gathered up by the large glass AB. If then we can
only make this light _available_, it is clear that we shall have
acquired a large increase of _light_ from the distant object. Now it
will be noticed that the light which has converged to _p_, diverges from
_p_ so that an eye, placed that this diverging pencil of rays may fall
upon it, would be too small to receive the whole of the pencil. Or, if
it did receive the whole of this pencil, it clearly could not receive
the whole of the pencils proceeding from other parts of the image _emf_.
_Something_ would be gained, though, even in this case, since it is
clear that an eye thus placed at a distance of ten inches from _emf_
(which is about the average distance of distinct vision) would not only
receive much more light from the image _emf_, than it would from the
object EMF, but see the image much larger than the object. It is in this
way that a simple object-glass forms a telescope, a circumstance we
shall presently have to notice more at length. But we want to gain the
full benefit of the light which has been gathered up for us by our
object-glass. We therefore interpose a small convex glass _ab_ (called
an eye-glass) between the image and the eye, at such a distance from the
image that the divergent pencil of rays is converted into a pencil of
parallel or nearly parallel rays. Call this an emergent pencil. Then all
the emergent pencils now converge to a point on the axial line _m_M
(produced beyond _m_), and an eye suitably placed can take in all of
them at once. Thus the whole, or a large part, of the image is seen at
once. But the image is seen inverted as shown. 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