But this is not the only objection to the Galilean Telescope.

It is obvious that if the part C D of the object-glass were covered, the point P would not be visible, whereas, in the astronomical arrangement no other effect is produced on the visibility of an object, by covering part of the object-glass, than a small 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.