1831.] On a Peculiar Class of Optical Deceptions. 299 



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 

 5 or 6 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 background being beyond, it will spread a tint, as it 

 were, over the space through which it moves (fig. 12). A simi- 

 lar 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 quadran- 

 gular 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 card- 

 board rods (fig. 13). 



This appearance is visible even when the rods are several 

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

 ally 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 

 looking 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 ; but the 

 explanation is not quite the same, though nearly so it will 

 suggest itself. Two bright pins or needles produce the effect 

 very well in diffuse daylight ; and the line produced by the 

 shadow of one on the other, and that belonging to the intersec- 

 tion, are easily distinguished and separated. 



If, whilst a single bar is moved in one hand, several bars or 



