ontributed 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 mirror. When the small mirror, on the surface
of which the second reflection is effected, is plane, and inclined at an
angle of 45 deg. 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.

Both in the one and in the other of these two telescopes, the small
mirror interposed between the object and the great mirror forms relative
to the latter a sort of screen which prevents its entire surface from
contributing towards forming the image. The small mirror, also, in
regard to intensity, gives some trouble.

Let us suppose, in order to clear up our ideas, that the material of
which the two mirrors are made, reflects only half of the incident
light. In the course of the first reflection, the immense quantity of
rays that the aperture of the telescope had received, may be considered
as reduced to half. Nor is the diminution less on the small mirror. Now,
half of half is a quarter. Therefore the instrument will send to the eye
of the observer only a quarter of the incident light that its aperture
had received. These two causes of diminished light not existing in a
refracting telescope, it would give, under parity of dimensions, four
times more[19] light than a Newtonian or Gregorian telescope gives.

Herschel did away with the small mirror in his large telescope. The
large mirror is not mathematically centred in the large tube that
contains it, but is placed rather obliquely in it. This slight obliquity
causes the images to be formed not in the axis of the tube, but very
near its circumference,