Note 3 — Chapter IV

A word is needed in amplification of what was said on p. 26 as to the blending of successive images produced on the retina of the eye by the bioscope or cinematograph or by the old "wheel of life." The point which is of importance is not the length of time during which the stimulation of the retina caused by an image endures—becoming weaker and weaker as fractions of a second pass—but it is this: How long will a stimulus last in undiminished brightness? How soon must it be followed by another stimulus (another image) so that there may be fusion or continuity, the one succeeding the other before the earlier has had time, not to disappear, but to decline. If it has had time to decline in intensity, the appearance of flickering results. That is what the cinematographer has to avoid. It is found that a quicker succession—a shorter interval—is necessary with strong light than with weaker light in order to produce continuity. With a faint light the interval may be as great as one-tenth of a second; with a strong light it must not exceed one-thirtieth (or with still stronger light, one-sixtieth) of a second. With the stronger light there is a more rapid and a greater loss of the initial intensity of the impression or effect of stimulus, and though each successive effect remains as long, or longer, in dwindling intensity, you get want of continuity, or "flicker."