SUNBURN 513 



for graphical integration when deaHng with a spectrally continuous source 

 such as sunlight within the limits u, v, of the erythemal spectrum 



/-i 1 /"" Dhc d\ ,,^ ,^, 



^ = '"" ].. [e-Hl - .-~'-)gl7X - ('^-'8' 



We may WTite the corresponding eciuation from Eq. (13-lG), when the 

 preventive is in place, and describe the ratio of thresholds with and with- 

 out the preventive as follows: 



, f" DhcdX 



he 



Qr ['- DhcdX ■ ^ ^ ^ 



Quantities which could be canceled out in obtaining Eq. (13-18) now lie 

 within the integral and cannot be canceled. Thus there must be uncer- 

 tainty involved in the use of the ratio P as an index of the protection 

 afforded against sunburn by sunlight or any other polychromatic source. 

 This might account for a considerable amount of the variation found 

 experimentally in the values of P obtained for different persons. The 

 systematic variation with individual threshold, which is illustrated in 

 Fig. 13-10, could be accounted for if we invoke the variation of D with 

 wave length, for which the evidence has been discussed. If it is assumed 

 that with no preventive the threshold is determined principally by the 

 amount of absorption in the corneum, it may be assumed that in persons 

 with high threshold a smaller proportion of the incident longer wave 

 lengths of the erythemal spectrum penetrate to the vessels of the dermis 

 where such radiation inhibits the dilation of the minute vessels. Thus the 

 inhibitory effect should be less in the person with a thick corneum who will 

 also have a high threshold, tending to decrease the difference between the 

 erythemal threshold of high- and low-threshold persons. The same thick- 

 ness of a given preventive should diminish the intensity of the radiation 

 reaching the dermis in the same proportion in the case of the high- and the 

 low-threshold persons, but would have a greater absolute effect on the 

 latter than the former. This could account for the lower values of P 

 found for high-threshold persons, which is illustrated in Fig. 13-10. On 

 the other hand, if the threshold is considerably affected by the amount of 

 dilator substance formed in the corneum, the picture may be changed 

 somewhat. 



The thing of practical importance is that the amount of protection 

 measured by P varies with the threshold. Radiation from a carbon arc 

 was used in making the measurements shown in Fig. 13-10, but the same 

 systematic variation was found when similar measurements were made 



