BEHAVIOR OF STOCK AND INBRED ALBINO RATS 285 



After the rats had learned to discriminate between the open 

 box and the box covered with black cardboard, a sheet of milk 

 glass was laid over the open box. To increase the difficultness 

 of discrimination thereafter sheets of ordinary typewriter paper 

 were added one by one to the milk glass, cutting off, increas- 

 ingly, the amount of light entering the light box. The various 

 conditions of discrimination are given thus: 



A, B, Preference Series, No Shock 



Black cardboard versus open box. 



Black cardboard versus milk glass. 



Black cardboard versus milk glass and 1 paper. 



Black cardboard versus milk glass and 2 papers. 



Black cardboard versus milk glass and 3 papers. 



Black cardboard versus milk glass and 4 papers. 



Black cardboard versus milk glass and 5 papers. 



Black cardboard versus milk glass and 6 papers. 



Black cardboard versus milk glass and 4 papers. Review. 



Black cardboard and paper under, versus milk glass + 4 papers. 



Review. 

 Black cardboard and paper under, versus milk glass. Review. 

 Black cardboard and paper under, versus milk glass. Memory. 



Table 5 presents the records of the six male rats (stock and 

 inbred) ; table 6, the six female rats, tested by the earlier work- 

 ers; tables 7 and 8, similarly, give the records of those tested 

 by the present writer. 



In the tables each horizontal black line indicates a change in 

 condition. For conditions 1-5 the rats were required to present 

 perfect records (5 right, wrong) for four successive series of 

 five trials each. Later, when discrimination became more diffi- 

 cult, this was reduced to three, and then to two. When any 

 rat, after making perfect records under the previous easier con- 

 ditions, began to make one or more mistakes each day, the 

 discrimination was deemed too difficult and the experiment was 

 terminated. When all had reached the limit of their discrim- 

 inating ability a review series was given to all with only four 

 papers added to the milk glass; then a series of five trials in 

 which a sheet of white paper was laid under the black card- 

 board to determine whether the rats, by any chance, were choos- 

 ing because of the difference in the black and white covers rather 

 than by the difference in lightness and darkness. 



Finally, after twenty-eight days, two series were given on 

 successive days, as a memory test, under the same conditions 

 as the latest previous test, but with no punishment for failure. 



