502 li \l)l \l H)\ HlOLOflV 



III I lie model (icsciilx'd. i ccipiocity (/ X / = a constant) has been 

 assumed : it is specilicnily stated in F>(|. (13-3) and is implicit in Im|. (13-()). 

 I"'xperimentally. recipiocity d<)(\s hold over a wide i:iii«i;e of dosr's when 

 monochiomatic radiation is used to determine the threshold, as has heeii 

 shown by Haiissoi' and \'ahle (1922), C'oblentz el al. (li)32j, and Jilum and 

 Terus ( l!)4()b). On the other hand, when polychromatic radiation is used 

 there may be a wide de\iation tVom i-eci))rocity, as shown by Schall and 

 Alius (192(). 1928a) and by Blum and Terus (194()1)). This would seem 

 to reflect complex time relationships in the mechanism of vascular 

 dilation. 



The existence of more than one factor active in determining the ery- 

 themal spectrum also explains certain of Hausser's findings (1928) that 

 had always remained puzzling. When he studied the erythemal effec- 

 tiveness in terms of the intensity of the erythema, Ilausser found that for 

 shorter wave lengths a proportional increase in dosage produced a rela- 

 tively greater increase in intensity of the erythema than did a comparable 

 increment of the longer wave lengths. This is difficult to luiderstand in 

 terms of a simple model, but is not surprising in a complex system involv- 

 ing processes with different wave-length dependence. 



\'ari()us factors have been reported to affect the erythemal threshold, 

 but when these reports are based on minor differences they should be 

 accepted with caution. Threshold measurements are of limited accuracy 

 at best, and the great individual variation makes statistical treatment 

 uncertain in view of the obvious complexity of the erythemal mechanism. 

 Heat is one of the factors that might be expected to affect the erythemal 

 response, but, although numerous studies have been made, the results are 

 somewhat conflicting. Schall and Alius (1928a) and Clark (1936) both 

 found that the temperature diu'ing exposure to ultra\'iolet radiation had 

 very little effect on the threshold. Clark found, on the other hand, that 

 the rate of appearance of the erythema increased with temperatiu'e. This 

 is explained if we assume that the primary photochemical reaction is, as 

 might be expected, virtually independent of temperature, but that subse- 

 quent parts of the erythemal process are temperature dependent. Fail- 

 ure to separate these two phases of the response may account for some of 

 the apparent disagreement in results obtained by other investigators. 

 Recently Ilelmke (1948-49) reported that infrared radiation lowers the 

 erythemal threshold and shortens the time to appearance of erythema in a 

 majority of indi\'iduals. 



SUNTAN 



Pigment Migration and Formation. Observed grossly, the erythema of 

 sunburn appears to fade almost imperceptibly into svmtan; i.e., there is a 

 gradual color change from red to brown. But this does not represent an 

 immediate relation between these two aspects of sunburn. Whereas the 



