570 PHOTOTROPIC RESPONSE OF NECTURUS 



and the dish parallel. One of the Mazda glowers was fixed above, 

 so that the beam of light (10 cm. in diameter) centered on the head. 

 When the glower was turned on, the animal crawled posteriorly until 

 the head was out of the beam of light. This interval was designated 

 the reaction time, and was measured by a stop-watch to the nearest 

 second. The light was then turned out, and the animal given a rest 

 period of 1 minute before the next trial began. Four intensities of 

 light were used at four different temperatures on six animals. Ten 

 trials were sufficient to obtain accurate averages before fatigue 

 became apparent. 



It was soon discovered that the reaction time of Necturus does not 

 change much with the variations of light intensity from 1,875 candle 

 meters to 8,000 candle meters. At a constant temperature of 22°C. 

 and illumination of 1,875 candle meters, the average of sixty trials 

 (ten on each animal) was 2.8 seconds while at 8,000 candle meters it 

 was 3.1 seconds. At 2°C. the average for 1,875 candle meters was 8.1 

 seconds, and for 8,000 candle meters it was 8.3 seconds. The reaction 

 time was not analyzed as to its exposure and latent periods, which 

 were very short. Crawling began almost immediately after the light 

 was turned on, so that the reaction time is really a measure of the time 

 necessary for the animal to crawl out of the lighted area. Since it is 

 improbable that this constitutes an exception to the Bunsen-Roscoe 

 law, it is evident that in all the trials the maximum amount of stimu- 

 lating substance was produced in a very short time, even at the lowest 

 intensity. This would be true if the velocity of the photochemical 

 reaction is high enough at 1,875 candle meters. The determination 

 of the exposure period in its relation to the photic intensity was re- 

 served for future investigation. In all the temperature tests only 

 one intensity of light was used, 4,200 candle meters. 



From Fig. 1 it will be seen that the reaction time varies inversely 

 to the temperature. Between 2° and 32°C. the temperature coeffi- 

 cient gradually diminishes, as it does in other animals (Hecht, 1919), 

 although its numerical value is not especially significant, since the 

 several chemical processes underlying the reaction time of Necturus 

 have not been analyzed. At 32°C. the animal probably moves as 

 fast as its anatomy will allow, without showing the spasmic movements 

 characteristic of temperatures above 35°C. The curve expresses in 



