1831.] On a Peculiar Class of Optical Deceptions. 305 



wheel put on, and then a second piece of cork ; the end of the 

 pin was then stuck into a quill or a pencil, and thus was formed 

 an apparatus very like a child's windmill, except that it had 

 two sets of vanes, each revolving in opposite directions. On 

 walking across a room towards a window, or a candle, with 

 this little toy in the hand, or blowing at it slightly from the 

 mouth, the lines were beautifully seen, being either stationary 

 or moving, according to the relative velocity of the two wheels. 

 This could be altered at pleasure by inclining the vanes more 

 or less, or by blowing towards the centre of the wheels, or 

 towards the edges when the larger hind wheel received more 

 propulsive force. 



Spinners or whirligigs formed of discs of cardboard stuck 

 upon pins, and upon which radii, either straight or curved, or 

 other forms, had been drawn in bold lines with black ink, when 

 spun upon a sheet of paper, and then looked at through the 

 moving fingers or through equidistant bars of pasteboard moved 

 before them, show a great many of the effects. 



Finally, a couple of open radial wheels (fig. 1) upon pins or 

 wires, if revolved between the fingers in different positions and 

 directions, show a great many of these effects extremely well. 

 Their shadows may be thrown upon each other, or upon the 

 wall ; one may be held near the eye, when it acts like a grate 

 with parallel bars ; and if one side of each wheel is black whilst 

 the other is white, still greater variety may be obtained. They 

 will be quite sufficient, when employed in a few experiments, 

 to make anything in this description clear, which I may have 

 left obscure. 



The curious appearance exhibited by the wheel animalcule 

 has such a resemblance to some of those described in this paper, 

 that they inevitably associate in the mind of a person who has 

 witnessed both effects. This little insect has been well described 

 by Mr. Baker* and others, and can only be viewed distinctly 

 under a high magnifying power ; it then presents an elongated 

 sack-like form (fig. 17), either attached by the posterior part 

 to the side of the vessel containing the water in which it exists, 

 or else floating in the fluid. When the effect in question is 

 observable, there is seen the appearance of two wheels, one on 



* Baker on the Microscope, vol. ii. p. 266; see slso Leeuwenhoek, Phil. 

 Trans., Nos. 283, 295, 337 ; and Adams on the Microscope, p. 548. 



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