1831.] On a Peculiar Class of Optical Deceptions. 301 



When the motion of the wheels upon the machine is in the 

 same direction, the velocities equal, and the eye placed in the 

 prolongation of the axis of the wheels, no particular effect takes 

 place. If it so happens that the cogs of one coincide with those 

 of the other, the uniform tint belonging to one wheel only is 

 produced. If they project by the side of each other, it is as if 

 the cogs were larger, and the tint is therefore stronger. But 

 when the velocities vary, the appearances are very curious ; the 

 spectrum then becomes altogether alternately light and dark, 

 and the alternations succeed each other more fapidly as the 

 velocities differ more from each other. 



When wheels with radii are put upon the machine, it is easy 

 to observe, in perfection, the optical appearance already referred 

 to, as exhibited by carriage wheels, &c. (fig. 2). They should 

 be looked at obliquely, so as to be visually superposed only in 

 part ; and provided the wheels are alike, and both revolving in 

 the same direction with equal velocity, they immediately assume 

 the form described, passing in curves from the axis of one wheel 

 to the axis of the other, and much resembling in disposition 

 the curves formed by iron filings between two opposite poles 

 of a magnet. 



If the wheels revolve in opposite directions, then the spectral 

 lines, originating at each axis as a pole, have another disposi- 

 tion, and instead of running the one set into the other, are dis- 

 posed generally like the filings about two similar magnetic poles, 

 as if a repulsion existed : not that the curves or the cause are 

 the same, but the appearances are similar. A very little atten- 

 tion will show that all these lines are the necessary consequence 

 of the travelling of successive intersecting points ; and any one 

 of them may be followed out by experimenting with the two 

 pasteboard rods already described, these being moved in the 

 hand as if each were the spoke of a wheel. 



All these effects may be simply exhibited by cutting out two 

 equal pasteboard wheels without rims, passing a pin as an axis 

 through each, spinning one upon a mahogany or dark table, 

 and then spinning the other between the fingers over it, so that 

 the two may be visually superposed. If the appearances are 

 observed by a lamp or candle, the wheels should be so held to 

 the light that the shadow of the tipper may not fall upon the 

 lower, otherwise the effects are complicated by similar sets of 



