218 Mr. Faraday on a 



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

 means of reflection, in a very striking manner. If a white 

 cardboard wheel, with equal radii, he 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 gradually become straight, and when from 

 three feet to any greater distance from the glass, will see the 

 spectrum of the reflected image, having as many dark radii 

 upon it as there are radii in the wheel he is revolving. What- 

 ever the velocity, or however irregular the motion of the wheel, 

 these lines are perfectly stationary. The explanation of the 

 change of form and ultimate appearance of the whole, and of 

 the number and fixed position of the lines, will be so evident 

 when the experiment is made, in conjunction with what has 

 been said, as to require no further statement here. 



A very striking deception may be obtained in this way, by 

 revolving a single cog wheel (Fig. 6.) between the fingers before 

 the glass, when from twelve to fifteen or eighteen feet from it. 

 It is easy to revolve the wheel before the face so that the eyes 

 may see the glass through or between the cogs, and then the 

 reflected image appears as if it were the image of a cog-wheel, 

 having the same number of cogs, but perfectly still and every cog 

 distinct ; instead of being the image of one in such rapid motion, 

 that by direct vision the cogs cannot be distinguished from 

 each other, or their existence ascertained. The effect is very 

 striking at night if a candle be placed just before the face, and 

 near to it, but shaded by the wheel ; in the reflection the wheel 

 is then well illuminated, and the reflected face or shadow 

 forms a good back-ground against which to observe the effect. 



I have, perhaps, already rendered this paper longer than 

 necessary, but the singularity of the appearances and the 

 facility with which they may be observed, have induced me 



