384 Profs. Percy Frankland and Marshall Ward. 



life than before. Similarly, at 4 P.M., the blue was as before, while 

 the red had filaments several thousands of /t's in length. 



On November 18 spores were sown in normal gelatine at 8 A.M., 

 and kept at 22 C. till 12.30, when they had germinated to rodlets 

 8 9 fi long. Two cultures were then exposed at the south window, 

 the sun being off (behind a building) on microscopes, and over blue 

 and red glass respectively, the light reflected up from ordinary 

 mirrors. Controls as usual. The whole day was clear and cold. 



The following are the results : 



The doubling period for the red culture comes out (for the 

 mean) = 



24 -7549 -5 p = 1.30 P.M. 3.47 P.M. = 137 minutes at 15 '516 '415. 



Owing to the fact of my having the previously described cultures, 

 exposed to the open sky behind the same blue and red screens, but at 

 a lower temperature, it is not difficult to see what happened here. 

 The amount of light reflected from the small glass mirrors of the 

 microscopes is not sufficiently intense to kill the rodlets at these tem- 

 peratures, though it is sufficient to inhibit their growth perceptibly. 



This explains many of, my previous failures. If I arrange the 

 microscopes so as to throw a more intense daylight on the cultures, 

 then the heat rays produce difficulties, because the screens transmit 

 them in different proportions, and I either find the measured filament 

 growing longer than can be measured before the inhibition sets in, 

 or the difference in temperature between the two cultures so great 

 that doubts arise as to how much of the inhibition is due to lower or 

 higher temperature, and how much to the light action. 



For, as we now see, the temperature is effective at once, but the 

 light action lakes a considerable time to make its effects visible on 

 the growth-curve, and over and over again I have found cultures just 

 beginning to show the retarding, brake-like action of the light injury 

 at, or even after, the conclusion of the short growing period I am able 

 to quantitatively examine and record. Of course, though one may 

 be convinced by inspection that, of two cultures, one has formed a 

 smaller crop than the other, in the absence of measurements the 

 statement wants the definiteness I have been trying to attain. 



On December 6, spores in normal gelatine were sown, and exposed 

 at once at 11 A.M. to the hazy, winter sun, at south window of my 

 house. 



The " red " stood over bichromate ; the " blue " over water tinged 

 with CuS0 4 , both in quartz cells. 



The temperature of controls ran as follows during exposure : 





