peculiar Class of Optical Deceptions. 219 



to suppose that many persons would like to repeat the expe- 

 riments, 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 tlfat 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 

 wheel put on, and then a second piece of cork ; the end of the 

 pin was then stuck into a quill or a pencil, and thus was formed 

 an apparatus very like a child's windmill, except that it had 

 two sets of vanes, each revolving in opposite directions. On 

 walking across a room towards a window, or a candle, with 

 this little toy in the hand, or blowing at it slightly from the 

 mouth, the lines were beautifully seen, being either stationary or 

 moving, according to the relative velocity of the two wheels. This 

 could be altered at pleasure by inclining the vanes more or less, 

 or by blowing towards the centre of the wheels, or towards the 

 edges when the larger hind wheel received more propulsive force. 



Spinners or whirligigs formed of discs of cardboard stuck 

 upon pins, and upon which radii either straight or curved, or 

 other forms, had been drawn in bold lines with black ink, when 

 spun upon a sheet of paper, and then looked at through the 

 moving fingers or through equidistant bars of pasteboard 

 moved before them, shew a great many of the effects. 



Finally^ a couple of open radial wheels (Fig. 1.) upon pins 

 or wires, if revolved between the fingers in different positions 

 and directions, shew a great many of these effects extremely 

 well. Their shadows may be thrown upon each other, or upon 

 the wall ; one may be held near the eye, when it acts like a 

 grate with parallel bars ; and if one side of each wheel is black 

 whilst the other is white, still greater variety may be obtained. 

 They will be quite sufficient, when employed in a few experi- 

 ments, to make, in this description, anything clear which I 

 may have left obscure. 



