The last of these figures is the rather complicated one, like the back of an “engine-turned” watch. This is called the Chromatrope, or Wheel of Colour, and is always a favourite object in a magic lantern. It consists of two circular plates of glass, one upon the other, and painted in variously coloured curved lines, as seen in the illustration. When the image is thrown upon a screen, and the glass plates turned in opposite directions, a most singular and beautiful effect is produced. The lines, unless the eye follows them very closely, disappear, and torrents of coloured spots seem to pour from the centre to the circumference, or _vice versâ_, according to the direction in which the glass wheels are turned. So perfect is the illusion, that it is almost impossible to believe that the movement is only circular, and not spiral. * * * * * NOW we will pass from Art to Nature. The figure on the left hand of the same illustration represents part of one of the Wheel Animalcules, so called because they look exactly as if the fore-part of their bodies were furnished with two delicate wheels, running rapidly round, and evidently moving or stopping at the pleasure of the owner. Soon after the powers of the microscope became known, these Wheel-bearers were discovered, and for a long time they were thought to have a pair of veritable revolving wheels upon their heads. They were naturally held in high estimation, as, although almost every kind of lever can be found in the animal world, a revolving wheel had never been seen. However, as the defining powers of the microscope improved, the so-called wheels were found not to be wheels at all, but stationary organs, and that their apparent revolution was nothing but an optical delusion. The wheels are, in fact, two discs, around the edges of which are set certain hair-like appendages, called “cilia,” from a Latin word signifying the eyelashes. Each of the cilia has an independent motion of its own, and, as they bend in rapid and regular succession, they produce an effect on the eye similar to that of a revolving body. As for the animal itself, they produce a double effect, either acting as paddles, and forcing the animal through the water, or, when it is affixed to some object, causing a current which drives into its mouth the minute beings on which it feeds. The particular species of Wheel-hearer whose mouth is here shown is called scientifically _Limnias ceratophylli_. It derives the latter name from the fact that it is mostly found on the submerged stems and leaves of the Hornwort (_Ceratophyllum_), which is very common in ponds and slow streams. The creature is, however, to be found on the water-growing plants, and Mr. Gosse, in his “Evenings with the Microscope,” gives a very full and graphic account of itself and its habits.