Different Kinds of Strain 111
"Did you have an ice-cream soda?"
"No," said the child; but the telltale shadow moved in a direction opposite to that of the mirror, showing that she had become myopic and was not telling the truth.
The child blushed when I told her this and acknowl edged that the retinoscope was right; for she had heard of the ways of the uncanny instrument before and did not know what else it might do to her if she said any thing more that was not true.
So sensitive is this test that if the subject, whether his vision is ordinarily normal or not, pronounces the initials of his name correctly while looking at a blank surface without trying to see, there will be no error of refrac tion; but if he miscalls one initial, even without any consciousness of effort, and with full knowledge that he is deceiving no one, myopia will be produced.
Mental strain may produce many different kinds of eyestrain. According to the statement of most author ities there is only one kind of eyestrain," an indefinite thing resulting from so-called over-use of the eyes, or an effort to overcome a wrong shape of the eyeball. It can be demonstrated, however, that there is not only a differ ent strain for each different error of refraction, but a different strain for most abnormal conditions of the eye. The strain that produces an error of refraction is not the same as the strain that produces a squint, or a cataract,1 or glaucoma,2 or amblyopia.3 or inflammation of the con junctiva4 or of the margin of the lids, or disease of the optic nerve or retina. All these conditions may exist
1 An opacity of the lens.
2 A condition in which the eyeball becomes abnormally hard.
8 A condition in which there is a decline of vision without apparent cause. 4 A membrane covering the inner surface of the eyelid and the visible part of the white of the eye.