322 LIGHT AND THE BEHAVIOR OF ORGANISMS 



(" zitternde Bewegung ") in some swarm-spores. ''Die 

 blauen indigofarbigen und violetten Strahlen sind allein 

 auf die phototaktischen Schwarmer von Einfluss und liegt 

 das Maximum der Wirkung im Indigo " (p. 623), These 

 conclusions in general support those of Cohn, but it appears 

 quite probable that in higher light intensity the swarm- 

 spores would have been found positive in the longer waves 

 as well as the shorter, since, as Strasburger states, the red 

 and yellow produced indefinite reactions in the low inten- 

 sity in which they were exposed. It is worthy of note 

 that Strasburger and Wiesner both obtained practically 

 the same effect on reactions with slightly impure colored 

 light produced by means of filter screens as they did with 

 monochromatic light produced by means of prisms. And 

 the same appears to be true in all other experiments on 

 organisms without eyes, in which such a comparison has 

 been made, as, e.g., in the observations of Harrington and 

 Learning on Amoeba and in my own work on the same 

 organism referred to in detail later. This indicates that 

 the reactions of these organisms in waves of a given length 

 are not appreciably affected by the presence of waves of a 

 different length and that extreme precautions to eliminate 

 all foreign rays are not necessary in studying their reac- 

 tions to different colors. In more sensitive forms, however, 

 especially in those with well-developed eyes, monochro- 

 matic light is indispensable. 



2. Engelmann s Experiments 



Engelmann's experiments on unicellular forms in spec- 

 tral colors are of the highest character and the greatest 

 interest. He exposed the organisms in monochromatic 

 light of solar, gas, and electric microspectra thrown on the 

 slide by means of a prism attached to a microscope, and 

 noted not only the regions in which they aggregated, but 

 also the reactions during the process of aggregating. He 

 divides these creatures into three classes based upon their 

 different reactions, as follows: 



