I2O The Dancing Mouse 



This apparatus has the following advantages. First, the 

 electric-boxes, between which the mouse is expected to dis- 

 criminate by means of their difference in brightness, are 

 illuminated from above and the light therefore does not shine 

 directly from the lamps into the eyes of the animal, as it 

 approaches the entrances to the boxes. Choice is required, 

 therefore, between illuminated spaces instead of between 

 two directly illuminated surfaces. Second, the amount of 

 illumination of each electric-box can be accurately measured 

 by the use of a photometer. Third, since the same kind of 

 lamp is used in each box, and further, since the lamps may 

 be interchanged at any time, discrimination by qualitative 

 instead of quantitative difference in illumination is excluded. 

 And finally, fourth, the tests can be made expeditiously, 

 conveniently, and under such simple conditions that there 

 should be no considerable error of measurement or of ob- 

 servation within the range of brightness which must be used. 



It was my purpose in the experiment with this apparatus 

 to ascertain how great the difference in the illumination of the 

 two electric-boxes must be in order that the mouse should 

 be able to choose the brighter of them. This I attempted 

 to do by fixing an incandescent lamp of a certain known 

 illuminating power at such a position in one compartment of 

 the light-box that the electric-box below it was illuminated 

 by what I call a standard value, and by moving the incandes- 

 cent lamp in the other compartment of the light-box until 

 the illumination of the electric-box below it was just suffi- 

 ciently less than that of the standard to enable the dancer 

 to distinguish them, and thereby to choose the brighter one. 

 The light which was changed from series to series I shall call the 

 variable, in contrast with the standard, which was unchanged. 



The tests, which were made in a dark-room under uni- 

 form conditions, were given in series of fifty each ; usually 



