REGULATION OF REACTIONS 269 



from the point where the organism became negative. The 

 light intensity at this point was therefore 1 1,1 11 ± ca. m. 

 But the colony was still negative after having crossed the 

 aquarium, a distance of nearly 8 cm., or nearly 23 cm. 

 from the arc, i.e., in an intensity of 4726 ± ca. m., \vhich 

 is 6385 ± ca. m. less than the intensity in which it first be- 

 came negative. Similar results are represented in path 5, 

 but unfortunately the distances between the sources of light 

 and the aquarium, in this exposure, were not recorded. 



The colony which produced path B was positive to the 

 light from the arc when first put into the aquarium at c, 

 but after moving toward the source of light a few centi- 

 meters it became negative, turned about and moved in the 

 opposite direction. When it reached c' the glowers were 

 exposed and the colony promptly changed its direction of 

 motion and proceeded on a course directed from a point 

 between the two sources of light. This point, however, was 

 much nearer the arc than the glowers, the light from the 

 former being much more intense than that from the latter. 

 When the light from the arc was cut off at w, the colony 

 was found to be negative to the comparatively weak light 

 from the glowers. It consequently changed its course and 

 moved from this source; but after continuing about 3 cm. 

 it became positive, turned about and moved toward the 

 glowers to n' , and probably would have continued farther 

 had it not been prevented from doing so by the wall of the 

 aquarium. It will be noticed that the point 11' , where the 

 colony was still positive at the end of its course, was about 

 3 cm. nearer the glowers than n, where it proved to be nega- 

 tive, and nearly 7 cm. nearer than the point where it 

 changed its course from negative to positive. That is, the 

 organism was positive at n' in a much higher light intensity 

 than that in which it was negative at n and at the point 

 where it changed from negative to positive. 



This shows that there were very striking changes in the 

 optimum in these colonies. It also shows that the reversal 

 in sense of reaction was not due to an effect produced by 



