iiot so easy to understand why they do not connect themselves: 

 in the imagination, as in other cases of broken lines, so as to 

 convey the impression of a straight spoke. The idea at first 

 suggests itself that the portions of one spoke, thus seen sepa- 

 rately, 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 Httle attention to the phsenomena 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 behind the bars, of a number of radii, each having the 

 curvature 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 por* 

 tions of one and the same line, seen through the intervals of 

 the bars, form on the retina the images of so many different 

 radii. 



The true principle, then, on which this phaBUomenon depends, 

 is the same as that to which is referable the illusion that occur* 

 xvhen a bright object is wheeled rapidly round in a circle, giving 

 tise to the appearance of a hne of light throughout the whole 

 circumference ; namely, that an impression 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 princi- 

 ple in the present case, it will be best to take the phaenomenon 

 m 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 through a 

 single narrow vertical aperture which is moving horizontally in 

 a given direction PQ. Let us also assume that the progressive 

 motion of the aperture is just equal to the rotatory motion of the 

 circumference of the wheel. It is obvious that if, at the time of 

 the transit of the aperture, the radius should happen to occupy 

 either of the vertical positions VO or OW, the whole of it would 

 be seen at once through the aperture, in its natural position ; 

 but if, while descending in the direction VR, it should happen 

 to be in an oblique position RO, terminating at any point of the 

 circumference at the moment the aperture has, in its progress 

 horizontally, also arrived at the same point R, the extremity of 

 the radius will now first come into view, while all the remaining 

 part of it is hid. By continuing to trace the parts of the radius 

 that are successively seen by the combined motions of the aper- 

 ture and of the radius, we shall find that they occupy a curve 

 "R. a be d generated by the continued intersection of these two 

 finest Thus, when the aperture has moved to A, the radiut 



