LIGHT REACTIONS AND PHOTOTAXIS 595 



were distributed in eight glass tubes of 12 mm. diameter, arranged 

 side by side in a small stand. A spectrum obtained from an 

 arc lamp by means of a carbon bisulphide prism, was thrown 

 obliquely from above on the row of glasses. Before beginning 

 the experiment I made the animals, who were collected near 

 the surface, adapt themselves to weak light reflected from the 

 ceiling. As soon as the arc lamp was switched on, the animals 

 began to sink in the green and yellow-green, after that in the 

 blue-green, blue and yellow and finally in the violet and the 

 red. The lower border of the part of the tubes filled with ani- 

 mals formed a blunt cone having its lowest point in the green 

 and yellow-green. The same experiment could also be made 

 in the reverse way. I made the animals adapt themselves to 

 the different spectrum colors. When a strong white light was 

 switched on at some distance from the side of the glasses, the 

 animals in the green part of the spectrum would sink last, as 

 they were adapted to the strongest light, those in the red and 

 violet first. 



By another experiment it can be shown, that the same rays 

 that bring about the strong reactions to changes of the inten- 

 sity of illumination have also the strongest orienting power. If 

 a narrow cuvette with parallel sides, filled with larvae, is exposed 

 to the spectrum, the long axis being cut at right angles by the 

 direction of the rays, the major part of the animals accumulates 

 in the yellow-green and green. One can observe the animals 

 swimming toward this part from all sides, leaving the rest of 

 the glass nearly free. In the yellow-green they gradually sink 

 to the bottom and collect there in large numbers near the front 

 pane. The negative animals are found in the violet and red 

 near the rear pane, while the green part remains free of negative 

 animals. These experiments, frequently repeated, confirm the 

 results obtained by Hess, who found the maximum of reaction 

 to light in a great number of invertebrates to occur in the green 

 and yellow-green parts of the spectrum. It is important to 

 keep in mind, that the same rays have the strongest effect on 

 the reactions to changes of intensity of illumination described 

 above. I find it necessary to state in parenthesis that the ani- 



