The Eye.
What is that part of the eye called, which is blue, gray, or brown, in different persons?
It is called the iris.
What is the pupil of the eye?
The pupil is the black spot or hole in the centre of the iris.
Fig. 11.
What is the use of the pupil?
The pupil is the window of the eye, and the rays of light which enter the eye must pass through it.
What power has the iris over the pupil?
The iris has the power of making the pupil larger or smaller, according to the amount of light which it receives.
How does a bright light affect the iris?
It causes the iris to contract, so that the pupil becomes smaller.
How does a faint light affect the iris?
It causes the iris to dilate, so that the pupil becomes larger.
Of what advantage is the iris to the eye?
The iris acts like a sentinel to protect the eye from any sudden light.
How does a sudden light affect the eye?
A sudden light causes pain to the eye.
Of what shape is the pupil of the eye?
In man, the pupil of the eye is circular.
What is the size of the pupil of the eye?
The pupil varies from the one-twentieth to the one-third of an inch in diameter, depending upon the brightness of the light entering the eye.
How else are our eyes protected from any sudden light?
By means of our eyelids, which may be closed, and thus shut out all the light.
When we pass from a well-lighted room into the open air, at night, why does it seem darker at first than it does afterwards?
Because the pupil is contracted at first; but it soon dilates, and allows more rays of light to pass into the eye, so that the night seems less dark.
Why does the pupil become larger in the twilight?
So that more rays of light may enter the eye, whereby objects may be more distinctly seen.
Why can an owl or a bat see at night?
Because the pupil of the eye in them is quite large, and admits much light.
Why do they stay in dark places through the day?
Because the bright light of day hurts their eyes.
May the image of an object be retained in the eye after the eyelids are closed?
It may for a very short time, as any one can prove by first looking at some bright object, and then closing the eyelids.
Why does a burning coal moved rapidly around, seem like a circle of fire?
Because the light from it is retained a short time by the eye, thus seeming to form a complete circle.
Why cannot we count the posts in a fence, when riding rapidly in a car?
Because the light from each post falls in such quick succession upon the eye, that it cannot distinguish one post from another.
Can objects be seen distinctly when placed near the eye?
No; when objects are within six inches of the eye they cannot be seen distinctly.
Can objects be seen when at a great distance from the eye?
Yes; objects may be seen even when millions of miles away from us; thus, the sun is seen although ninety-five millions of miles from the earth, and some of the stars which we behold in the heavens, are still farther away than the sun.
Of what use is the eye to us?
The eye gives us ideas of the size, the shape, the color, the place, the distance, and the movements of things around us, so that we can use them the better to promote our comfort and our happiness.
What may we remember about the light?
And God said, “Let there be light;” and there was light. Thus the day was separated from the night, and the sun was made to send forth his silvery rays upon hill and valley, field and forest, causing the rarest buds and most beautiful blossoms to come forth from the lifeless earth, although no eye, save the All-Seeing One, was there to behold them!
