Page:Popular Science Monthly Volume 67.djvu/743

Rh the variations, only one two-hundredth or one four-hundredth of an inch from the normal in the dimensions of the eyeball or in its corneal curves which may prevent the accurate photographic 'definition' of the retinal image. To this must be added a knowledge of the means or lack of means possessed by the eye to overcome or neutralize these results of deviations from the standard of size and contour. What are the kinds of deviations that may be cancelled, how far the neutralizing is possible, what kinds may not be overcome, and the mechanism of the overcoming, these are all pretty well understood.

In the briefest way we may say that if an eyeball is too long (from the cornea to the retina) it is near-sighted or myopic. One can not see well at a distance, for everything appears blurred and hazy. The focus of the image is in front of the retina, and there is no device of the unaided eye which can transpose it to its proper position upon the retina. Any effort to do so is by the nature of the conditions a negative one, an endeavor, if one may so speak, and of course, unconsciously, to lessen muscular effort. There is as result no eyestrain, no morbid, or wearying, or hurtful attempt at muscular exertion.

The supposition, in this case, is that a pure or uncomplicated myopia exists, and that it is of the same degree or amount in both eyes. In truth, however, that supposed condition never exists. No oculist probably ever tested a pair of eyes having no astigmatism, and having exactly the same over-length, or myopia. And this astigmatism, or the difference in lengths of the two eyeballs, or both factors combined, brings always the possibility of 'eyestrain.' For eyestrain is the name given to any unphysiologic, i. e., pathologic ocular action or function which is wearying, excessive or unnatural. It thus becomes clear how it is that the two eyes by reason of the presence of a difference in their relative lengths, or because of astigmatism in one or both, may result in morbid effort or strain, although if both were alike in over-length, and without other optical defect, there could be no such strain. This general fact makes evident the truth that in general those with (moderately) myopic eyes have far less ocular diseases and pains in the eyes, less headaches and other general disorders, than those whose eyes are hyperopic, or 'far-sighted.' For in all 'far-sighted' eyes there is a never-to-be-renounced effort to overcome the trouble. But this freedom from pain and other symptoms in myopia may lull the patient with a false security and costly neglect. The great danger is that without the attention of a skilled specialist the myopia may increase, become 'malignant,' as it is termed, and the eyeball continue to elongate, with imperilled or fatal loss of vision. The myopic eye is one abnormally elongated, enlarged or stretched, and once having lost its tonicity or normal measurement, it tends to extremes of enlarging.

The far-sighted or hyperopic eye is the reverse of the myopic eye: