598 WOLFGANG F. EWALD 



was due to the ultra-violet rays. The glass plate had also a 

 visibly retarding effect on the negativating process. 



Returning to the effect of hght on the process of negativation, 

 I will first give a description of the visible phenomena connected 

 with this process. The animals normally collect near the sur- 

 face on the side of the vessel nearest to the hght ('positive pole' 

 of the vessel) provided the rays strike the vessel obliquely or 

 from the side. If the hght intensity is not inframinimal for 

 negativation, the animals begin after some time to sink to the 

 bottom and to collect there likewise at the positive pole. It 

 is not before a considerable time that the second phase of the 

 process begins, the nauplii reversing their orientation and, in 

 consequence, collecting round the negative pole. According to 

 the Hght intensity the first phase will last for a longer or shorter 

 span of time. 



To decide the efiect of the different wave lengths of visible 

 Hght on the process of negativation I proceeded by two different 

 methods. First I used an arrangement similar to that described 

 above for testing the effect of different wave lengths on the pho- 

 totactic reflexes and orientation. The animals, equally divided 

 in thirteen small tubes, were brought into the strong primary 

 spectrum of a Rowland grating. SunHght, projected into the dark 

 room by means of a heHostat, was used for illumination. When 

 the glass tubes were brought from the dark into the Hght of the 

 spectrum, one could again observe the animals sinking most 

 stroDgly in the green, less so in the blue, still less in the yeUow, 

 while they remained close to the surface in the red and the violet. 

 This difference was observed to persist, if the Hght rays struck the 

 glasses about horizontally, this being a case of permanent regu- 

 lation of the position by Hght intensity. If the naupHi were not 

 too old — younger than forty-eight hours — they would begin to 

 become negative after some time, first in the green, shortly after 

 in the violet. Later on the animals in the blue-green and blue 

 foHowed, finally those in the yeHow. In the red part only a very 

 slight negativating effect could be observed. One glass served as 

 a control and was placed outside the spectrum. Here the ani- 

 mals would not become negative. As soon as the animals becam^e 



