be visible. It must be _large_ enough to be appreciated by the eye, and
it must _send light_ enough. Thus increase of distance may render an
object invisible, either through diminution of its apparent size, or
through diminution in the quantity of light it sends to the eye, or
through both these causes combined. A telescope, therefore, or (as its
name implies) an instrument to render distant objects visible, must be
both a magnifying and an illuminating instrument.

[Illustration: _Fig. 1._]

Let EF, fig. 1, be an object, not near to AB as in the figure, but so
far off that the bounding lines from A and B would meet at the point
corresponding to the point P. Then if a large convex glass AB (called an
_object-glass_) be interposed between the object and the eye, all those
rays which, proceeding from P, fall on AB, will be caused to converge
nearly to a point _p_. The same is true for every point of the object
EMF, and thus a small image, _emf_, will be formed. This image will not
lie exactly on a flat surface, but will be curved about the point midway
between A and B as a centre. Now if the lens AB is removed, and an eye
is placed at _m_ to view the distant object EMF, those rays only from
each point of the object which fall on the pupil of the eye (whose
diameter is about equal to _mp_ suppose) will serve to render the object
visible. On the other hand, every point of the image _emf_ has received
the whole of the light 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