--2; -- 



crrnpletely , followed "by those "behind 74 and then 73. It WT.S < f oen 

 quite impos ible to detect ny dit'terenoe in the response behind 75 

 and 76. 



It was noticeable in every series exposed that in the -vhite 

 light of the control complete aggregation of the colonies as the re- 

 sult of their phototropism was no*- so rapid as in A=*to f -i^o /*-P- 

 Repeatedly the observation was made that the control locked more like 

 the culture in fr* 1 - VR /+yt^ T:.e expl.-unticn in prob-ibly 



either that the intensity was fso raiich greater that some individuals 

 were caused to respond negatively, or were neutral; or that the great- 

 er reflection from the back of the dish ma e all parts of the watr 

 more nearly ali>-e in intensity of illumination and so tended to 

 equalize the Rtiraulus acting on the different sides cf the orginism. 



When a series of cultures was xpooed in the same way with the 

 sun hidden by clouds no respcns -> could be obtained behind the red, 

 cringe ^nd yellow filters. 



Theae experiments srow the greater stimulating eiticiency of th e 

 blue end nf the apectrura since comparison of the order of efficiency 

 of the screens in produciri/-; the positive response is not in agreement 

 with the order of the intensity of the light behind them, (see table 1 ) 

 In fact the stimuli of least energy produce the most rapid response. 

 They also show that with a longer exposure or more intense stimulus 

 the less effective wave lengths may produce the s-ime degre " of r spons e * 

 In none of the exposures to sunlight were there any negative reactions 

 or reversals of the response. 



On April 6, the experiments ^re repeated -vith the electric 

 arc as the source of li ht, and with the dark boxes at such distances 

 from it that the intensity behind all was the same. The results are 



