upon the axe, and, being held sufficiently firm by the friction of the cork, turned with them. By these arrangements the axes could be changed, or the wheels shifted, or the velocities altered without the least delay.
The beauty of many of the effects obtained by this apparatus has induced me to describe it more particularly than I otherwise should have done. The appearance which I first had shown to me by Mr. Maltby was exhibited very perfectly; two equal cog-wheels (fig. 6) were mounted, so as to have equal opposite velocities; when put into motion, which was easily done by the thumb and finger applied to the upper pulley of the horizontal copper plate, they presented each the appearance of a uniform tint at the part corresponding to the series of cogs or teeth, provided that the eye was so placed as to see the whole of both wheels; but when a position was taken up so that the wheels were visually superposed, then, in place of a uniform tint, the appearance of teeth or cogs was seen—misty but perfectly stationary, whatever the degree of velocity given to the wheel. By cutting the cogs or teeth in the wheel nearest to the eye, deeper (fig. 7), the eye could be brought into the prolongation of the axes of the wheels, and then the spectral cog-wheel appeared perfect (fig. 8). The number of intervals thus occurring was exactly double the number of teeth in either wheel: thus a wheel with twelve teeth produced twenty-four black, and twenty-four white alternations. When one wheel was made to move a little faster than the other, by shifting the wooden roller on its axis, then the spectrum travelled in the direction of' that wheel having the greatest velocity; and with more rapidity the greater the difference between the velocities of the two wheels. When the wheels were looked at so that they visually superposed each other in part, the effect took place only in those parts: and it was striking and extraordinary to observe two uniform tints mingling, and instantly breaking out into the alternations of light and shade which I have described. There are many variations in the curvature and other appearances obtained by altering the position of the eye, which will be at once understood when observed, and which for brevity's sake I refrain from describing.
Wheels were then fixed on the machine, consisting of radii
