towards the
great one, is placed in the axis of the tube and forms an image which is
viewed through an aperture in the middle of the great mirror. A similar
plan is adopted in Cassegrain's Telescope, a small convex mirror
replacing the concave one. In Newton's Telescope a small inclined-plane
reflector is used, which sends the pencil of light off at right-angles
to the axis of the tube. In Herschel's Telescope the great mirror is
inclined so that the image is formed at a slight distance from the axis
of the telescope. In the two first cases the object is viewed in the
usual or direct way, the image being erect in Gregory's and inverted in
Cassegrain's. In the third the observer looks through the side of the
telescope, seeing an inverted image of the object. In the last the
observer sees the object inverted, but not altered as respects right and
left. The last-mentioned method of viewing objects is the only one in
which the observer's back is turned towards the object, yet this method
is called the _front view_--apparently _quasi lucus a non lucendo_.

[Illustration: _Fig. 3._]

It appears, then, that in all astronomical Telescopes, reflecting or
refracting, a _real image_ of an object is submitted to microscopical
examination.

Of this fact the possessor of a telescope may easily assure himself;
for if the eye-glass be removed, and a small screen be placed at the
focus of the object-glass, there will appear upon the screen a small
picture of any object towards which the tube is turned. But the image
may be viewed in another way which requires to be noticed. If the eye,
placed at a distance of five or six inches from the image, be directed
down the tube, the image will be seen as before; in fact, just as a
single convex lens of short focus is the simplest microscope, so a
simple convex lens of long focus is the simplest telescope.[1] But a
singular circumstance will immediately attract the observer's notice. A
real picture, or the image formed on the screen as in the former case,
can be viewed at varying distances; but when we view the image directly,
it will be found that for distinct vision the eye must be placed almost
exactly at a fixed distance from the image. This peculiarity is more
important than it might be thought at first sight. In fact, it is
essential that the observer who would rightly apply the powers of his
telescope, or fairly test its performance, should understand in what
respect an image formed by