would otherwise fall upon a screen. When the two equal cogwheels 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 distinguishing the shadow of each wheel, and showing how beautifully the spectrum breaks out where they superpose. When the spoke-wheels are revolved they also cast a shadow, presenting 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. 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 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 between the fingers before a glass, so that the wheel and its reflected 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 gradually recede from the glass, but still look through the wheel in his hand at the reflected image, i. e. still retain them superposed, which is best done by bringing the revolving wheel close to the eye, he will see the lines or radii of the reflected image
