ject
when that object is near enough to subtend a certain angle, or, in
popular language, to show itself a certain size--the rays of light
must converge--in fact, the eye cannot single out and appreciate
parallel rays: could it do this, objects would not appear to grow
smaller as they are removed. A pencil might be removed to the Moon,
240,000 miles away, and would still appear to the eye the same size as
it does here close to you; with perfect vision there would be no such
thing as perspective, but, with our present conditions of sight, the
result would be inconvenient. We should never be able to see, at one
and the same time, anything larger than the pupil of our eye. The
beauties of the landscape would be gone, and our dearest friends would
pass us unheeded and unseen; everyday life would resolve itself into a
task similar to that of attempting to read our newspaper every morning
by means of a powerful microscope; we should commence by getting on to
a big black blotch, and, after wandering about for half an hour, we
might perhaps then begin to find out that we were looking at the
little letter "e," but anything like reading would be quite out of the
question. We may, therefore, with our limited aperture of sight, be
thankful that our eyes have the imperfection of not appreciating
parallel rays. But we will now consider how this imperfection may be
remedied by science.

There are two different ways of doing this--viz., first, by increasing
the amount of light received, by means of telescopes of great
aperture; and secondly, by employing an artificial retina a thousand
times more sensitive than the human. Now, the human retina receives
the impression of what it looks at in a very minute fraction of a
second, provided of course that the eye is properly focussed, and no
further impression will be made by keeping the eye fixed on that
object; but in celestial photography, when the telescope is turned
into a camera, the sensitive plate, having received the impression in
the first second, may be exposed not only for many seconds, or
minutes, or hours, but for an aggregate of even days by re-exposure,
every second of which time details on that plate new objects, sunk so
deep in the vast depths of space as to be immeasurably beyond the
power of the human eye, even through telescopes hundreds of times more
powerful than the largest instruments that science has enabled us to
construct; and yet here is laid before us a faithful