464 'Journal of Comparative Neurology and Psychology. 



boxes differed from one another in brightness. For set I this 

 difference was medium, in comparison with later conditions, and 

 discrimination was therefore of medium difficultness. For set II 

 the difference was great, and discrimination was easy. For set III 

 the difference was shght, and discrimination was difficult. It is 

 clear, then, that the series of words, medium, great, slight, in the 

 table refers to the amount by which the electric boxes differed 

 in brightness, and the series medium, easy, difficult, to the demand 

 made upon the visual discriminating ability of the mice. 



For the sake of obtaining results in this investigation which 

 should be directly comparable with those of experiments on the 

 modifiability of behavior in the dancer which have been conducted 

 during the past three years, it was necessary for us to use the same 

 general method of controlling the visual conditions of the experi- 

 ment that had previously been used. This we decided to do, not- 

 withstanding the fact that we had before us methods which were 

 vastly superior to the old one with respect to the describability of 

 conditions and the accuracy and ease of their control. To any 

 experimenter who wishes to repeat this investigation with other 

 animals we should recommend that, before recourse is had to the 

 use of cardboards for the purpose of rendering the boxes distin- 

 guishable, thorough tests be made of the ability of the animal to 

 discriminate when the boxes are rendered different in brightness 

 by the use of a screen which excludes a measurable amount of 

 Hght from one of them. We have discovered that the simplest and 

 best method of arranging the conditions for such experiments with 

 the dancer as are now to be described is to use two electric boxes 

 which are alike in all respects and to control the amount of light 

 which enters one of them from the top. It is easy to obtain satis- 

 factory screens and to measure their transmitting capacity. We 

 regret that the first use which we wished to make of our results in 

 this investigation forced us to employ conditions which are rela- 

 tively complicated and difficult to describe. 



For the sake of the scientific completeness of our paper, how- 

 ever, and not because we wish to encourage anyone to make use of 

 the same conditions, we shall now describe as accurately as we 

 may the conditions of visual discrimination in the several sets of 

 experiments. 



The cards at the entrances to the electric boxes were the same 

 in all of the experiments. Each card (the black and the white) 



