second. It is obvious that this time is longer than the period of pupillary inactivity that follows the initiation of stimulation. If we wish to redefine latency period as the time from photic stimulation to the first detectable response, we must re-examine our data. Figure 5 shows that the voltage level used for a single sample measurement was reached approximately 0.034 sec. after the moment of first noticeable change. Though it would have been preferable to trigger the "Stop" circuit of the counter by this first minute voltage change, it was necessary to chose the higher voltage in order to operate beyond the noise level of the system.
Consciously or unconsciously, most authors have run into similar difficulties. For example, it is impossible to detect with the unaided eye the exact moment of initiation of the contraction movement, especially when the stimulus intensity is low and the reactions shallow. Since, in addition, the latency period increases with decreasing stimulus brightness (Lowenfeld and Lowenstein, 1959), the large range of values for the pupillary latency period reported by competent authors is not surprising.
The prolongation of the latency period beyond its true value depends upon the steepness of the slope of increasing contraction velocity. For this reason mainly, we chose the maximal light intensity conveniently available for our experiment on aging. The values obtained on the readout of the Digital Recorder and those marked on the Memoscope at the -0.35 volt cutoff point agreed very well (maximal discrepancy ±4 milliseconds), and since all of our normal subjects responded to the powerful light flashes by reactions with steep velocity slopes, the differences between the true latency periods and the mean times measured at the -0.35 volt cutoff point varied only little among individuals.
LATENCY PERIOD AND LIGHT INTENSITY
Even at the high brightness levels used in this experiment, the latency period tended to lengthen when the stimulus intensity was reduced. Figure 8 shows the responses of the pupil to five different flash light intensities ranging in intensity ratios from 1 to 80.
The logarithms of the relative intensities (10 log relative intensity in db) are 0, 6.0, 12.0, 16.8, and 19.0. Peak intensity
