306 



Profs. Percy Frankland and Marshall Ward. 



Comparative Measurements. 



The foregoing results led to the attempts now to be described-- 

 to grow two filaments side by side, one in the light and the other in 

 the dark, to see if the action of light could be detected by any change 

 in the growth curve. 



Before passing to these experiments, I obtained satisfactory evidence 

 that two cultures, side by side and under the same conditions, behave 

 similarly.* 



On February 18th I started the following comparative cultures, to 

 see how far I could test the action of daylight rendered so probable 

 by some of the foregoing results. 



Spores were sown in broth at 9 A.M. in two hanging drop cultures, 

 each with a layer of water at the bottom to prevent rapid changes of 

 temperature or drying up. The two cultures were then put under a 

 dark bell-jar, covered with foil and brown paper, at 18 C., rising to 

 21 C., close to the south window where I intended to start the 

 experiment. The cultures were left thus so that the spores should 

 have time to germinate out normally, which they began to do about 

 noon ; they were left undisturbed till 2 P.M. 



Meanwhile, I had selected two similar bell-jars and placed these a 

 the window, each over a microscope provided with a thermometer ; 

 one bell-jar was darkened with tin "foil and brown paper, the other 

 not. The thermometers were examined, and readings taken from 

 time to time. The differences observed at first, and due to the shading 

 action of the covering, began to get less and less as the bell-jars, 

 table, and microscopes, were warmed by the sun, which was bright 

 and hot ; and by noon the temperatures were very nearly the same 

 in each jar. 



At 12.30 the thermometer under the light jar indicated a tempera- 

 ture of 25'5 C. ; that under the darkened one =24 - 6 C.,f and this 

 difference of nearly 1 C. was maintained till about 1.30, the readings 

 being as follows : 



About 2 P.M. the sun began to sink behind trees and the roof of a 

 building facing the window, and now it was possible to use the bright 

 light from the sky without danger of direct insolation. 



* Better proofs of this were obtained later, however, and are given at pp. 359 361. 

 f These are all air temperatures, unless the contrary is specially stated. 



