132 Dr. Roget’s explanation of 
The most remarkable circumstance relating to this visual 
deception is, that the convexity of these curved images of 
the spokes is always turned downwards, on both sides of the 
wheel ; and that this direction of their curvature is precisely 
the same, whether the wheel be moving to the right or to 
the left of the spectator. The appearance now described is 
represented in Plate XL fig. i.* 
In order to discover a clue to the explanation of this phe- 
nomenon, it was necessary to observe the influence which 
certain variations of circumstances might have upon it ; and 
the following are the principal results of the experiments 1 
made for this purpose. 
1 . A certain degree of velocity in the wheel is necessary 
to produce the deception above described. If this velocity be 
gradually communicated, the appearance of curvature is first 
perceptible in the spokes which have a horizontal position : 
and as soon as this is observed, a small increase given to the 
velocity of the wheel, produces suddenly the appearance of 
curvature in all the lateral spokes. The degree of curvature 
remains precisely the same as at first, whatever greater velo- 
city be given to the wheel, provided it be not so great as to 
prevent the eye from following the spokes distinctly as they 
revolve : for it is evident, that the rapidity of revolution may 
be such as to render the spokes invisible. It is also to be 
noticed that, however rapidly the wheel revolves, each 
• The appearance In question has been noticed by an anonymous writer in the 
Quarterly Journal of Science (Vol. X. p. 282) who gives, however, no explanation 
of the phenomenon. It would have been impossible, indeed, to reconcile the 
facts as they are there stated, with any theory that could be imagined for their 
solution. 
