3. The illumination should always be sufficient. Nothing is more
injurious to the eyes than reading by a poor light. Many persons
strain their eyes by reading on into the twilight as long as they
possibly can. They become interested and do not like to leave off.
Some read in the evening at too great a distance from the source of
light, forgetting that the quantity of light diminishes as the square
of the distance from the source of light increases. Thus, at four
feet, one gets only one-sixteenth part of the light upon his page that
he would at one foot. It is the duty of parents and others who have
charge of children to see to it that they do not injure their eyes by
reading by insufficient light, either daylight or artificial light.
There is a common notion that electric light is bad for the eyes. The
only foundation I can think of for such a notion is that it is trying
to the eyes to gaze directly at the bright electric light. It is bad
to gaze long at any source of light, and the brighter the source of
light gazed at, the worse for the eyes, the sun being the worst of
all. I have seen more than one person whose eyes were permanently
injured by gazing at the sun, during an eclipse or otherwise. As a
matter of fact, nothing short of sunlight is better than the
incandescent electric light to read by or to work by.