FIG. 202. -Principle and arrangement of Sir W. HerschrTs (.front view) telescope. 



formation of a reversed aerial or real image, /;, a. By using the eye- 

 piece 0, arranged in front of the principal focus of the mirror on the 

 lower edge of the tube, the eye sees the image B'A' magnified, but re- 

 versed. This arrangement is only possible in telescopes with a mirror 

 of large aperture, so that the head of the observer, who turns his 

 back to the portion of the heavens observed, does not to any large 

 extent intercept the rays falling on the mirror. For this reason a 

 position slightly inclined to the axis of the tube is given to the 

 mirror. In a very large telescope the portion of the head which 

 encroaches on the aperture of the tube is but a small fraction of the 

 surface of the mirror ; this would not be the case in a telescope of 

 small dimensions. Telescopes of this kind are known as front-view 

 telescopes, a name given to them by Sir W. Herschel liimself. 

 The largest made by the illustrious astronomer of Slough on this 

 model is that represented in Fig. 203. It was 39 feet 4 inches 



