Explanation of an Optical Deception. 4G9 



perpendicular. The two spokes 'A-hich arrive at the vertical 

 position, above and below the axle, are seen of their natural 

 shajje, that is, without any curvature. Those on each side of 

 the upper one appear slightly curved ; those more remote, still 

 more so: and the curvature of the spokes increases as we fol- 

 low them downwards on each side till we arrive at the lowest' 

 spoke, which, like the first, again appears straight. 



The most remarkable circumstance relating to this visual 

 deception is, that the convexity of these curved images of the 

 spokes is always turned downwards on both sides of the wheel ; 

 and that this direction of their curvature is precisely the same, 

 whether the wheel be moving to the right or to the left of the 

 spectator. 



In order to discover a clue to the explanation of this phasno- 

 menon, it was necessary to observe the influence which certain 

 variations of circumstances might have upon it ; and the fol- 

 lowing are the principal results of the experiments 1 made for 

 this purpose. ^ 



1. A certiiin degree of velocity in the wheel is necessary to 

 produce the deception above described. If this velocity be 

 gradually comnmnicated, the appearance. of curvature is first 

 perce))tible in the spokes which have a horizontal jiosition : 

 and as soon as this is observed, a small increase given to the 

 velocity of the wheel produces suddenly the appearance of cur- 

 vature in all the lateral spokes. The degree of curvature re- 

 mains precisely the same as at first, whatever greater velocity be 

 ^iven to the wheel, provided it be not so great as to prevent 

 the eye from following the spokes distinctly as they revolve ; 

 for it is evident that the rapidity of revolution maybe such as 

 to render the spokes invisible. It is also to be noticed that, 

 however rapidly the wheel revolves, each intlividual spoke, 

 appears, during the moment it is viewed, to be at rest. 



2. The number of spokes in the wheel makes no difference 

 in the degree of curvature they exhibit. 



3. The appearance of curvature is more perfectly seen when 

 the interval between the bars through which the wheel is 

 viewed are narrow, provided they are sufHciently wide to allow 

 of the distinct view of all the parts of the wheel in succession, 

 as it passes along. For the same reason, the phacnomenon is 

 seen to the greatest advantage when the bars are of a dark co- 

 lour, or shaded, and when a strong light is thrown upon the 

 wheel, 'i'he deception is in like manner aided by every cir- 

 cumstance which tends to abstract the attention from the bars, 

 aiul to fix it upon the wheel. 



4. If the mnnber of bars be increased in the same given 

 space, no other difference will result than a greater multiplica- 

 tion 



