1122 BIOLOGICAL EFFECTS OF RADIATION 



the application of light to the treatment of disease. As previously indi- 

 cated, even prior to the time when definite wave-lengths of light were 

 isolated for the purpose of determining accurately their biological effects, 

 it had become apparent that ultra-violet was an important factor if not 

 a chief factor, in the reported lethal effects of sunlight, and of light from 

 artificial sources as well, on microorganisms. Arsonval (6), Ward 

 (155, 156), and others eventually arrived at this conclusion during the 

 progress of their work. Strebel (146) isolated regions of the spectrum, 

 using cadmium and aluminum spark sources, and showed that strong 

 killing action for bacteria was confined to the ultra-violet. The region 

 of lethal action, so far as studied, was determined by Barnard and 

 Morgan (9) to lie between X3287 and 2265 A. Although, in the literature, 

 the lower wave-length limit of bactericidal action may be thus definitely 



o 



stated, little consideration should be given to this below 2400 or 2300 A 

 in the absence of intensity measurements, owing to the increasing absorp- 

 tion in the instrument, apparatus, and materials. 



As a result of his earlier investigations, Bie (16) had regarded all 

 wave-lengths (infra-red effects were not studied) as inactivating, but 

 shorter wave-lengths were more effective. Subsequently he computed 

 the lethal effects of X2950 to 2000 A to be at least 10 or 12 times more 

 effective than the longer wave-lengths tested, and he may be regarded as 

 having clearly established the dominant action of ultra-violet. These 

 results were obtained with a carbon arc, a quartz monochromator, and 

 the bacteria exposed in a small moist chamber covered with a quartz 

 window. At about the same time, Bang (8), working with B. prodigiosus, 

 reported that in the ultra-violet there exist two bactericidal maxima; 

 first in the region of X3600 to 3400 A and then a second maximum rising 

 sharply at about 3000 A to a higher value and continuing horizontally 

 toward 2000 A. Later (86) he placed the highest lethal action, for 

 wave-lengths shorter than 3000 A, at around X2500 A. An explanation 

 of his results in the region of the longer ultra-violet remains unconfirmed. 

 The maximum at 2500 A has been regarded (Mme. and M. Henri, 73) 

 as indicating greater intensity in that part of the ultra-violet. 



The determination of incident energies in the different spectral regions 

 as applied to biological studies was the advance made by Hertel (75). 

 With a quartz prism and quartz lenses he utilized the spectrum from 

 spark gaps of several metals. Intensity measurements were made with a 

 thermopile and galvanometer. His results were definite in determining 

 that bactericidal action was directly related to energy furnished and 

 absorbed, and within limits inversely related to wave-length. However, 

 he gives data for only six spectral lines, between X4400 and 2100 A. It 

 seems rather remarkable that following this work there appear to be no 

 other studies on bacteria in which intensities were measured for a period 

 of years. 



