1831.] On a Peculiar Class of Optical Deceptions. 293 



are furthest from the upper straight line. These curves are 

 the same in form as those already described and explained by 

 Dr. Roget*, and the appearance itself is produced in a similar 

 manner ; but the phenomena are distinct and the causes dif- 

 ferent. The effect at present referred to is best observed 

 when the velocities are great, whereas that explained by Dr. 

 Roget takes place only when the velocities are moderate. It 

 is probable that some of the appearances briefly mentioned by 

 an anonymous writer in the ' Quarterly Journal of Science,' 

 First Series, vol. x. p. 282, and already referred to by Dr. 

 Roget, were of the kind now to be explained ; for though the 

 description is not accurate either for the effects which form 

 the object of this paper, or that explained by Dr. Roget, 

 and is, indeed, inconsistent with the observation or explanation 

 of any of the phenomena, it probably had its origin in the 

 occurrence of some of both kinds under the eyes of the writer. 

 The effect is easily obtained by revolving a white pasteboard 

 wheel before a black or dark ground, and then, whilst re- 

 garding the wheel fixedly, traversing the space before it with 

 a grate also cut out of white pasteboard. By altering the 

 position of the grate and direction of its motion, it will be seen 

 that the straight lines in the wheel are always parallel to the 

 bars of the grate, and that the convexity of the curved lines is 

 always towards that side of the grate where its motion coin- 

 cides in direction with the motion of the radii of the wheel. 

 By varying the velocity of the wheel and grate, the curves 

 change in their appearance, and the whole or any part of the 

 system, as described and figured by Dr. Roget, may be ob- 

 tained at pleasure. 



I have had a very simple apparatus constructed, by which 

 these and many other analogous appearances can be shown in 

 great perfection and variety. One board was fixed upright 

 upon the middle of another, serving as a base ; the upright 

 board was cut into the shape represented in fig. 4 ; the middle, 

 and the two extreme projections, forming points of support, 

 were supplied with little caps cut out of copper-plate and bent 

 into shape (fig. 5), so that, when in their places, they offer 

 four bearings for the support of two axes, one on each side the 

 middle. The axes are small pieces of steel wire tapered at the 

 * Philosophieal Transactions, 1825, p. 131. 



