CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 659 



will be sharp without changing the position of the screen. And 

 this is now known to be what happens in the accommodation of 

 the eyes in the higher animals and in man. Furthermore, it 

 has been found that for near objects there must be a muscular 

 effort to make the crystalline more convex, while if the object is 

 distant, the eye forms a perfect image without effort. 



REFRACTIVE EYE DEFECTS 



923. For a person with normal eyes it is almost impossible 

 to understand the difficulties under which one labors if the eyes are 

 defective. The difficulties become especially trying for those who 

 must do close work in the trades or in school work, and in exacting 

 professional work. 



From the examination of tens of thousands of school children 

 in our own and other lands it is found that over 10% of them have 

 eye defects of some kind. And in a careful examination of 5,000 

 college students 45% to 50% had ocular defects of a kind that 

 made the use of spectacles desirable, and for many of them abso- 

 lutely necessary. 



It is believed that if those with normal sight had anything like a 

 proper realization of the difficulties of those with eye defects every 

 effort would be made to give relief. 



It is hoped that these demonstrations, which are so easily made 

 and show so strikingly the simpler refractive eye defects, will be of 

 service in helping to give an understanding of the facts and the 

 means for relief. 



Great care has been exercised in selecting demonstrations which 

 shall show the common defects, and those of moderate severity, 

 not the unusually severe or rare. From the personal experience of 

 the senior author, it is known that the appearances shown for 

 presbyopia are not exaggerated; and friends with the other eye 

 defects have assured us that the appearances given in the demon- 

 strations are not uncommon. 



923a. For a discussion of the eye defects in school children see : Hermann 

 Cohn, Die Sehleistungen von 50,000 Breslauer Schulkindern, 1899; Dr. M. 

 Dresbach, Examinations of the Eyes of College Students, The Medical Record, 

 Aug. 3, 1912, also in the Educational Review, Dec., 1913. In Dr. Dresbach's 

 papers are many references to the work of others. 



