peculiar Class of Optical Deceptions. 21 3 



revolution. Fig. 11. represents the fixed spectrum produced 

 by two equal wheels of eight radii each. When the radii or 

 spokes are narrow, the difference in the intensity of tint be- 

 tween the middle and the edges of each image of a spoke is so 

 slight as to be scarcely perceptible. But as this circumstance 

 and many others will explain themselves immediately they are 

 experimentally observed, it is unnecessary to dwell minutely 

 upon them here. 



A very simple experiment will render the whole of these 

 effects perfectly intelligible. If a little rod of white cardboard 

 five or six inches long, and one-thirtieth of an inch wide, be 

 moved to and fro from right to left before the eye, an obscure 

 or black back-ground being beyond, it will spread a tint, as it 

 were, over the space through which it moves (Fig. 12.) A 

 similar rod held and moved in the other hand will produce the 

 same effect ; but if these be visually superposed, i. e., if one be 

 moved to and fro behind the other, also moving, then in the 

 quadrangular space included within the intersection of the two 

 tints will be seen a black line sometimes straight, and connecting 

 the opposite angles of the quadrangle; at other times oval or 

 round, or even square, according to the motions given to the 

 two cardboard rods (Fig. 13.) 



This appearance is visible even when the rods are several 

 inches or a foot apart from each other, provided they are 

 visually superposed. It is produced exactly as in the former 

 case, and the black line is in fact the path of the intersecting 

 point of the moving rods. As their motions vary, so does the 

 course of this point change, and wherever it occurs, there is 

 less eclipse of the black ground beyond than in the other parts, 

 and consequently less light from that spot to the eye than from 

 the other portions of the compound spectrum produced by the 

 moving rods, 



In this experiment the eye should be fixed, and the part 

 looked at should be between the planes in which the rods are 

 moved. The variation produced by using black rods, and look- 

 ing at a white ground, will suggest itself. Those who find it 

 difficult to observe the effect at first, will instantly be able to 

 do so if the rod nearest the eye is black, or held so as to throw 

 a deep shade : the line is then much more distinct j but the 



