or spokes, each having twelve, equal in length and width (fig. 1). When revolved alone, each wheel gave, with a certain velocity, a perfectly regular tint; but when visually superposed, there appeared a fixed wheel, having twenty-four spokes, equal in dimensions to the original spokes. Variations of the position of the eye, or of the relative velocity of the two wheels, caused alterations similar to those I have referred to with the cog-wheels.
In observing these effects, either the wheels should be black or in shade, whilst the part beyond is illuminated; or else the wheels should be white and enlightened, whilst the part beyond is in deep shade. The cog-wheels present nearly a similar appearance in both cases, though in reality the parts of the spectrum which appear darkest by the one method are lightest by the other. The spoke-wheels give a spectrum having white radii in the first method and dark radii in the second. Placing the wheels between the eye and the clouds, or a white wall, or a lunar lamp, answers well for the first method; and for the second, merely reversing the position and allowing the light to shine on the parts of the wheel towards the eye, whilst the background is black, or in obscurity, is all that is required. Strictly, the phenomena should be viewed with one eye only, but it is not often that vision with two eyes disturbs the effects to any extent.
The cause of these appearances, when pointed out, is sufficiently obvious, and immediately indicates many other effects of a similar kind, and equally striking, which are dependent upon it. The eye has the power, as is well known, of retaining visual impressions for a sensible period of time; and in this way, recurring actions, made sufficiently near to each other, are perceptibly connected, and made to appear as a continued impression. The luminous circle visible when a lighted coal or taper is whirled round—the beautiful appearances of the kaleidophone—the uniform tint spread by the revolution of one of the spoke- or cog-wheels already described —are a few of the many effects of this kind which are well known.
But during such impressions, the eye, although to the mind occupied by an object, is still open, for a large proportion of time, to receive impressions from other sources; for the original
