522 RADIATION BIOLOGY 



end of the solar spectrum — shorter than 0.32 /x it should not be forgotten 

 that pigment darkening is hiought ai)out by somewhat longer \va\'e 

 lengths extending into the visible at about 0.42 /x. Henschke and 

 Schultze (1939b) showed that when suntan is produced by natural sun- 

 light a good deal of the color may be due to pigment darkening rather than 

 the production of new pigment, because of the high intensity of the wave 

 lengths between 0.32 and 0.42 /x as compared to those shorter than 0.32 /j 

 (see Fig. 13-8). This should be particularly true when the sun is far from 

 zenith as in the later afternoon and early morning, and at noon in late fall 

 or early spring in temperate latitudes. Differences in the pigment- 

 darkening effect may explain why some persons retain a rather dark sun- 

 tan throughout the winter months while others do not. In the early 

 spring, when there is relatively little of the erythemal radiation in sunlight 

 but a considerable amount of the pigment-darkening radiation, darkening 

 of the skin by the latter may be mistaken for new suntan. The belief that 

 this darkening is accompanied by immunity to sunburn may lead to over- 

 exposure when the erythemal radiation increases with the progress of 

 the season. 



FACT AND FANCY 



The convincing fact that one has been severely sunburned Avhen he 

 thought he was adequately protected from exposure looms large against 

 any explanation that may be offered in terms of such a complex of factors 

 as has been discussed. In the past, when still less was known about the 

 subject, it was a natural tendency to invent factors — sometimes with little 

 respect for physics or physiology — which would seem to explain such 

 puzzling occurrences. In a book published in 1905 b}^ Giles it is suggested 

 that there are penetrating "Y" and "Z" rays in sunlight which are 

 analogous to X rays and which account for the alleged injurious effects of 

 tropical sunlight. While we ma}^ smile today at this naive invention, we 

 may wonder how much influence this book had in generating popular 

 fear of tropical sunlight and in establishing such practices as the 

 wearing of red spine pads in the tropics, which continued at least up to the 

 time of World War II.-'- Another wholly uncritical book reflecting 

 this fear was that of Woodruff" (1905), on which Jack London based his 

 diagnosis of his own breakdown during the Cruise of the Snark.-^ I recall 

 too, what difficulty I had only a few years ago in convincing a practitioner 

 of medicine that the "actinic" rays of the sun would not pass through the 

 metal roof of an automobile. Certainly some of the difficulty one encoun- 

 ters in this respect comes from failure to recognize the importance of 

 indirect radiation from the sky. 



2^ An ainiisiiifz; account of field tosts in the Philip)pint>s to dotoiinine the value of 

 oranf!;c-r('(l underwear (IMialen, I'.tlOj shows how .seriou.sly such ideas were taken at 

 one time in some quarters. 



^^ The problem of tropical sunlight has l)eeii discussed elsewhere (Blum, 1945). 



