C 131 ] 
V. Explanation of an optical deception in the appearance of 
the spokes of a wheel seen through vertical apertures. By 
P. M. Roget, M. D. F, R. S. 
Read December 9, 1824. 
A CURIOUS optical deception takes place when a carriage 
wheel, rolling along the ground, is viewed through the inter- 
vals of a series of vertical bars, such as those of a palisade, 
or of a Venetian window-blind. Under these circumstances 
the spokes of the wheel, instead of appearing straight, as 
they would naturally do if no bars intervened, seem to have 
a considerable degree of curvature. The distinctness of this 
appearance is influenced by several circumstances presently 
to be noticed ; but when every thing concurs to favour it, the 
illusion is irresistible, and, from the difficulty of detecting its 
real cause, is exceedingly striking. • 
The degree of curvature in each spoke varies according 
to the situation it occupies for the moment with respect 
to the perpendicular. The two spokes which arrive at the 
vertical position, above and below the axle, are seen of 
their natural shape, that is, without any curvature. Those 
on each side of the upper one appear slightly curved ; those 
more remote, still more so ; and the curvature of the spokes 
increases as we follow them downwards on each side till we 
arrive at the lowest spoke, which, like the first, again appears 
straight. 
