This is the Telescope, as it was first discovered, and such an arrangement would now be called a _simple astronomical Telescope_.
Let us clearly understand what each part of the astronomical telescope does for us:-- The object-glass AB gives us an illuminated image, the amount of illumination depending on the size of the object-glass.

The eye-glass enables us to examine the image microscopically.
We may apply eye-glasses of different focal length.

It is clear that the shorter the focal length of _ab_, the nearer must _ab_ be placed to the image, and the smaller will the emergent pencils be, but the greater the magnifying power of the eye-glass.

If the emergent pencils are severally larger than the pupil of the eye, light is wasted at the expense of magnifying power.

Therefore the eye-glass should never be of greater focal length than that which makes the emergent pencils about equal in diameter to the pupil of the eye.