an optical deception, &c. 133 
Individual spoke appears, during the moment it is viewed, 
to be at rest. 
2. The number of spokes in the wheel makes no difference 
in the degree of curvature they exhibit. 
3. The appearance of curvature is more perfectly seen when 
the intervals between the bars through which the wheel is 
viewed, are narrow ; provided they are sufficiently wide to 
allow of the distinct view of all the parts of the wheel in 
succession, as it passes along. For the same reason, the phe- 
nomenon is seen to the greatest advantage when the bars are 
of a dark colour, or shaded, and when a strong light is thrown 
upon the wheel. The deception is, in like manner, aided by 
every circumstance which tends to abstract the attention 
from the bars, and to fix it upon the wheel. 
4. If the numbers of bars be increased in the same given 
space, no other difference will result than a greater multi- 
plication of the curved images of the spokes ; but if a cer- 
tain relation be preserved between the angles subtended at 
the eye by the whole intervals of the bars, and of the extre- 
mities of the spokes, this multiplication of images may be 
corrected. The distance of the wheel from the bars is of no 
consequence, unless the latter are very near the eye, as in 
that case the apertures between them may allow too large a 
portion of the wheel to be seen at once. 
5. If the bars, instead of being vertical, are inclined to the 
horizon, the same general appearances result ; but with this 
difference, that the spokes occupying positions parallel to the 
bars, are those which have no apparent curvature : while the 
curvatures of the other spokes bear the same relations to 
these straight spokes, and to each other, that they did in the 
