The change that came over my eyes is that which these two pictures
illustrate: the black, round spot in the centre is an opening covered
with a transparent window, by which light enters the eye; the black
spot is called the pupil, and nature has provided a beautiful
contrivance by which the pupil can get larger or smaller, so as to make
vision agreeable. When there is a great deal of light we limit the
amount that enters by contracting the pupil so as to make the opening
smaller. Thus the picture with the small pupil represents the state of
the collier’s eye when he is above ground in bright sunlight. When he
descends into the pit, where the light is very scanty, then he wants to
grasp as much of it as ever he can, and consequently his pupil enlarges
so as to make a wider opening, and this is what he calls getting his
“pit-eyes.”
But you need not go down a coal-mine to see the use of the iris--for so
that pretty membrane is called which surrounds the pupil. Every time
you pass from light into darkness the same thing can be perceived.
When we turn down the lights in a room, so that we are in comparative
darkness, our pupils gradually expand. As soon as the lights are
turned up again, then our pupils begin to contract. Other animals have
the same contrivance in their eyes. You may notice in the Zoölogical
Gardens how quickly the pupil of the lion contracts when he raises his
eyes to the light. The power of rapidly changing the pupil might be
of service to a beast of prey. Imagine him crouching in a dense shade
to wait for his dinner; then of course the pupil will be large from
deficiency of light; but when he springs out suddenly on his victim, in
bright light, it would surely be of advantage to him to be able at once
to see clearly. Accordingly his pupil adjusts itself to the altered
conditions with a rapidity that might not be necessary for creatures of
less predaceous habits.
These changes of the pupil explain how the telescope aids our eyes
when we want to discern any faint objects, like the little planets.
Such bodies are not visible to the unaided eyes, because our pupils
are not large enough to grasp sufficient light for the purpose. Even
when they are opened to the utmost, we want something that shall enable
them to open wider still. We must therefore borrow assistance from
some device which shall have an effect equivalent to an enlargement
of the pupil beyond the limits that nature has actually assigned to
it. What we want is something like a funnel which shall transform a
large beam of rays into a small one. I may explain what I mean by the
following illustration: Suppose that it is raining heavily, and that
you want to fill a bucket with water. If you merely put the bucket out
in the middle of a field, it will never be filled; but bring it to
where the rain-shoot from a house-top is running down, and then your
bucket will be running over in a few moments. The reason, of course,
is that the broad top of the house has caught a vast number of drops
and brought them together in the narrow shoot, and so the bucket is
filled. In the same way the telescope gathers the rays of light that
fall on the object glass, and condenses them into a small beam which
can enter the eye. We thus have what is nearly equivalent to an eye
with a pupil as big as the object glass. Thus the effect of a grand
telescope amounts to a practical increase of the pupil from the size of
a threepenny-piece up to that of a dinner-plate, or even much larger
still.