386 ANNUAL EEPOKT SMITHSONIAN INSTITUTION, 1932 



of a small area of the sky is much lower than direct simlijrht, since 

 the solid angle subtended by the whole visible sky is 92,000 times 

 that subtended by the sun, the intesrrated amount of ultra-violet 

 from the whole sky is very appreciable. 



The sun, with its accompanying factors of environment, can hardly 

 be compared to artificial sources of light. The exact physiologic 

 effects of light or of the air bath alone are not clearly understood, 

 nor is the effect of light on single cells. In application, dosage has 

 been difficult to control, and marked variation in the effects comes 

 from a small stimulative or a larger destructive dose of light. Simi- 

 larly, the technic of application with most workers has been different. 

 Published experiments lack specific details in many instances, espe- 

 cially those pertaining to the spectrum, such as its limits and the dis- 

 tribution and the character of the radiant energy employed. These 

 must be defined accurately instead of attributing results merely to 

 " ultra-violet energy." Perhaps this is the cause of the contradictory 

 nature of many of the results published. 



Many claims made for the use of radiation are still based on empiric 

 results, and practical applications have been made without scientific 

 support, chiefly because the action of radiation on the living cell is 

 not clearly understood and the fundamental principles of the bio- 

 physics and physiology of radiant energy are still unsolved. Per- 

 haps when monochromatic sources of ultra-violet energy in sufficient 

 intensity have become available to repeat experiments, more exact 

 information will be obtained. To name two specific reactions of 

 exact wave lengths, one may cite the ordinary erythema or sunburn 

 production and the direct production of antirachitic effect. Aside 

 from these, despite claims, the exact wave lengths alleged to increase 

 hemoglobin, prevent or cure the common cold, and cure forms of 

 extrapulmonary tuberculosis, or even superficial ulcers, have not been 

 established. 



So it is evident that confusion must still exist. Controversies 

 constantly take place between the proponents of the use of simlight 

 and those of artificial sources. The value of sunlight for one form of 

 disease against another, for instance pulmonary tuberculosis as 

 against the extrapulmonary forms, is a subject for debate. The ad- 

 vantages of different artificial sources of energy are still open ques- 

 tions. The workers in high altitudes are still enthusiastic in ex- 

 pounding their clinical results in contrast to those in the lowlands. 

 This difference of opinion appears in part due to the fact that, in 

 the development of the use of light for disease, only empiric results 

 were known for many years before accepted laboratory evidence was 

 produced which placed light therapy on a seientific basis. The broad 

 scope, therefore, of this whole field will allow me only brief reference 

 to such fundamental facts. 



