154 On Light in some of its Relations to Disease. 



without its compensating shadow. But to be more specific, 

 and to come as directly as possible to the subject of dis- 

 cussion to-night, let me say at once that I propose only to 

 consider briefly the important role which light plays in 

 producing or in predisposing to certain classes of disease, 

 as it passes through the transparent media of the human 

 eye. 



It will be necessary, before advancing to the main dis- 

 cussion of such a subject, to review briefly some elementary 

 principles which are well established in regard to the 

 classes of troubles which will engage our attention, as well 

 as to glance hastily at some points in the physiology of 

 vision. 



We frequently meet with affections of the conducting 

 nerves in which pains, convulsions, loss of sensibility, or 

 other unnatural phenomena exist, but in which the patho- 

 logist, when he examines the affected nerves, or their sur- 

 roundings, or even the more central parts from which 

 these nerves are derived, finds either no appearance of 

 morbid change, or, if any such changes are found, they 

 are not constant and cannot be regarded as characteristic. 

 Such aflfections are called functional nervous affections, or 

 neuroses. Among them are neuralgia, the more common 

 forms of headache, epilepsy, chorea, more commonly 

 called St. Yitus's dance, hysteria, and insanity. Patholo- 

 gists have exercised great patience and industry in vain 

 endeavors to find some constant organic condition which 

 should lead to a better understanding of some one or all 

 of these distressing and often alarming affections. 



Failing in this, many able and earnest observers have 

 devoted themselves to the careful study of the history and 

 attending phenomena of these neurosefe, with the result of 

 collecting and arranging a great many most valuable facts. 

 Among the most important and most interesting facts re- 

 lating to the cause of these neuroses are those which show 



