40 LIGHT AND THE BEHAVIOR OF ORGANISMS 



manns and others who used this method of producing light 

 of graded intensity assumed that the Hght rays in the 

 aquarium under such conditions were parallel with each 

 other and perpendicular to the side through which they 

 entered, and that the change in direction of motion when 

 the prism was put into place was due not to the direction 

 of the rays but to difference in light intensity. Oltmanns 

 does not make it clear in what sense he uses these terms. 

 He does not say whether he means difference of intensity in 

 the field or difference on the surface of the organism, ray 

 direction in the field or ray direction through the organism. 

 No matter, however, in which sense these terms were used, 

 the conclusion was not warranted, for it is clear from a 

 theoretical as well as from a practical standpoint, that the 

 rays of light, in the aquarium were neither parallel with each 

 other nor perpendicular to the side through which they 

 entered. The India-ink mixture contains numerous solid 

 particles of carbon in suspension, which, together with 

 particles in suspension in the water in the aquarium, 

 unquestionably diffuse the light in such a way that the rays 

 in the aquarium coming from the more highly illuminated 

 end are more numerous than those coming from the other 

 end, and so if the direction of the rays were the control- 

 ling factor one might expect the organisms to go toward 

 either end. 



After reviewing the work of the preceding authors and 

 presenting some original experiments similar in method to 

 those of Strasburger, Davenport (1897) agrees with Loeb 

 in assuming two dissimilar sorts of locomotor responses 

 to light. These he designates phototaxis and photopathy. 

 Phototaxis he defines " as migration in the direction of the 

 light rays, and photopathy as migration toward a region 

 of greater or less intensity of light." He accepts the theory 

 of orientation as outlined by Sachs and formulates another 

 which is in all essentials like that of Loeb. He says (p. 209) : 

 " Let us first think of the way in which light acts on the 

 negatively phototactic (and photopathic?) earthworm. 



