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 ·1 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 ·1 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 according 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 ·1 second,
