9 o GROWTH 



rickets is by no means closed. In the rat, for example, it seems 

 that rickets may be prevented, if both vitamin D and ultra- 

 violet rays are absent, by a correct ratio of mineral elements. 

 Zucker, Johnson, and Barnett 36 have stated that by making the 

 diet acid producing, there was protection against rickets. 



77. Sunlight and Nutrition 



It has been generally believed for years that sunlight has a 

 favorable influence on the animal organism, and apparently 

 most of our earlier ideas were advanced by clinicians. No sys- 

 tematic effort to study the role of the sun's rays, however, was 

 made until comparatively recent times, about thirty years ago. 

 Since then a voluminous literature has been accumulated and the 

 subject has been studied from various viewpoints, but the mode 

 of reaction of the living cell to light is as yet almost entirely 

 unsolved. 



In the earlier work on this topic there was much conflicting 

 evidence, for though the rays of the sun seemed in some cases 

 to be very favorable, there were others in which no such action 

 was observed, and in fact darkness seemed to have no untoward 

 results. For example, Blessing 37 made a report from observa- 

 tions on the Fram when the ship was used by Nansen on an 

 arctic exploration. The party did not develop anemia or suffer 

 any other bad results. 



Among the earlier reports on the action of light, the one by 

 Finsen 38 is by far the most important. In 1896 he founded a 

 Light Institute at Copenhagen, and that organization has made 

 notable contributions to the subject. Their chief contribution 

 from a practical standpoint has been the successful treatment 

 of lupus by heliotherapy. 



So far as man is concerned, the visible rays of the spectrum 

 lie between 760 and 380 /*ja, though the limits are somewhat 

 variable. Rays of a wave length between 380 and 295 /xfi are 



