Mast, Light Reactions iii Lower Organisms. i^J 



By means of the light grader referred to several times in the 

 preceding pages, I was able to subject colonies to rays which were 

 nearly parallel but decreased in intensity from one end of the field 

 to the other, so that when the longitudinal axes of the colonies 

 were parallel with the rays, one side was more strongly illuminated 

 than the other; and I found that this intensity difference did affect 

 the direction of motion, as will be shown in the following detailed 

 account of the experiment. 



The light grader was so arranged that the Nernst glower was 

 vertical and the rays and the long axis of the lens horizontal. The 

 plate glass aquarium was so placed that the rays were parallel 

 with the bottom. Now by fastening over the lens a screen, which 

 contained an opening in the form of two truncated triangles with 

 their apices in contact, a field of light- was produced which was of 

 high intensity at either end and gradually' became lower toward 

 the middle. Two methods were used in ascertaining the direction 

 of movement in such a field of light. 



In the first method a large number of colonies were taken up in 

 a pipette and half of them introduced into the aquarium near the 

 side farthest from the glower at a fixed point some distance from 

 one end of the field, and the other half in a similar place near the 

 opposite end. Thus the organisms in one group as they swam 

 across the field were more intensely illuminated on the right side, 

 while those in the other group were more intensely illuminated 

 on the left side. 



In the second method a single colony was selected and allowed 

 to cross the aquarium toward the source of light several times, 

 first near the right end of the field so that the lower light intensity 

 was to the left and then near the left end of the field so that the 

 lower light intensity was to the right. This alternating process 

 was continued until the path in the two different positions was 

 definitely established. The angles of deflection were read and 

 recorded as described on p. 117. Those obtained by the first 

 method may be found in Table II and those by the second method 

 in Table III. The negative numbers indicate deflection to the 

 left of the ray direction and the positive to the right. 



Table IV represents the effect of difference in light intensity, 

 on deflection in graded light. The course taken by the colonies 

 was obtained by studying the reactions of single colonies, just as 

 in the experiments of Table III. This table shows that an increase 



