156 . The Dancing Mouse 



of experimentation was practically the same as that already 

 described. A mouse which had been placed in A by the 

 experimenter was permitted to enter B and thence to return 

 to A by entering one of the electric-boxes, the red or blue or 

 green one, as the case might be. Mistakes in choice were 

 punished by an electric shock. One further point in the 

 method demands description and discussion before the re- 

 sults of the tests are considered, namely, the manner of 

 regulating and measuring the brightness of the lights. 



Regulating brightness with this apparatus was easy enough ; 

 measuring it accurately was extremely difficult. The experi- 

 menter was able to control the brightness of each of the two 

 colored lights which he was using by changing the position 

 or the power of the incandescent lamps in the light-box. 

 The position of a lamp could be changed easily between tests 

 simply by moving it along toward or away from the electric- 

 box in the slit which served as a lamp carrier. As the dis- 

 tance from the entrances of the electric-boxes to the further 

 end of the light-box was 120 cm., a considerable range or 

 variation in brightness was possible without change of lamps. 

 Ordinarily it was not necessary to change the power of the 

 lamps, by replacing one of a given candle power by a higher 

 or lower, during a series of tests. Both the candle power of 

 the lamps and their distance from the filters were recorded 

 in the case of each test, but for the convenience of the reader 

 I have reduced these measurements to candle meters * and 

 report them thus in the descriptions of the experiments. 



But measuring the actual brightness of the red light or 

 the green light which was used for a particular series of tests, 

 and the variations in their brightnesses, was not so simple 

 a matter as might appear from the statements which have 

 just been made. The influence of the light filters themselves 



1 The illuminating power of a standard candle at a distance of one meter. 



