135 
of an optical deception, &c. 
one spoke, thus seen separately, might possibly connect 
themselves with portions of the two adjoining spokes, and so 
on, forming by their union a curved image made up of parts 
from different successive spokes. But a little attention to the 
phenomena will show that such a solution cannot apply to 
them : for when the disc of the wheel, instead of being 
marked by a number of radiant lines, has only one radius 
marked upon it, it presents the appearance, when rolled be- 
hind the bars, of a number of radii, each having the curva- 
ture corresponding to its situation ; their number being 
determined by that of the bars which intervene between the 
wheel and the eye. So that it is evident, that the several 
portions of one and the same line, seen through the intervals 
of the bars, form on the retina the images of .^o many dif- 
ferent radii. 
The true principle, then, on which this phenomenon de- 
pends, is the same as that to which is referable the illusion 
that occurs when a bright object is wheeled rapidly round in 
a circle, giving rise to the appearance of a line of light 
throughout the whole circumference : namely, that an im- 
pression made by a pencil of rays on the retina, if sufficiently 
vivid, will remain for a certain time after the cause has 
ceased. Many analogous facts have been observed with 
regard to the other senses, which, as they are well known, 
it is needless here to particularize. 
In order to trace more distinctly the operation of this prin- 
ciple in the present case, it will be best to take the pheno- 
menon in its simplest form, as resulting from the view of a 
single radius, (fig. 2.) OR of the wheel VW, revolving steadily 
upon its axis, but without any progressive motion, and seen 
