﻿BOTANICAL GAZETTE 



[JANUARY 



slide was a regular microscope slide with a hole i . 5 cm. in diameter 

 bored through it. A disk of fluorite was cemented into the hole 

 with its upper surface flush with the upper surface of the slide. 

 This slide was held in the regular mechanical stage of the microscope 

 and the fluorite window of the discharge tube was thus brought into 

 contact with the fluorite disk in the slide. 



When the tube was excited for any great length of time it became 

 hot, and sufficient heat was conducted to the microscope slide to 

 vitiate the results. It was found, however, that the light was so 

 destructive that during a single exposure of sufficient length to kill 

 the organisms, the temperature did not increase more than i° C. 

 The discharge tube was moved away from the microscope slide 

 immediately after each exposure. The temperature of the drop of 

 water which contained the organisms was measured by means of a 

 thermal junction made of copper and constantin. The sensitive- 

 ness of the galvanometer used was such that, with these junctions, 

 one division on its scale corresponded to o?o5 C. The constant 

 junction was kept packed in ice in a thermos bottle. The vari- 

 able junction was flattened out very thin and was attached to a 

 flexible support in such a manner that it could be placed beside the 

 organisms under the cover glass. The junction was held in place 

 on the slide by the capillary pressure of the cover slip, and was in 

 the field of view of the microscope during the entire experiment. 

 If the temperature of the drop of water was raised more than i° C. 

 by the exposure to the light, the experiment was discarded. The 

 arrangement of the tube, slide, and thermal junction is shown in 

 fig. 2. 



The length of time required for killing varied both with the 

 species and with the individual organisms. In general, a small 

 organism was killed more quickly than a large one. With a given 

 light intensity, an exposure of several minutes was not sufficient to 

 kill such organisms as rotifers and lumbricoid worms, while 

 Sphaerella-]ik.e swarm spores, which contain both chlorophyll and 

 an "eye spot," were killed almost instantly. The swarm spores 

 were killed so quickly that there was not sufficient change in tem- 

 perature to be indicated by the thermal junction. In some of the 

 experiments the intensity of the light was reduced until an exposure 



