6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8/ 



case of ordinary glass mercury arc caution should be used to avoid 

 too great exposure as it may produce a lasting injury in the nature 

 of an actual burn on the retina. 



In order to be able to correlate these physical characteristics which 

 we have indicated with the direct observations of biological material, 

 the lower portion has been devoted to characteristics for which data 

 is available. Curve a shows the transmission of flesh, having been 

 corrected for surface absorption. On the long wave-length side un- 

 doubtedly water is most important in setting the limit. On the short 

 wave-length side other constituents of the living matter account for 

 the fact that this transmission drops off rapidly on passing into the 

 visible range. It will be noticed, therefore, that the region of the 

 maximum transmission of flesh occurs roughly in the range emitted 

 from a water-filtered high-temperature light. The near infra-red thus 

 constitutes a region of relatively penetrating radiation for therapeutic 

 purposes. Curve b shows relative visibility of light for the human eye. 

 Curve c shows the relative phototropic response of an oat seedling to 

 light. It will be noticed that it is insensitive to red and a considerable 

 portion of the yellow, the maximum occurring in the blue. Curve (/ 

 shows the absorption band that seems to be correlated with vitamin A. 

 Radiation that is damaging to the vitamin value causes a weakening in 

 this band. Curve e shows the absorption band that seems to be corre- 

 lated with ergosterol and vitamin D. Radiation that seems to produce 

 activation destroys this absorption. Curve /, the full line curve, shows 

 the erythema response of the human skin in the case of a very light 

 erythema (zero degree). Here it will be seen that a minor maximum 

 occurs at .298/* and a great maximum in the region of .253111. In the 

 case of extreme erythema, fourth or fifth degree, the relative effective- 

 ness of these two regions is reversed, the great maximum occurring 

 at .300ja and the smaller maximum at the region of .253;^. It is, 

 however, very significant and perhaps important from a therapeutic 

 standpoint that a minimum of erythema occurs between these two 

 ranges and that this minimum coincides with the chief ergosterol 

 absorption. It may thus be possible to secure a maximum therapeutic 

 dosage with a reduction in resulting erythema by the use of mono- 

 chromatic light in this range. The magnesium spark lines at .280/x are 

 promising for this purpose. 



Another point of interest is that the lethal region for algae occurs 

 in this same range as ergosterol absorption, the threshold for this 

 effect being indicated by the arrow marked D. The solar energy falls 

 oft" rapidly at this jKiint. 



