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.