peculiar Class of Optical Deceptions. 207 



dfcular to the nave or axis will be straight, but the [others 

 curved ; and the curve will be greatest in those which are far- 

 thest 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 different. 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 pro- 

 bably 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 regarding 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 coincides 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 obtained at pleasure. 



I have had a very simple apparatus constructed, by which 

 these and many other analogous appearances can be shewn 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 sup- 

 port, were supplied w r ith little caps cut out of copper-plate and 

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



* Phiksophiwl Transactions, 1825, p. 121, 



P2 



