Page:Popular Science Monthly Volume 67.djvu/749

Rh contraction of the ciliary muscle, can never be renounced. This constitutes the predominant source of eyestrain.

There are several other misfortunes or imperfections of the accommodation mechanism which may not be neglected if we wish to understand the matter in all its bearings. The crystalline lens must of course be transparent, hence it can not be nourished directly by the blood with its red corpuscles. Its healthful action is, therefore, dependent upon nourishment by blood-serum alone—plainly a difficult task, especially as this serum must reach it indirectly by osmosis, filtration, etc. It has also no nervous connection with the brain, and the three conditions named conspire to bring about two most noteworthy faults in its life-history. It is prone to become non-transparent or opalescent and, finally, almost opaque in the old, and this is cataract. Its elasticity also decreases steadily from childhood until it is so inelastic at about 45 years of age that the 'range' or degree of accommodation becomes too limited to enable it to focus the images of objects clearly on the retina except by holding the book, for instance, too far from the eye. This is the beginning of 'presbyopia'; at about sixty all the elasticity of the lens is lost and accommodation is at an end. Moreover, oculists have been hitherto unmindful of the fact that the accommodation may be less than normal in many young patients, even for short tasks. It is always so for long and continuous ones. For presbyopia, there is no prevention and no cure. There is a makeshift device (spectacles) whereby we may supply the lost focusing power of the living lens, by glass lenses placed in front of the crippled 'crystalline lens.' Of cataract, however, there is a pretty sure method of prevention, and this, again, is spectacles.

The science and art of correcting or neutralizing these optical defects of the eyes—myopia, hyperopia and astigmatism—is by means of artificial optical lenses. First, be it noted, it is a medical art and science, which no optician can compass. He has neither the legal nor the ethical right to attempt it, and surely he has not the scientific and medical knowledge requisite for its accomplishing. However poorly the medical man has executed his task, the optician will do it far more blunderingly. This verbum sapienti should be sufficient warning until, as with the druggist, we have also with the optician, passed laws to prevent him from attempting to fill the office of physician. There will then not be so many ruined eyes, and far less suffering from eyestrain.

Spectacle lenses have the power of changing the shape and direction of the image-forming cone or bundle of rays of light entering the eye so that its faulty optical construction and powers are neutralized, and the image is at last accurately focussed,focussed [sic], and perfectly pictures the object. The outside lenses are in a way reversely unnatural, so that the inner eye-defect is met with an outer cancelling modification,