LIGHT 



127 



the flight of a group of birds gi\"es warning 

 to a fox or coyote. It is a far cry from such 

 reactions to patterns of light as a sign of 

 unseen things to the gleaning of ideas from 

 a printed page, yet the two reactions are 

 not altogether dissimilar. 



ULTRAVIOLET RADIATION 



Even under optimum conditions, the at- 

 mosphere is opaque to ultraviolet rays 

 shorter than about 2900 A. Hence we are 

 concerned here with the ecology of the 

 small fraction of radiant energy that lies 

 between the shortest visible wavelengths of 



and the situation in Chicago is shown in 

 Figure 24. The floor of the beech-maple 

 forest under the full canopy of summer re- 

 ceives less than 1 per cent of the ultraviolet 

 radiation to be found in direct sunlight 

 (Strohecker, 1938). 



The lethal action of ultraviolet radiation 

 from the sun is important. The bactericidal 

 effect begins at about 3650 A and is 

 stronger in the shorter wavelengths. The 

 energy necessary to kill bacteria at wave- 

 lengths of 3650 A is 10,000 times that 

 needed to kill at wavelengths shorter than 

 3000 A. Enzyme action may be destroyed 

 by 3300 A (lipase) or shorter wavelengths. 



O 



< 



o 



_l 



7MG.(H0URLY) 



6MG. 



AVERAGE PER HOUR 



O 



u 



y 5MG. 



O 



< 4MG. 



O MINIMAL ERYTHEMA DOS 



< 3MG. 



2MG. 



I MG. 



CLEAR POINTS 



NORTH DISTRICT 



SOUTH DISTRICT 



WEST DISTRICT 



LOOP STREET LEVEL 



98% 



84% 



70% 



5 6% 



42% 



28% 



14% 



1928 



NOV. DEC. 



FEB. 



MAR. 



1929 

 APR. 



JUNE JULY AUG. 



Fig. 24. Average intensities of ultraviolet radiation in Chicago between 11 A.M. and 3 

 (Redrawn from Tonney and DeYoung. ) 



Z 



UJ 



U 



cr 



UJ 



Q. 



P.M. 



violet light (3900 A) and the limit of at- 

 mospheric transmission. This band contains 

 about 1 to 5 per cent of the total radiation 

 from the sun that is received at the suiface 

 of the earth. Often the atmospheric cut- 

 off comes at longer wavelengths; when the 

 December sun in Chicago stands about 20 

 degrees above the horizon, transmission 

 stops at 3050 to 3100 A. The smoke pall 

 that hangs over many cities acts as do the 

 forest leaves in nature to eliminate much 

 of the ultraviolet that would otherwise be 

 received at ground level. Baltimore is esti- 

 mated to lose half of its potential supply. 



Bacteria, fimgi, nematode eggs, and viruses 

 can be inactivated by radiation between 

 3400 and 4400 A, a region in which sun- 

 hght is intense. This lethal action of visible 

 light, as well as the similar effect of ultra- 

 violet radiation, is an important factor in 

 the low survival of infective agents in 

 nature. The mechanism of inactivation dif- 

 fers; the bactericidal influence of white or 

 blue light depends on the presence of 

 oxygen, while ultraviolet kills Staphylococ- 

 cus albiis equally effectively in air and in 

 high vacua (Duggar, 1936; Buchbinder, 

 1942; HoUaender, 1942). 



