peculiar Class of Optical Deceptions. 215 



the spectrum was clear and distinct, but travelled in the direc- 

 tion of the wheel having the greatest number of teeth. When 

 the other wheel was made to move so much faster as to bring 

 an equal number of cogs before the eye or rather any one 

 part of the eye, in the same time as the other, the spectrum 

 became stationary again. The explanations of these variations 

 will suggest themselves immediately the effects are witnessed. 



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 rapidly 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 super- 

 posed C;nly in part ; and provided the wheels are alike, and both 

 revolving in the same direction with equal velocity, they imme- 

 diately assume the form described, passing in curves from the 

 axis of one wheel to the axis of the other, and much resem- 

 bling in disposition those curves formed by iron filings between 

 two opposite poles of a magnet. 



If the wheels revolve in opposite directions, then the spec- 

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

 position, and, instead of running the one set into the other, are 

 disposed 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 attention will shew 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. 



