No. 495] NOTES AND LITERATURE 209 



the signs. The mouse in repeated experiments tries at first the 

 simple plan of returning to the right or left door according 

 as he has found that to be correct. When he finds that the 

 correct portal is being alternated, he quickly learns to alternate 

 in his choices. But when he finds that there is no regularity in 

 the alternations, he begins to pay careful attention to the signs 

 posted about the portal; "to run from one to the other, poking 

 its head into each and peering about cautiously, touching the 

 cardboards at the entrances, apparently smelling of them, and 

 in every way attempting to determine which box could be 

 entered safely." Often the mouse runs from one portal to the 

 other twenty times or more, before deciding which to enter. 

 Now, it is in this state of uncertainty and concern that the 

 mouse is ready to give interesting results in animal education 

 and in sense physiology. He uses all his senses to the best of 

 his ability in determining which is the "right" door to enter, 

 so there is opportunity, which Dr. Yerkes has skilfully used, 

 to test his senses, and at the same time to study his ability to 

 learn. When the two portals are indicated, the "right" one 

 by a light card, the "wrong" one by a dark card, the mouse 

 learns to choose the correct card. If for both cards are substi- 

 tuted others that are of deeper shade, but have a similar relative 

 brightness, the mouse continues to choose the one of lighter 

 shade. He has learned, not that a particular card, or a partic- 

 ular shade, is the right one, but that the ligh ter of the two is the 

 one to choose ; he often runs back and forth many times, seeming 

 to compare them carefully. By accurately grading the difference 

 in brightness between the two portals, it was possible to deter- 

 mine just what differences the mice could discriminate, giving 

 an opportunity for work on Weber's law. The mice rapidly 

 learned to discriminate finer and finer shades of difference. A 

 certain mouse, in a first series of experiments, could discriminate 

 only when the difference in brightness was practically half the 

 greater brightness. In a later series he could discriminate when 

 the difference was but one fifth and, after much more practise, 

 when the difference was only one tenth. Such educability at 

 first carried confusion into the data designed to test Weber's' 

 law. When it was finally taken into account, the law was found 

 to hold. 



In a similar way Yerkes made extensive studies of color vision 

 in the mouse. He found that apparently they do not see colors, 



