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 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 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 intersection, are easily distinguished and separated.
If, whilst a single bar is moved in one hand, several bars or
