4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8/ 



month's time the surface of the agar plate was covered with a quite 

 uniform green growth of algal cells. 



The cover was removed, and the lower part of the petri dish was 

 immediately covered with clear cellophane that had been soaked in 

 99 per cent alcohol. The culture was then placed in position in the 

 spectrograph. It is necessary that no absorbing medium shall be pres- 

 ent between the measured incident energy and the exposed algae. 

 However, Johnson ( 1931) found that the percentage transmission of 

 cellophane as compared to air is close to 100. Browning and Russ 

 (1917) have demonstrated that no difference can be detected in the 

 density of the growth of bacteria over the irradiated and non-irradi- 

 ated portions of agar ; consequently, ultra-violet irradiation has no 

 appreciable effect upon agar. 



After exposure of 21 minutes to the spectrum the cover of a sterile 

 petri dish was placed over the cellophane-covered lower dish and the 

 petri dish culture was returned to the bell jar in diffuse light. 



Xo change was observed in the growth of the algae on this first plate 

 until one week after exposure. Then white lines resulting from the 

 complete decolorization of the chlorophyll and death of the green cells 

 corresponded to the typical mercury lines for all wave lengths shorter 

 than 3,000 A. just as they would be seen on a photographic positive 

 (see pi. i). 



A slightly different technique was developed for the preparation of 

 subsequent plates for exposure in the spectrograph. The surface of a 

 glass plate of dimensions 8 by 10 cm was ground so as to retain the 

 agar poured on it. The plate was placed in a large petri dish 15 cm 

 in diameter and covered with Detmer 1/3 agar 2 per cent, sterilized, 

 and inoculated as described above with a suspension of green cells of 

 Chlorella vulgaris. After a month's time the agar plate covered with 

 green cells was cut out of the surrounding agar in the petri dish and 

 placed upright in a closed sterile brass container with a quartz window. 

 A decker was arranged in front of the slit of the spectrograph to per- 

 mit the exposure of different portions of the plate for different lengths 

 of time. 



The second plate was subjected to five irradiation periods of 6 and 20 

 minutes, i, 3, and 18 hours. When the plate was removed from the 

 spectrograph at the end of 222 hours the effect of the 18-hour exposure 

 was clearly visible. Three lethal regions of the 3-hour exposure were 

 also visible. Plate 2, Figure i is a photograph made of the plate as 

 soon as it was removed from the spectrograph. The algal plate was 

 placed in a sterile petri dish in a bell jar in diffuse light. Within two 

 days the results of all five exposures were evident. 



