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* 
