304 On a Peculiar Class of Optical Deceptions. [1831. 



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. Whatever the velocity, or 

 however irregular the motion of the wheel, these lines are per- 

 fectly 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 reflexion 

 the wheel is then well illuminated, and the reflected face or 

 shadow forms a good background against which to observe 

 the effect. 



I have, perhaps, already rendered this paper longer than 

 necessary ; but the singularity of the appearances, and the faci- 

 lity with which they may be observed, have induced me to 

 suppose that many persons would like to repeat the experi- 

 ments, and must be my excuse for some further variations in 

 the mode of experimenting. 



A disc of cardboard, about two inches and a half in diameter, 

 was cut into a wheel like fig. 16; another disc, rather larger, 

 was cut into a similar wheel, and then the radii of one were 

 twisted obliquely like the wings of a ventilator, and the radii 

 of the other similarly set, but in the opposite direction : a small 

 hole being made in the centre of each, a large pin was passed 

 through that of the smaller wheel, and then a small piece of 

 cork passed on to the pin to hold the wheel near the head, but 

 free to turn ; two or three beads were then added, the second 



