608 WOLFGANG F. EWALD 



tion of copper acetate with addition of methyl green and Hthium 

 carmine for the blue filter. In both colors the animals stained 

 with Bismarck brown became negative first, those stained with 

 methylene blue second and the unstained ones last. I was 

 therefore not able to state that the complementary colors chiefly 

 absorbed by the dye (yellow for the methylene blue and blue 

 for the Bismarck brown) had a stronger effect than those of the 

 same color. This is, however, what ought to have been the 

 case, if I had succeeded in producing a 'sensitisation.' More- 

 over, the difference in the velocity of negativation between 

 stained and unstained animals was not considerable, the negati- 

 vating effect of the dyes being very small in comparison to most 

 of the agents mentioned in this paper. The effect of heat was 

 not, however, to be made responsible for the result; special 

 measurements showed that the differences never exceeded 0.4 

 to 0.5 of a degree (C), the water being warmer sometimes in 

 the colored and sometimes in the uncolored water. The^poison- 

 ous effect of the dyes made it impossible to use higher concen- 

 trations. All these observations indicated the probability that 

 the effect of methylene blue and Bismarck brown had nothing 

 to do with their color but with their containing some chemical 

 agent, which would make the animals negative in small con- 

 centrations and kill them in stronger ones. It is well known 

 that methylene blue is poisonous for living protoplasm, especially 

 in the light, and the same is true, according to Straub, for eosine, 

 which is said to form a hypothetical poisonous eosine peroxide 

 under the influence of light. In my experiments all the dyes 

 mentioned proved to be toxic even in the dark in concentrations 

 slightly higher than those used for rny purposes. To obtain 

 a final answer to the question at issue I made use of the reaction 

 to changes of intensity of illumination, mentioned in the begin- 

 ning of this paper. Three glass tubes with brown, blue and 

 unstained nauplii were made to adapt themselves to weak light 

 coming from above. After some time, the strong monochro- 

 matic hght was turned on, the source of light being at a level 

 with the surface of the water in the tubes. The nauplii sank 

 every time with equal velocity in all three tubes, whether I 



