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- 



