EFFECTS OF RADIATION ON BACTERIA 1121 



series of papers by Ward (155, 156) beginning in 1892. At this time 

 there was very httle careful work on the efficiency of the different spectral 

 regions (cf. Arloing, 4, 5; Janowski, 81; and others). Ward's studies 

 eventually included the effects of radiation from the carbon arc, but 

 his earlier experiments were made with sunlight and the solar spectrum. 

 Having obtained what seemed convincing evidence, on plate cultures 

 with gelatin, that the spores of Bacillus anthracis are killed by the direct 

 action of sunlight and not by increased temperature, likewise that a 

 reasonable time exposure did not spoil the substrate as a food source, 

 he proceeded to do more critical work on light quality. Using both 

 glass and solution filters, spectroscopically tested, he found no inhibition 

 of growth behind screens transmitting red, orange, and yellow. Bacteri- 

 cidal action was marked behind screens transmitting most or all of the 

 blue-violet, whether other wave-lengths were present or were excluded. 

 In view of the doubt still existing in respect to lethal action of ultra- 

 violet radiation longer than about X3100 A, it should be noted that with 

 his unmeasured intensities lethal action through the ordinary glass of a 

 Petri plate was definite in the blue, blue-violet, and beyond. Even 

 though the intensities in these ranges may have been considerable, the 

 results are not easily explained. 



Ward's best technique, like that of Buchner (20), was to distribute 

 the bacteria in the melted agar, pour the plate, and subsequently expose 

 the closed dish covered with black paper except for a test circle or 

 stenciled letter. Colony growth served to indicate the extent of the 

 lethal effect. In his later experiments quartz windows were introduced, 

 the spectrum was "photographed" on the plate through the relative 

 abundance of the colonies appearing, and with an uncovered electric 

 arc light the ultra-violet region was found particularly effective as a 

 lethal agent. 



During the earlier periods of these sunlight studies, as also of light 

 from artificial sources, no attention was paid to the measurement of 

 light intensities. In view of this circumstance, and further, since so 

 many well known factors involving time, place, climate, and local 

 atmospheric conditions influence intensity (cf. Brackett, Paper IV of 

 this book), no particular interest attaches to the details of these earlier 

 or of later qualitative experiments aside from the part they have played 

 in the development of this subject, 



ULTRA-VIOLET 



Earlier Studies. — Notable impetus was given to heliotherapy and to 

 investigations dealing with the effects of radiation, especially of ultra- 

 violet, by the establishment at Copenhagen in 1896 of the Finsen Institute 

 (Finsens Medicinske Lysinstitut). The establishment of this institute 

 is to be attributed to the recognition of the work of Niels R. Finsen in 



