714 VERHOEFF AND BELL. 



seasons of the year, usually disappearing during the winter months. 

 For this reason sunlight has been suggested as the etiological factor 

 in the disease. 



This hypothesis was first advanced by Schiele ^^^ and advocated 

 by Kreibich ^^^ who showed that an occlusive bandage had a favorable 

 effect upon the symptoms. This effect, however, may be explained 

 in other ways than by the shutting out of light. Birch-Hirschfeld ^^ 

 repeatedly exposed the conjunctiva of a rabbit within a period of 

 18 months to the "Uviol lampe" of Schott and obtained changes 

 stated to be not unlike those of vernal catarrh. He does not, however, 

 accept the view that the latter is due to ultra violet light. No doubt 

 similar changes could be produced by other irritants frequently 

 applied. 



The evidence for the view that vernal catarrh is due to the action 

 of sunlight, therefore amounts to little more than the fact that the 

 symptoms are most pronounced in the spring and summer. This 

 fact, however, is accounted for even better on the more recent theory 

 that the disease is due to pollen. Moreover the following objections, 

 that to us seem insurmountable, may be urged against sunlight as a 

 cause. In the first place if vernal catarrh is due to sunlight the lower 

 lid, which is not only more exposed, but thinner and more trans- 

 parent, should be more affected than the upper lid, whereas, as a 

 matter of fact, it entirely escapes involvement. In this connection 

 it may be noted that in cases of trachoma, a somewhat similar disease, 

 the lower lid also usually escapes and here the disease is undoubtedly 

 due to some infectious agent. Similarly this theory is inconsistent 

 with the fact that the bulbar conjunctiva, which is directly exposed 

 to tiie light, is but seldom affected, and almost never affected in associ- 

 ation with the palpebral form of the disease. 



Finally, the possibility of abiotic action is ruled out by the fact 

 that it is impossible for abiotic waves to pass through the entire 

 thickness of the lid, if only on account of its rich vascularization. 

 This objection does not apply to possible heat effects produced by 

 visible or infrared rays, but in this case it would be necessary to 

 assume exposure of the eyelid to direct sunlight for considerable 

 periods of time as well as special sensitiveness of the conjunctiva to 

 heat, neither of which conditions seems possible. 



