We have been hitherto talking about large planets, which, if not as big
as our earth, are at least as big as our moon. But now we have to say a
few words about a number of little planets, many of them being so very
small that a million rolled together would not form a globe so big as
this earth. These little objects you cannot see with your unaided eye,
and even with a telescope they only look like very small stars.
I have often been asked why it is that a telescope enables us to
see objects, both faint and small, which our unaided eyes fail to
show. Perhaps this will be a good opportunity to say a few words on
the subject. I think we can explain the utility of the telescope by
examining our own eyes. The eye undergoes a remarkable transformation
when its owner passes from darkness into a brilliantly lighted room
(Fig. 64). Here you see two views of an eye, and you notice the great
difference between them. They are not intended to be the eyes of two
different people, or the two eyes of the same person; they are merely
two conditions of the same eye. They are intended to illustrate two
different states of the eye of a collier. The right shows his eye when
he is above ground in bright daylight; the left is his eye when he has
gone down the coal-pit to his useful work in the dark regions below. I
remember when I went down a coal-pit I was lowered down a long shaft,
and when the bottom was reached a safety lamp was handed to me. The
gloom was such, that at first I found some little difficulty in guiding
my steps, but the capable guide beside me said in an encouraging voice,
“You will be all right, sir, in a few moments, for you will get your
_pit-eyes_.” I did get my “pit-eyes,” as he promised, and was able to
see my way along sufficiently to enjoy the wonderful sights that are
met with in the depths below.
[Illustration: This shows the Eye in the Dark.
This shows the Eye in the Daylight.