SELIG HECHT 



547 



At a temperature of 20°C. it is approximately 1.3 seconds. The 

 sensitization period, however, is very small, resembhng in dura- 

 tion the exposure periods associated with photographic processes. 



The best demonstration of the composition of the reaction time is, 

 in fact, by the use of a photographic shutter. An animal is kept 

 in a dark room, and observations are made with the aid of a ruby 

 lamp to which My a is comparatively insensitive. By means of a 

 camera shutter the animal is exposed to hght for very short inter- 

 vals, and its reaction time noted. It was found that with an illu- 

 mination of 2,000 meter candles, an exposure of j^ second was 



TABLE II. 



more than enough to cause a response of the animals in the usual 

 reaction time. 



3. Within a moderate range of intensities, and at a constant tem- 

 perature, the latent period remains the same. This may be demon- 

 strated by determining the reaction time of an animal to a series of 

 intensities which require sensitization periods of less than 0.1 second. 

 Under such conditions, the sensitization period and its variations 

 due to the changes in intensity will not be noticeable in the reaction 

 time as measured with a stop-watch. Table II records the reaction 

 time of two animals exposed to three intensities. 



It will be seen that the variations in the reaction time, and con- 

 sequently of the latent period, are well within those of the experi- 



