falls from behind you or from one side. If not, by turning a little in
your seat, you can get the light from over your shoulder.

Notice how the light falls upon the blackboard. When the light comes
from the windows behind you, or from one side, you can see what is
written there quite plainly. But if the blackboard happens to be
between two windows, and especially if this is the lightest side of
the room, you will find that the light dazzles you so that you cannot
see the writing clearly.

You must have noticed, too, that if, after you have been reading from
the blackboard you look down again suddenly to the page of your book,
for an instant you will not see the letters plainly. Then, almost
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,