1831.] On a Peculiar Class of Optical Deceptions. 303 



would otherwise fall upon a screen. When the two equal cog- 

 wheels are revolved so as to have the shadows cast upon a 

 white screen, that shadow exhibits all the appearances and 

 variations observed when the eye is looking by the wheels in 

 shade at a white background. The shadow is light where the 

 wheels appear dark, for there the light has passed by the cogs ; 

 and dark where the wheels appear light, for there the cogs have 

 intercepted most of the rays. The screen should be near to 

 the wheels, that the shadow may be sharp ; and it is convenient 

 to have one wheel of rather smaller radius than the other, or 

 else to place them obliquely to the sun for the purpose of dis- 

 tinguishing the shadow of each wheel, and showing how beau- 

 tifully the spectrum breaks out where they superpose. When 

 the spoke-wheels are revolved they also cast a shadow, pre- 

 senting either the appearance of fixed or moving radii according 

 to the circumstances already described. When the two small 

 spoke-wheels upon one pin are revolved in an oblique direction, 

 their shadow exhibits very beautifully the lines often seen in 

 the wheels of carriages. 



During these experiments the attention cannot but be drawn 

 to the observation of the figures produced by the shadow of 

 one wheel upon the face of the other. These are frequently 

 very beautiful, and combining as they often do with the designs 

 produced, as already described, are occasionally more striking 

 than any of the appearances yet spoken of. Mr. Wheats tone 

 is, however, engaged in an inquiry of a much more general and 

 important kind, which includes these effects, and which, I trust, 

 he will soon give to the public. 



Several of the effects with wheels already described, and 

 some new ones, may be obtained with great simplicity, by means 

 of reflexion, in a very striking manner. If a white cardboard 

 wheel, with equal radii, be fixed upon a pin, and rotated be- 

 tween the fingers before a glass, so that the wheel and its re- 

 flected image may visually superpose in part, the fixed lines 

 will be seen, like those of fig. 2, passing in curves between the 

 axis of the wheel and the reflected image. If the person gra- 

 dually recede from the glass, but still look through the wheel 

 in his hand at the reflected image, i. e. still retain them super- 

 posed, which is best done by bringing the revolving wheel close 

 to the eye, he will see the lines or radii of the reflected image 



