Polytechnic Association Proceedings. 909 



persistence of vision. 



A new scientific toy has lately been added to the already numer- 

 ous devices for producing optical illusion by rapid motion. It 

 consists of a wire bent to coincide with the outline of one-half of 

 a decanter, vase, or cup, which, when attached to a vertical axis, 

 and made to revolve rapidly by means of gearing, gives a view of 

 the whole object intended to be shown. This phenomenon depends 

 upon the fact that the visual perception of an object does not ter- 

 minate at the instant the rays reflected from it have ceased to enter 

 the eye. It is accounted for very clearly by the undulatory theory 

 of light, if we assume what is true in many other instances, that 

 the organ acted upon by a vibratory force is, itself, set into vibra- 

 tion. Experiments have shown that visual perception continues 

 about one-eighth of a second after the actual disappearance of the 

 object, so that its reappearance within that time would prevent the 

 eye from detecting its absence. It is, therefore, only necessary to 

 revolve the wire of the toy alluded to so fast that every point in 

 the outline of the object will be shown eight times per second; in 

 other words, eight revolutions of the wire within a second will 

 give the impression of the solid. It might, at first sight, be inferred 

 that the parts near the axis, revolving so much slower than those 

 most distant from it, would not be as clearly represented, but the 

 problem has nothing to do with the actual distance made by the wire 

 in space; the simple requisite being that every part of the outline 

 shall reappear eight times per second. The period of persistence 

 seems very short, yet the actual number of vibrations made by the 

 visual organs during that time is so large as to excite the incredulity 

 of those who have not fully investigated the subject. The most 

 common case of persistence is the line of light which boys at play 

 sometimes produce by rapidly rotating a firebrand. The same 

 effect is seen in the fliajht of a rocket, and on a grander scale in the 

 zig-zag fiery path of the lightning spark. 



A white spot on a dark wheel, revolving at high speed, portrays 

 a white ring, and spots of diflerent colors will unite and give the 

 same resultant hue, which would be made by mixing similarly 

 colored paints. One of the most curious optical illusions is pre- 

 sented when radial lines on different revolving discs assume the 

 form of curves. 



Persistence of vision can be most readily illustrated by drawing 

 the upper half of the human face or form on one side of a small 

 card, and the lower half immediately underneath, but on the oppo- 



