mall loss of illumination. In other words, the dimensions of the field
of view of a Galilean Telescope depend on the size of the object-glass,
whereas in the astronomical Telescope the field of view is independent
of the size of the object-glass. The difference may be readily tested.
If we direct an opera-glass upon any object, we shall find that any
covering placed over a part of the object-glass _becomes visible_ when
we look through the instrument, interfering therefore _pro tanto_ with
the range of view. A covering similarly placed on any part of the
object-glass of an astronomical telescope does not become visible when
we look through the instrument. The distinction has a very important
bearing on the theory of telescopic vision.

In considering the application of the telescope to practical
observation, the circumstance that in the Galilean Telescope no real
image is formed, is yet more important. A real image admits of
measurement, linear or angular, while to a _virtual_ image (such an
image, for instance, as is formed by a common looking-glass) no such
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