REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 67 



intensity ; movement toward the light into a region of greater intensity. 

 In the well-known experiments of Strasburger (1878) and others, this 

 condition is modified by passing the light through a wedge-shaped 

 prism filled with a solution that cuts out part of the light. 



When a drop of water or a culture dish is placed beneath such a 

 prism, and the latter is so situated that its surface is perpendicular to 

 the light rays, the intensity of the illumination is greatest behind the 

 thin edge of the prism, and thence decreases gradually toward the 

 opposite end, while the rays of light all come directly from above. 

 Under these conditions Strasburger (1878, p. 36) found that the positive 

 swarm-spores remained equally distributed throughout the drop, not 

 collecting at the lighter end. Now, the only difference between this 

 experiment and the one illustrated in Fig. 1 1 of the present paper is 

 that in Strasburger's experiment the decrease in illumination is very 

 gradual. We have seen above (p. 52) that a very gradual change in 

 illumination produces no reaction. Hence the organisms may wander 

 from one side of the drop to the other without reaction, the difference 

 in illumination at two successive instants never rising to the necessary 

 threshold of stimulation. If the relation of stimulus to reaction follows 

 Weber's law, the result is just what we should expect, provided the 

 change in illumination is sufficiently gradual. When the difference in 

 illumination from above is great, Strasburger's own experiments (/. c., 

 p. 33) show that the organisms do react. 



On the other hand, Holt & Lee (1901), using a similar prism, 

 found, under similar conditions, that the negative organism, Stentor, 

 does, on the whole, tend to gather at the darker side of the drop. 

 This shows that the difference in illumination between neighboring 

 points in this particular experiment was not below the threshold of 

 stimulation for the organism in question. If, as Holt & Lee sup- 

 pose, a certain amount of light was reflected from the lighter end of 

 the vessel, then the inclination to go to the darker side would be rein- 

 forced by Stentor's tendency to turn when the light falls upon its 

 anterior end (see p. 43). The fact that in Strasburger's experiments 

 the organisms remained scattered throughout the drop seems to indi- 

 cate that this reflected light played no part in his results. 



In another set of experiments Strasburger placed his prism over the 

 swarm-spores in such a way that the light came obliquely from the 

 direction of the thick end of the wedge. If the positive organisms now 

 go toward the thicker end of the wedge, they pass toward the source 

 of light, but into a region of decreased illumination ; if they go toward 

 the thin end they pass away from the source of light, but into a region 

 of higher illumination. Which will they choose? 



Strasburger found that the positive swarm-spores pass toward the 



