354 journal of Comparative Neurology and Psychology. 



not seem to cause any marked advance in the time of beginning 

 their activity. In other w^ords, the periods of activity seem quite 

 regular and are not materially affected by incidental circum- 

 stances. 



In this experiment the period of activity extended between 

 the hours of 8 p.m. and 4 a.m. — a period of eight hours. The 

 remainder of the 24 hours, with the exceptions noted above, 

 was devoted to rest. This is beautifully shown in Fig. 6 which 

 represents the graphic records for three successive days, extending 

 from 8 a. m., June 24, to 8 a. m., June 27. The ages of the rats 

 during this interval were 70, 71 and 72 days. By comparing 

 these records with those of Fig. 5 one is impressed with the marked 

 regularity of activity at this later age. They began their work 

 with a noticeable degree of punctuality. Rat No. 4 was especially 

 prompt and usually began within a few minutes of 8 o'clock each 

 evening. 



Fig. 7. The graphic records of the activity of each rat for a period of 24 hours at the age of 77 

 days, showing the effect of continuous hght. The electric Hghts were left burning during the entire 

 night. 



To what is this nocturnal activity due .? Is it due to the absence 

 of light or to some other cause ^ In order to see if light had any 

 effect on their activity the electric lights were left burning during 

 the night of July 3. The graphic record for that period is shown 

 in Fig. 7. A great difference is observed between this record and 

 those of Fig. 6. The lack of regularity and the great reduction in 

 the amount of activity are very noticeable. The marked difference 

 in activity is also seen in Table IV, which illustrates the average 

 number of revolutions for day and night. On the nights of July 3 

 and July 6 the lights were left burning and the readings the next 

 morning at 8 o'clock are indicated. I can give only two sugges- 

 tions for the difference between these two nights' work. From 

 midnight on during the night of July 3-4 there was an almost 

 continual roar and racket from the explosion of giant firecrackers. 



