An Optical Deception. 283 



bars, the spokes assume the peculiar curvatures, respectively, 

 which are represented in the annexed wood-cut 



in a general manner. The upper and lower spokes, as they 

 pass the vertical, are seen in their natural form, or perpen- 

 dicular ; but in all the rest a curvature will be observed, directed 

 upwards on each side of the vertical, evanescent towards the 

 upper one in particular, and attaining a maximum, as it appears 

 to me, somewhere about that one on each side which occupies 

 the middle of the two lowest quadrants. 



It is indifferent for the production of this effect whether the 

 wheel is running along a plane as it revolves, as in the case of 

 a carriage driving through a street, and viewed through the 

 ordinary iron railing, or whether, as in a fly wheel seen through 

 a similar railing, generally erected in the stair -case of a steam 

 engine, it merely revolves on its own axis. But I must observe 

 that a certain relative distance between the two objects and the 

 eye respectively is convenient; and that the effect is not very 

 perceptible unless the velocity of the wheel exceeds a certain 

 limit. With that velocity, the curvature of the spokes in- 

 creases to a maximum, which is only limited by the total 

 disappearance of the spokes in consequence of their rapidity 



The general principles on which this deception is to be ex- 

 plained will immediately occur to your mathematical readers, 

 but a perfect demonstration will probably prove less easy than 

 it appears at first sight. 



I am, Sir, your obedient servant, 



J. M. 

 London, Dec. 1, 1820. 



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