CHAP. IV ] 



THE TELESCOPE. 



269 



In Gregory's telescopes, the magnified image is erect : this instru- 

 ment may therefore be used as a terrestrial telescope. By the use of 

 a rod outside, the small mirror can bs moved so as to obtain 

 accurate focus, the eye-piece being fixed. This adjustment is also 

 necessary when an observation is first taken on a celestial object 

 and then on a terrestrial one more or less distant from the 

 observer. 



Cassegrain's telescope is arranged in a somewhat similar manner. 

 It has the same inconveniences and some advantages. In this 

 construction, the small mirror, which is convex, is placed between the 

 large mirror and the image. 



It remains for us to speak of the form suggested by Newton, 

 Fig. 206. The mirror m which receives the rays from the object 

 mirror M is placed, as in Cassegrain's, in front of the principal 



FIG. 204 Principle and arrangement of Gregory's telescope. 



focus where the real image is formed. But it is a plane mirror 

 inclined at an angle of 45, so that it deviates the beam in a 

 direction at right angles to the axis of the instrument. An aperture 

 is made in the side of the tube, and the eye-piece tube is placed 

 in it to examine the magnified image. 



Instead of a plane mirror, a rectangular prism may be used, the 

 rays are reflected to the eye- piece by the back surface at the angle of 

 total reflection. 



Sir W. Herschel constructed a number of telescopes for his own 

 observations ; he ground and polished the mirrors, and was most 

 skilful in these long and delicate operations. 



