﻿BOTANICAL GAZETTE 



relative intensity has been approximated from their effect on a photo- 

 graphic plate. It is known that the spectrum from a hydrogen 

 discharge tube contains a number of bright lines in the neighbor- 

 hood of wave length 1600 Angstrom units, while if the hydrogen is 

 extremely pure there are no lines between wave lengths 2000 and 

 1675 Angstrom units. In this study the destructive action of light 

 including the wave lengths in the region of 1600 Angstrom units has 

 been compared with the destructive action of light from which these 

 waves have been filtered out by means of screens. The hydrogen 

 discharge tube previously described was 

 used as a source of light. Three 

 different methods were used. 



In the first method the hydrogen 

 discharge tube was placed upright with 

 the fluorite window above. The rock 

 salt screen A (fig. 3) was laid upon 

 the fluorite window. Glass plates were 

 coated with nutrient agar and set aside 

 in a sterile closet until the agar became 

 air dry. Spores of Penicilliun were 

 placed upon the agar surface, and the 

 plate C placed, spores downward, on 

 the ring support B. The lower surface 

 of the plate was about 0.05 mm. from 

 the salt screen. After the exposure, the 

 plates were placed in Petri dishes lined with damp filter paper 

 and the Petri dishes set in the incubator. The agar absorbed water 

 and the uninjured spores germinated. 



Exposures of various lengths were made, and the shortest expo- 

 sure which would kill determined. The amount of current flowing 

 through the discharge tube was measured and kept constant during 

 all the experiments. The rock salt screen cut out the light of wave 

 lengths shorter than 1800 Angstrom units (26). Control experi- 

 ments were made by removing the rock salt screen and laying in its 

 place a screen of fluorite having the same thickness. The fluorite 

 was transparent to waves longer than 1250 Angstrom units. By 

 using the fluorite screen the distance between the spores and the 



Fig. 3— Arrangement of dis- 

 charge tube for studying the 

 relation between wave length 

 and the destructive action of 

 Schumann rays: A, rock salt 

 screen; B, brass ring; C, cover 



