process can be applied. In simple observation the only noticeable effect
of this difference is that, whereas in the astronomical Telescope a
_stop_ or diaphragm can be inserted in the tube so as to cut off what is
called the _ragged edge_ of the field of view (which includes all the
part not reached by _full pencils of light_ from the object-glass),
there is no means of remedying the corresponding defect in the Galilean
Telescope. It would be a very annoying defect in a telescope intended
for astronomical observation, since in general the edge of the field of
view is not perceptible at night. The unpleasant nature of the defect
may be seen by looking through an opera-glass, and noticing the gradual
fading away of light round the circumference of the field of view.

The properties of reflection as well as of refraction have been enlisted
into the service of the astronomical observer. The formation of an image
by means of a concave mirror is exhibited in fig. 3. As the observer's
head would be placed between the object and the mirror, if the image,
formed as in fig. 3, were to be microscopically examined, various
devices are employed in the construction of reflecting telescopes to
avoid the loss of light which would result--a loss which would be
important even with the largest mirrors yet constructed. Thus, in
Gregory's Telescope, a small mirror, having its concavity 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