What causes an object to become invisible as its distance increases?
Two things are necessary that an object should 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.