SITNBURISr 499 



"standard" erythemal spectrum in the first place, but would not account 

 for the discrepancies when different spectral sources were used. ^I'lic 

 source of these discrepancies must be sought elsewhere. 



In all this discussion it is tacitly understood that the threshold is deter- 

 mined by the elaboration of a given amount, D^, of the dilator substance, 

 which presumably brings about a minimal detectable dilation of the 

 minute vessels. If this were true at the threshold condition, 



D, = D, + Do + • • • + Di. (13-14) 



This assumption seems obvious enough since it would not be anticipated, 

 on first consideration, that the amount of response of the vessels to a given 



Wi ^ m. 



■^ ^ U 



Fig. 13-5. Diagram showing the result of exposure to a series of doses of "long-wave- 

 length" (filtered through pyrex) mercury arc radiation as follows: 



1: 2.4 ergs cm~^ X 10* of "erythemal effective energy" 



2: 2.7 ergs cm~- X 10* of "erythemal effective energy" 



3: 3.4 ergs cm"^ X 10* of "erythemal effective energy" 



4: 5.3 ergs cm"^ X 10* of "erythemal effective energy" 



5: 8.0 ergs cm~^ X 10* of "erythemal effective energy" 



6: 12.0 ergs cm"^^ y^ jg* of "erythemal effective energy" 



7: 18.1 ergs cm^^ X 10* of "erythemal effective energy" 



8: 26.G ergs cm^^ X 10* of "erythemal effective energy" 



9: 40.0 ergs cm"^ X 10^ of "erythemal effective energy" 



The amount of shading indicates the degree of erythema 2 hr after the exposure. 

 Note that the erythema is less pronounced for the highest doses than for some of the 

 intermediate doses. {After Blum and Terus, 1946a.) 



amount of dilator substance varies with the wave length. However, it is 

 apparently necessary to assume the latter in order to account for the 

 discrepancies in Table 13-1. 



Blum and Terus (1946a) offered an explanation of this discrepancy 

 based on their finding that large doses of the longer wave lengths of the 

 erythemal spectrum could cause inhibition of the erythema brought about 

 by these or shorter wave lengths. Figure 13-5 indicates diagrammatically 

 the various degrees of erythema 2 hr after exposure to a graded series of 

 doses of ultraviolet radiation including both the longer and shorter wave 

 lengths of the erythemal spectrum. The degree of erythema increases 

 with increasing dose up to a certain point and then falls off as higher doses 

 are given. This apparent optimum was not observed when onl}^ shorter 

 wave lengths of the erythemal spectrum were used (i.e., 0.2537 n). The 



