278 STUDIES IN GENERAL PHYSIOLOGY 



is covered with a red or blue glass, the transformation does 

 not take place. I tried to discover now whether at a tem- 

 perature of 7 C. and less the sunlight would still be able to 

 make the positive animals negative. I again took some 

 animals which had become positively heliotropic in the north 

 room, and convinced myself first of all that at a constant 

 temperature of 20 0. they would become negatively helio- 

 tropic in direct sunlight in a few minutes. I then returned 

 them to the north room, and here the animals again became 

 positively heliotropic at the same temperature in the course 

 of fifteen minutes. After this I filled the space between the 

 dish containing the animals, and the outer vessel, while still 

 in the north room, with ice and salt, and waited until the 

 temperature in the dish containing the animals had fallen to 

 8 C. During this procedure the animals had only become 

 more intensely positively heliotropic. I now brought the 

 entire apparatus into a south room and exposed it to the 

 direct sunlight. The animals which had been disturbed 

 by being carried from room to room at once moved to the 

 window side of the dish when exposed to sunlight. Not one 

 animal became negative or indifferent. The temperature 

 dropped to 5 C. The animals remained strongly positively 

 heliotropic. I kept the temperature of the water in the 

 vessel containing the animals between 5 C. and 3 C. for an 

 hour. Greater differences in temperature in the dish con- 

 taining the animals were prevented through constant stirring 

 of the fluid in the outer vessel. The animals remained posi- 

 tively heliotropic permanently. I now allowed the tempera- 

 ture of the dish containing the animals to rise rapidly by 

 stopping the addition of ice. In fifteen minutes the tem- 

 perature reached 7 C., and a few animals had already be- 

 come negative at this point. In the next fourteen minutes 

 the temperature rose to 9 C., and more than half of the 

 animals became negatively heliotropic. Fifteen minutes later, 



