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before you have time to notice it, you feel a little change take place
inside your eyes, and the print upon the page of your book becomes
quite plain. This is because your eye has to change the shape of one
of the parts inside it, called the _lens_, before you can see clearly
the things that are near you. This change, which is called
_accommodation_, is made by a little muscle of the eye; and if you
keep your eyes working at close work, like reading or writing or
fancy-work, too long at a time, or if your eyes need glasses to make
them see clearly, and you haven't them on, this little muscle becomes
tired. Then the print of your book, or your writing, or the stitches
you have taken begin to blur before your eyes. Your eyes begin to feel
tired, and your head begins to ache. This is what we call _eye
strain_.

Sometimes this eye strain upsets your appetite or your digestion and
makes you sleepless and worried. The trouble may be caused by your own
carelessness: you may have been reading too long, or in a poor light,
or with the light shining right in your face instead of coming over
your shoulder. But sometimes it is caused by the fact that your eyes
are not just the right shape; and then the only way to relieve it is
to have proper glasses, or spectacles, fitted, which will make up for
this too flat or too round shape, or too large or too small size, of
your eyes.

If you cannot see clearly what is written on the blackboard when the
light falls upon it from behind you, or above; or if, in a good light,
you cannot read the words in your book quite easily, without straining
at all, when you hold the book either at arm's length or a foot from
your face; or if your head aches or your eyes begin to feel tired or
uncomfortable, or the letters begin to blur, after you have read
steadily--say, for half an hour,--it is a pretty sure sign that there
is some trouble with your eyes. Then you had better have them examined
at once by your family doctor or by the school doctor. In many schools
now there are doctors to test the children's eyes, and ears, too, so
that each child may have a chance to see and hear everything that the
other children can see and hear.

Not very many years ago people thought that glasses were only for old
people, but now we know that many children's eyes need glasses, too. I
knew a little girl whose sight was so poor that when she was standing
and looked down at the grass, she couldn't see the green bl