The book of lantern ; being a practical guide to the working of the optical (1888)

178 THE BOOK OF THE LANTERN. of retaining the image of anything seen for at least one- eighth part of a second after the eye ceases to see that object. As an example of this, let me remind my readers, that although in the ordinary course of things, we are con- tinually "winking," an operation which is necessary to lubricate the eyeball, we are quite insensible of the cir- cumstance that for the time occupied in doing so, we are placed in absolute darkness. Although the eyelids are closed and the light is shut out, we have no perception of darkness, simply because of this curious property possessed by the retina of retaining the image of the object last seen, for at least the eighth part of a second. It is for this reason,—I may also point out in passing,—that so-called instantaneous photographs of moving objects, such as a " trotting horse," &c., appear to us to exhibit such very unnatural attitudes. As a matter of fact the photographic camera records movements which the human eye, on account of this "persistence of vision," cannot appreciate. It is evident that if this doctrine be true, the eye cannot appre- ciate a movement which takes place in less time than the eighth part of a second, and it is because the photographic lens can grasp and record the movements which take place in a mere fraction of that time, that the attitudes it depicts appear to us so highly unnatural. The human eye has never seen such attitudes, and never will see them. Perhaps the simplest illustration of " persistence of vision " is afforded by a burnt stick with a red hot end, which is turned rapidly round in front of the observer; <fco that observer the red spot of light looks like a con-