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



extremities ; each has upon it a little roller or disc of soft 

 wood, which, though it can be moved by force from one part 

 of the axis to another, still has friction sufficient to carry the 

 latter with it when turned round. These axes are made to 

 revolve in the following manner: A circular copper plate 

 about 4 inches in diameter has three pulleys of different 

 diameter fixed upon its upper surface, whilst its lower surface 

 is covered with a piece of fine sand-paper attached by cement. 

 A hole is made through the centre of the plate and pulleys, 

 and guarded by brass tube, so fitted as to move steadily but 

 freely upon an upright steel pin fixed in the middle of the 

 centre wooden support ; hence when the plate is in its place, 

 it rests upon the two rollers belonging to the horizontal axes, 

 whilst it is rendered steady by the upright pin. It can easily 

 be turned round in a horizontal plane, and it then causes the 

 two axes with their rollers to revolve in opposite directions, 

 and the velocities of these can be made either equal to each 

 other, or to differ in almost any ratio by shifting the rollers 

 upon the horizontal axes nearer to, or further from the centre 

 of the stand. 



To produce motions of the axes in the same direction, an 

 aperture was cut in the lower part of the upright board ; a 

 roller, turned for it, which loosely fitted within the aperture ; 

 and a steel pin or rod passed as an axis through the roller. 

 The roller hangs in its place by endless lines made of thread, 

 passing under it, and over little pulleys fixed on the horizontal 

 axes ; when, therefore, it is turned by the projecting pin, it 

 causes the revolution of the axes. The variation in velocities 

 is obtained by having the roller of different diameters in dif- 

 ferent parts, and by having pulleys of different dimensions. 

 This description will be easily understood by reference to the 

 figures. 



This apparatus had to carry wheels either with cogs or 

 spokes ; which was contrived in the following manner : The 

 wheels were cut out of cardboard, were about 7 inches in 

 diameter, and were formed with cogs or spokes at pleasure. 

 A piece of cork, being the end of a phial cork, about the tenth 

 of an inch in thickness, was then fastened by a little soft cement 

 to the middle of the wheel, and a needle run through both, 

 and then withdrawn, These wheels could at any time be put 



