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° C. 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 8° 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, 
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