gradually become straight, and when from three feet to any greater distance from the glass, will see the spectrum of the reflected image, having as many dark radii upon it as there are radii in the wheel he is revolving. Whatever the velocity, or however irregular the motion of the wheel, these lines are perfectly stationary. The explanation of the change of form and ultimate appearance of the whole, and of the number and fixed position of the lines, will be so evident when the experiment is made, in conjunction with what has been said, as to require no further statement here.
A very striking deception may be obtained in this way, by revolving a single cog wheel (fig 6) between the fingers before the glass, when from twelve to fifteen or eighteen feet from it. It is easy to revolve the wheel before the face so that the eyes may see the glass through or between the cogs, and then the reflected image appears as if it were the image of a cog-wheel, having the same number of cogs, but perfectly still, and every cog distinct; instead of being the image of one in such rapid motion, that by direct vision the cogs cannot be distinguished from each other, or their existence ascertained. The effect is very striking at night if a candle be placed just before the face, and near to it, but shaded by the wheel; in the reflexion the wheel is then well illuminated, and the reflected face or shadow forms a good background against which to observe the effect.
I have, perhaps, already rendered this paper longer than necessary; but the singularity of the appearances, and the facility with which they may be observed, have induced me to suppose that many persons would like to repeat the experiments, and must be my excuse for some further variations in the mode of experimenting.
A disc of cardboard, about two inches and a half in diameter, was cut into a wheel like fig. 16; another disc, rather larger, was cut into a similar wheel, and then the radii of one were twisted obliquely like the wings of a ventilator, and the radii of the other similarly set, but in the opposite direction: a small hole being made in the centre of each, a large pin was passed through that of the smaller wheel, and then a small piece of cork passed on to the pin to hold the wheel near the head, but free to turn; two or three beads were then added, the second
