BEHAVIORISTIC STUDY OF THE RAT 
21 
In the present experiments the attempt is made to determine 
whether the rat is nocturnal, and also just to what extent it is 
nocturnal; that is just how much more active it is in the dark 
than in the light. 
This experiment was carried on in the following way. The 
laboratory in which the records were taken was alternately 
illuminated and completely darkened every twelve hours. The 
amount of activity (in activity units) in these two periods was 
measured. Then the ratio of the amount of activity in the dark 
period to the amount in the light period was taken as a numerical 
indication of the active extent to which the animal may be said 
to be nocturnal. 
All conditions were kept the same in both periods, except 
the conditions of giving food. The animals were fed just once per 
day just as was done in all of the previous experiments. Records 
were taken in two series. In one series the animals were fed at 
the beginning of the dark period. In the other they were fed at 
the beginning of the light period. It proved necessary to take 
two series of records in this way because of the fact demonstrated 
in figure 5 above that spontaneous activity is not equally dis¬ 
tributed over the twenty-four hours with relation to the time of 
feeding. There it was shown that the amount of activity in the 
first twelve hours following the daily meal is very much greater 
than during the second twelve hours. Thus in the first series of 
records the effect of the food is added to the effect of the darkness, 
while in the second it is subtracted. Records obtained in this way 
are shown in figure 11. Curve A gives the results in the first 
series in which the animals were fed at the beginning of the dark 
period. The ratios of the amount of activity in the dark period 
to the amount in the light period are given on the ordinates and 
the ages of the animals are given on the abscissae. The ratios 
for animals fed at the beginning of the light period are given in 
Curve B. The great discrepancy between these two curves is 
quite obvious. 
The method by which the effect of food was eliminated may be 
brought out most simply by the following example. It will 
be noticed in the curves A and B in figure 11 that the animals 
