EFFECTS OF RADIANT ENERGY ON THE EYE. 707 



almost a tenth of this amount with the aUitude and sun favorable. 

 Assuming now that one quarter of 1% of this quantity, that is 250 

 ergs per square em. per second is within the abiotic region 295 to 305 /x/x 

 it is easy roughly to determine the exposure which is likely to produce 

 snow blindness. We have already seen that a well marked pho- 

 tophthalmia can be set up by a radiation in abiotic rays of about 

 2,000,000 erg seconds per square cm. 



Now assvnning that of the total radiation which would be received 

 direct, half, through direct and reflected action, reaches the eye of 

 one traveling among the high snow fields. The energy in total abiotic 

 radiations would be about 1250 ergs per square cm. per second. If 

 all of this cjuantity had the average abiotic effect on the conjunctiva 

 and the cornea a little less than 27 minutes exposure would be 

 required to make up the 2,000,000 ergs seconds just referred to and 

 to produce symptoms of photophthalmia. As a matter of fact 

 the region from 305 /x/x to 295 fxfj, has much less than the average 

 abiotic effect. Our crown glass screen ^ 7 cuts ofP the ultraviolet 

 at 295 /i/x substantially just at the end of the solar spectrmn. Experi- 

 ments made with this screen on the magnetite arc which is fairly 

 strong from 295 ^i/x to 305 ju^u showed that this screen increased the 

 exposure necessary to produce photophthalmia eighteen times. It 

 therefore appears that at a high altitude in the snow fields an exposure 

 of 7 to 9 hours under extreme conditions would be required to produce 

 photophthalmia as severe as that which we have here recorded as 

 typical, i. e., involving stippling of the corneal epithelium, Clinically 

 snow blindness very rarely reaches this phase, since, although the 

 exposures may be long the intensity of abiotic solar radiations 

 reaching the eye would be seldom as great as the maximum amount 

 just mentioned. For instance Schiess-Gemusens ^^* reports two cases 

 of ordinary snow blindness which fell under his own observation 

 in which the ordinary symptoms occurred after practically all day 

 exposures showing very marked conjuncti^'itis without any visible 

 effect on the cornea. Inasmuch as the exposures in casual climbing 

 on an all day trip are considerably less severe than with steady full 

 exposure to the snow fields, it is fair to assume that this latter condi- 

 tion might produce snow blindness in perhaps half the time previously 

 mentioned. This checks well with our experiment on solar erythema 

 where an exposure of 6 minutes at .5 meter from the magnetite arc 

 unshielded gave a slight l)ut definite erythema of the skin. At sea 

 level and under ordinary circumstances the critical exposure for 

 snow blindness would undoubtedlv run to manv hours. It is well 



