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310 but one or two varieties in the appearances, which have occurred to me since, are so striking, that I am glad of the opportunity of noticing them briefly in the same Number with the paper. At page 304 I have described the singular appearance produced when the reflected image of a revolving cog-wheel, held before a glass, is observed through the cog-wheel itself. If, in such a wheel, a little nearer the centre, a series of regular apertures be cut, so as to represent cogs and their intervals, but the number different by 1.2.3, or any small quantity, from the number of the cogs, then, upon making the experiment as before, that series of cogs in the revolving wheel through which the eye looks will appear to stand still, but the other series will travel in the spectrum: upon changing the eye to the other series of apertures, then the quiescent part of the spectrum will move, and the moving part become quiescent. If two or three series more of such apertures be cut in the wheel, concentric one to another, but the number of intervals varying in each, then a great variety of changes are produced, as the eye looks through one part or another of the wheel. The series of cogs in the spectrum move with different velocities, or in opposite directions, changing with the slightest motion of the eye. Two or three persons looking through different parts of the wheel see appearances entirely different; yet all these deceptive appearances result from a single reflexion of a single wheel, moving in a constant direction and with uniform velocity.

By the application of colours and coloured foils, very curious effects occur, which are endless in their variety. As an illustration, let a wheel with a single series of cogs at the edge, and with intervals equal to the cogs, have a circle of colour applied between the cogs and the centre of the wheel; let the part below the cogs be green, and the part below the spaces red; the coloured circle will consist of green and red alternately. If this wheel be revolved before the glass, the green and red mingle, and the reflexion observed in the ordinary way will exhibit one uniform colour; but if the reflexion he observed from between or behind the cogs, the green and red immediately separate, and besides having the appearance of fixed cogs, there is also the appearance of fixed unmingled colours.If the interval be equal to only half a cog, and three colours be applied, the three colours may, after being mingled by rotation,