Flashes of Light upon rapidly rotating Disks. 43 



Roget, " when a carriage wheel, rolling along the ground, 

 is viewed through the intervals 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 curva- 

 ture."— (Phil- Trans. 1825.) 



It was found that " the velocity of the wheel must not 

 be so great as to prevent the eye from following the spokes 

 as they revolve." So that Dr. Roget's experiment relates 

 simply to the curvature of the spokes of a wheel seen 

 through a narrow aperture ; and he accounts for this fact 

 by assuming the deception to arise from separate parts only 

 of each spoke being seen at the same moment ; the remain- 

 ing parts being concealed from view by the bars. He also 

 found that " when the disk 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 

 portions of one and the same line, seen through the inter- 

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

 different radii." 



My experiment differs from that of Dr. Roget, inas- 

 much, that the red and black disk may be made to revolve 

 with very great rapidity, by which the black is lost to the 

 eye, and the red alone reflected, slightly diluted with black. 

 The effect of viewing this disk during rotation through the 

 rotating slitted disk, is to decompose the former, and pre- 

 sent the black and red spaces as distinctly as when at rest, 

 except that the spaces are curved, and, under certain cir- 

 cumstances, increased in number. 



If a white disk be employed, with a single black space 

 passing from the centre to the circumference, and occupying 

 about 20° of the latter, the effect will not be as in the case 

 of the disk of the wheel with only one spoke giving the ap- 

 pearance of a complete wheel, as in Dr. Roget's experiment, 

 but the black space will be brought out in a curved form, 

 and sometimes divided into two. 



