CHANGES IN INTENSITY 99 



made or broken, but not while the constant current lasts. 

 Hence a rapidly alternating current throws the muscle 

 into tetanus, while the constant current has no effect. 

 If it is the rapid change in the intensity of light which 

 causes the swimming of a positively heliotropic Euglena 

 to the light, an intermittent light, of a sufficient number of 

 alternations per second, should be much more efficient 

 than a constant light; while in case the positive helio- 

 tropism is determined by constant illumination, this 

 should not be the case and the Bunsen-Eoscoe law should 

 hold. 



Mast 34S has recently published experiments on the 

 relative efficiency of the various parts of the spectrum 

 by a method based on the assumption of the validity of 

 the Bunsen-Eoscoe law for the heliotropic orientation of 

 these organisms. If his assumption b is correct, it con- 

 tradicts the theory which Jennings and Mast have de- 

 fended now for more than fifteen years ; if his assumption 

 is wrong, his experiments on the relative efficiency of 

 various parts of the spectrum cannot be correct. Since, 

 however, Mast's results with this method coincide with 

 those by Loeb and Wasteneys 312 obtained by a direct 

 method, it is very probable that the law of Bunsen and 

 Eoscoe holds for the heliotropic reactions of Euglena and 

 unicellular flagellates in general, and, if this is true, the 

 heliotropic reactions of unicellular algae {Euglena in- 

 cluded) are determined by light of constant intensity. 



''He does not seem to have noticed that his method was based on this 

 assumption. 



