"The first refers to the kalotrope, which may be briefly described as an arrangement of two concentric wheels, working nearly in contact and in contrary directions. Discs of various devices are provided for the hinder wheel, and a number of perforated black discs for the one in front. When a disc charged with twelve _black_ radii is placed on the hinder wheel, the six spokes of the front wheel, in passing rapidly across it, convert the twelve black radii into twenty-four apparently stationary _white_ radii upon a tinted ground. Here is a remarkable presentation of the complementary, inasmuch as it is placed permanently before the eye by persistence. "The second experiment is performed with the photodrome, which consists of an independent wheel to receive the device discs, and an apparatus (altogether apart, and, if desired, out of sight) by which flashes of light are thrown upon the disc in rapid and regular succession. Now, if a disc charged with twelve dark blue balls, nearly in contact, be placed upon the wheel, and a little natural light be allowed to fall [Page 318] upon it, so soon as it is thrown into rapid revolution, and flashes of artificial light (insulated in a lantern) are duly measured out upon it, we see twelve apparently stationary light-blue balls upon a zone of bright orange. Here, again, there is nothing for which we are not prepared; the complementary is suddenly presented, and it is maintained permanently before the eye by persistence. "A third experiment may prove interesting in its relation to Mr. Smith's ingenious theory. Place the kalotrope opposite a bright northern noonday sky, remove the front wheel, and affix to the hinder wheel one of the perforated black discs used for the kalotropic effects. The experimentalist stands at the back of the instrument, and can see the sky only through the apertures in the black disc. Cause these apertures to pass the eye at intervals varying from one-half to one-sixth of a second, and very remarkable presentations of colour are seen. Under the lower velocities the sky flashes, and assumes an unnatural brilliancy, and the intervals of the fourth and fifth of a second give it sometimes a crimson, at others a deep purple colour. Now, what are we to infer from this experiment? Certainly _not_ that the pulsations have absolutely produced variety of colour. At every pulsation the full natural light falls upon the eye, and the intervals between the pulsations give time for the reaction necessary to the suggestion of complementary colour, and that under manifold modifications arising out of the ever-changing condition of the eye during the experiment. If the apertures pass the eye with a velocity exceeding one-sixth of a second, the effect ceases. There is then perfect persistence, and the eye apprehends nothing but the ordinary light of the sky, reduced in intensity, with nothing to break its uniformity or give it a chromatic character.