lines which appear upon the lower wheel, and are produced by the shadow of the upper. These are the same in form and disposition as the former, and are even more distinct; they should be viewed, not through the upper wheel, but directly upon the lower; their explanation has in part been given, and will be sufficiently evident.
The form which the appearance occasionally assumes when a carriage wheel is revolving before upright bars, is exceedingly well shown by the little machine described (fig. 4), when mounted with a single wheel carrying several equal radii at equal distances. The bars of the grate should be equidistant, the intervals between them being about that between the extremities of two contiguous pokes of the wheel. The varied appearances produced by varying the motion of the wheel and grate, both in direction and velocity, will be better understood from a few easy experiments than from any description.
The lines which thus occur may any one of them be imitated by the two cardboard bars held and moved in the hand; the whole system may then be obtained at once if one of the independent wheels (fig. 1) be revolved by the pin between the lingers, and a single pasteboard bar (of equal width with the radii) passed once, not too rapidly, before it; by returning the bar the lines are seen a second time. Should the eye not readily catch the appearance, a black instead of a white single bar may be used, or a shadow be thrown by an opakc bar from a candle, or the sun, upon the revolving wheel; and then, to extend and follow out the forms, the bar should be moved to and fro slowly before the revolving wheel, to the extent of one half or the whole length of a radius, when it will immediately be seen that all the lines produced, even when a grate is used, are merely the courses of so many points of intersection between the radii of the wheel and the bars passing before or behind it.
A variation in the mode of observing many of these curious spectra, but which still further supports the explication given, is to cast the shadows of the revolving wheels, either by sun or candlelight, upon a screen, and observe their appearance. The way in which the cogs or radii of the wheels shut out more or less of a background from the eye, as already described, will enable them, to an equal degree, to intercept light, which
