280 The Evolution of Optics. 



the organ to withstand this increased strain. The result is 

 that the eyeball gives way beneath this pressure ; it becomes 

 elongated, and in its position of rest is now adapted for the 

 perception of near objects. It is an undoubted fact that 

 near-sightedness is on the increase among the more civil- 

 ized races. It develops usually in childhood, and statistics 

 show that in school-children the degree of near-sightedness 

 is directly in proportion to the number of hours of study. 



It might at first appear very desirable that the eye should 

 thus adapt itself to the work it has to do ; and so, no doubt, 

 it would be were these near-sighted eyes healthy and perfect 

 organs. But, unfortunately, this is not the case. The devel- 

 opment of near-sightedness is attended with grave dangers. 

 It means a giving away of the firm fibrous capsule of the 

 eye and always endangers the delicate nervous elements be- 

 neath. The near-sighted eye is a diseased eye and is always 

 liable to still further deterioration. The process is distinct- 

 ly hereditary, the children of near-sighted parents being 

 frequently born near-sighted, and when born with normal 

 eyes earlier develop more aggravated cases of near-sighted- 

 ness than the children of healthy parents. Thus Nature's 

 latest attempt to improve our visual organs is of but doubt- 

 ful utility. 



The problems of physiological optics thus far encountered 

 are much less involved than those which arise in connection 

 with the color-sense. Anything approaching a connected 

 and logical statement of this subject would require a much 

 more elaborate treatment than is here possible, and I shall 

 be obliged to omit many facts which I fear are almost essen- 

 tial to a rational presentation of the subject. 



From the time of Thomas Young to a comparatively re- 

 cent period the ability of the retina to differentiate color 

 has been practically unquestioned. Young saw the logical 

 necessity for retinal elements capable of receiving each 

 phase of vibration of a continuous spectrum, but believed it 

 impossible that so many different retinal elements could 

 exist. He therefore assumed arbitrarily the existence of 

 three sets of fibers corresponding to what he called the three 

 primary colors red, green, and violet. Another theory 

 somewhat similar was suggested by Herring, which, in place 

 of retinal elements, assumed the presence of three chemical 

 substances which were supposed to have certain positive and 

 negative reactions to light. Neither of these theories has 

 any discoverable foundation in the anatomical structure of 



