HIS IMPROVEMENTS IN THE TELESCOPE. 273 



examine that image, must necessarily place himself be- 

 yond the point where the rays that form it have crossed 

 each other ; beyond, let us carefully remark, means far- 

 ther off from the object-glass. The observer's head, his 

 body, cannot then injure the formation or the brightness 

 of the image, however small may be the distance from 

 which we have to study it. But it is no longer thus with 

 the image formed by means of reflection. For the im- 

 age is now placed between the object and the reflecting 

 mirror ; and when the astronomer approaches in order to 

 examine it, he inevitably intercepts, if not the totality, at 

 least a very considerable portion of the luminous rays, 

 which would otherwise have contributed to give it great 

 splendour. It will now be understood, why in optical 

 instruments where the images of distant objects are 

 formed by the reflection of light, it has been necessary 

 to carry the images, by the aid of a second reflection, out 

 of the tube that contains and sustains the principal mir- 

 ror. When the small mirror, on the surface of which the 

 second reflection is effected, is plane, and inclined at an 

 angle of 45 to the axis of the telescope ; when the 

 image is reflected laterally, through an opening made 

 near the edge of the tube and furnished with an eye- 

 piece ; when, in a word, the astronomer looks definitively 

 in a direction perpendicular to the line described by the 

 luminous rays coming from the object and falling on the 

 centre of the great mirror, then the telescope is called 

 Newtonian. But in the Gregorian telescope, the image 

 formed by the principal mirror falls on a second mirror, 

 which is very small, slightly curved, and parallel to the 

 first. The small mirror reflects the first image and 

 throws it beyond the large mirror, through an opening 

 made in the middle of that principal mirror. 



12* 



