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 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