700 VERHOEFF AND BELL. 



But substantially the whole of the solar spectrum which is capable 

 of producing abiotic action lies between 295 ^t^t and 305 ixjx, is evanescent 

 under most conditions, and only possesses pathological significance at 

 high altitudes and especially in extreme cold. Thei'e is good reason 

 to believe that the atmosphere is considerably more permeable to 

 ultra violet radiations at low temperatures than under ordinary con- 

 ditions, particularly as regards the extreme radiations. Figure 5 

 shows from the data of Abbot the distribution of energy in the so- 

 lar spectrum in curve (a) at Mt. Whitney for a zenith distance of 0°, in 

 curve (b) also at Mt. Whitney (14,000 ft.) but for zenith distance 60°. 

 Near the latter limit lies the general range of solar radiation as ob- 

 served at the surface. Two things in these curves are particularly 

 noteworthy, first that in both and especially at the higher altitude 

 the maximum radiation and indeed the bulk of the radiation in 

 general lies within the visible spectrum. Second, the maximum 

 energy lies not in the red, but in the case of the high altitude energy 

 fairly in the blue at about wave length 470 ixix and at the lower altitude 

 in the green at wave length about 500 /x/x. So far as the solar spec- 

 trum is concerned, therefore, the heat energy is chiefly within the 

 visible spectrum. No distinction therefore can be drawn between the 

 visible and the infra red spectrum on the ground of heat radiation and 

 all attempts to separate thermic effects by cutting out the visible 

 spectrum are therefore futile. So long as this reaches the eye it carries 

 with it the solar heat in its greatest intensity. From the area of the 

 curves here shown it appears that of the energy at high altitudes only 

 a very small proportion, of the order of magnitude of one quarter of 

 1% lies within the region 295 to 305 ^ipt. Even this small quantity is 

 evanescent at the sea level and at ordinary temperatures. It is to the 

 small remaining trace of abiotic rays here noted that the phenomena 

 of snow blindness are due. From the clinical standpoint snow blind- 

 ness is found to occur only as a photophthalmia of relatively very mild 

 degree and under exposures usually for a long period and either at 

 very high altitudes or very low temperatures or with both these condi- 

 tions concurring. On snow fields the exposure of the eye to solar 

 radiation, ordinarily greatly ameliorated by the obliquity of the inci- 

 dence, is rendered much more severe by the reflection from the snow 

 which is a good reflector down to the extreme ultra violet of the solar 

 spectrum. One would not go far wrong in estimating that the 

 radiation reaching the eye under such circumstances is of the order 

 of magnitude of a million ergs per square cm. per second. A single 

 square meter of snow at 2 meters distance would reflect to the eye 



