TIME-RELATIONS OF RESPONSE 37 



movement, we have seen that the successive dots in the 

 curve itself give time-intervals. When it is necessary to 

 measure short intervals, say of - i second, a resonant 

 vibrator accurately tuned to ten double vibrations per 

 second is employed. The successive dots in the curve then 

 represent intervals of a tenth of a second each. 



For the correct determination of the first two phases 

 of the responsive movement in which the time involved 

 is short — namely, the latent period and the apex time — it is 

 necessary to have the recording-plate moving at a rapid 

 rate. But for determining the time-relations during the 

 period of recovery, which is a matter of several minutes, 

 the recording-plate has to be moved at a relatively slow 

 rate. 



I give below records (fig. 12) which show these first two 

 elements in a typical manner. The record here, as explained 

 before, is reduced to two-thirds. 



Latent Period. — It will be noticed that there is a short 

 interval between the application of stimulus, represented 

 by the vertical line, and the initiation of response. The 

 movement is here seen to begin before an interval of 

 •1 second is completed. For more accurate determination 

 of the latent period a record must be taken on a faster- 

 moving plate. A detailed description will be found in a 

 later chapter, where it is shown that the average value 

 of the latent period may be taken as about -i second. 

 It will also there be observed that though in a' given 

 specimen the latent period is constant, it varies slightly in 

 different specimens. It is also appropriately modified accord- 

 ing to the physiological changes induced by temperature, 

 fatigue, and the influence of the season. 



The Apex Time. — It is seen from the upper of the two 

 curves in fig. 12 that the responsive fall practically attains 

 its maximum near the twentieth dot. This indicates that, 

 the value of the apex time in this case is 2 seconds. As 

 regards the rate of this responsive fall, the spacing of the 

 successive dots, each representing an interval of *i second, 



