EFFECTS OF RADIATION ON BACTERIA 1127 



VISIBLE LIGHT 



From the discussion thus far, it would no doubt be confusing to 

 attempt to draw any general conclusion regarding the effect on bacteria 

 of light within the visible spectrum. Speaking generally, most of the 

 earlier observers, using sunlight and filter systems, were confident of the 

 demonstration of lethal action (4, 5, 16, 155). With the utilization of 

 artificial sources of radiation and the greater application in biological 

 work of the quartz spectrograph and quartz monochromator, attention 

 centered upon the more intensive lethal effects in the ultra-violet. In 

 fact, with the safer establishment of the rule of the inverse relation of 

 kilhng to wave-length, within limits, and with the difficulty of obtaining 

 sufficiently high intensities at the longer wave-lengths in time intervals 

 sufficiently short to avoid multiplication by growth, the quantitative 

 studies have given only secondary attention to the visible region, or even 

 to the longer ultra-violet. Moreover, with very high energy levels, an 

 insignificant percentage of ultra-violet below X3000 A might afford the 

 necessary intensity in the more actively lethal region to account for any 

 effects observed. Increased purity, by means of a double monochromator, 

 is secured at the expense of intensity. In considering the earlier work, 

 where relatively intense sunlight was employed, the more recent data 

 with artificial sources presented by Bayne- Jones and Van der Lingen (10), 

 Coblentz and Fulton (29), and Duggar and Hollaender (42) are suggestive 

 of the possibiHty that further quantitative studies in the visible spectrum 

 may give evidence of lethal effects, or at least of other physiological effects 

 of fundamental interest. In this connection it should be noted that 

 absorption of radiation in the visible does occur (see section of this paper 

 on absorption). Further, in studies on the effects of visible light, there 

 should be included an adequate consideration of the possible influence 

 of oxygen, high temperature, and other environmental conditions. 



INFLUENCE OF CONDITIONS ON THE EFFECTIVENESS OF RADIATION 



Temperature. — That bactericidal effects are independent of tempera- 

 ture was reported by Cernovodeanu and Henri (25), but it is not clear 

 that their technique would have detected differences of the order charac- 

 teristic of photochemical or physical processes. In the light of the fact 

 just mentioned, Henri's conclusion that no change occurred in the speed 

 of reaction between 0° and 55°C. should be regarded as tentative. On 

 the other hand, Becquerel (12) reported bactericidal action with anthrax 

 spores in 2 to 3 min. at room temperature, whereas at the temperature of 

 Uquid air an exposure of 6 hr. was required for equivalent action. Unfor- 

 tunately, the conditions do not appear to be adequately stated. The 

 suggestion may be made, however, that at temperatures below freezing, 

 it is logical to assume dehydration of the protoplast, and since low water 



