peculiar Class of Optical Deceptions^, 217 



tersection between the radii of the wheel and the bars passing 

 before or behind it. 



A variation in the mode of observing many of these curious 

 spectra, but which still further supports the explication given, 

 is to cast the shadows of the revolving wheels, either by sun 

 or candle-light, upon a screen, and observe their appearance. 

 The way in which the cogs or radii of the wheels shut out more 

 or less of a back-ground from the eye, as already described, 

 will enable them, to an equal degree, to intercept light, which 

 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 back-ground. 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 shewing 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 direc- 

 tion, 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. Wheatstone 

 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 



