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 superposed only in part; and provided the wheels are alike, and both revolving in the same direction with equal velocity, they immediately assume the form described, passing in curves from the axis of one wheel to the axis of the other, and much resembling in disposition the curves formed by iron filings between two opposite poles of a magnet.
If the wheels revolve in opposite directions, then the spectral lines, originating at each axis as a pole, have another disposition, 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 show 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.
All these effects may be simply exhibited by cutting out two equal pasteboard wheel without rims, passing a pin as an axis through each, spinning one upon a mahogany or dark table, and then spinning the other between the fingers over it, so that the two may be visually superposed. If the appearances are observed by a lamp or candle, the wheels should be so held to the light that the shadow of the upper may not fall upon the lower, otherwise the effects are complicated by similar sets of
