306 NATURE'S TEACHINGS. 



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 versa, 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 stop- 

 ping 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. 



