137 
an optical deception, &c. 
to, leave in the eye the trace of a continuous curve line ; and 
the spokes will appear to be curved, instead of straight. 
The theory now advanced is in perfect accordance with 
all the phenomena already detailed, and is farther confirmed 
by extending the experiments to more complicated combi- 
nations. 
It readily explains why the image, or spectrum, as it may 
be called, of the spoke, is at rest, although the spoke itself 
be revolving : a circumstance which might escape notice, if 
the attention were not particularly called to it. 
Since the curved appearance of the lines results from the 
combination of a rotatory, with a progressive motion of the 
spoKes, in relation to the apertures through which they are 
viewed, it is evident that the same phenomena must be 
produced if the bars be at rest, and both kinds of motion 
be united in the wheel itself. For, whether the bars move 
horizontally with respect to the wheel, or the wheel with 
respect to the bars, the relative motion between them, and 
its effects, in as far as concerns^-l^e appearance in question, 
must be the same. The attention of the spectator should in 
both cases be wholly directed to the wheel, so that the motions 
in question should be referred altogether to it. Thus, in 
fig. 4, the real positions, at successive intervals of time, of 
the spoke A a, when zi\e wheel is rolling on the ground in 
the direction AZ, are expressed by the lines A^z, B6, Cc and 
T>d. While the spoke is iu these positions, the portions of 
it really seen through the hi^^ed aperture VW, are the parts 
a, /3, y, S, the impressions of which, being retained upon the 
retina, and referred to the wheel when in its last position,. 
MDCCCXXV. ' T 
