302 On a Peculiar Class of Optical Deceptions. [1831. 



lines which appear upon the lower wheel, and are produced by 

 the shadow of the upper. These are the same in form and dis- 

 position as the former, and are even more distinct ; they should 

 be viewed, not through the upper wheel, but directly upon the 

 lower ; their explanation has in part been given, and will be 

 sufficiently evident. 



The form which the appearance occasionally assumes when 

 a carriage wheel is revolving before upright bars, is exceedingly 

 well shown by the little machine described (fig. 4), when mounted 

 with a single wheel carrying several equal radii at equal di- 

 stances. The bars of the grate should be equidistant, the in- 

 tervals between them being about that between the extremities 

 of two contiguous spokes of the wheel. The varied appear- 

 ances produced by varying the motion of the wheel and grate, 

 both in direction and velocity, will be better understood from a 

 few easy experiments than from any description. 



The lines which thus occur may any one of them be imitated 

 by the two cardboard bars held and moved in the hand ; the 

 whole system may then be obtained at once if one of the inde- 

 pendent wheels (fig. 1) be revolved by the pin between the 

 fingers, and a single pasteboard bar (of equal width with the 

 radii) passed once, not too rapidly, before it ; by returning the 

 bar the lines are seen a second time. Should the eye not 

 readily catch the appearance, a black instead of a white single 

 bar may be used, or a shadow be thrown by an opake bar from 

 a candle, or the sun, upon the revolving wheel ; and then, to 

 extend and follow out the forms, the bar should be moved 

 to and fro slowly before the revolving wheel, to the extent of 

 one half or the whole length of a radius, when it will imme- 

 diately be seen that all the lines produced, even when a grate 

 is used, are merely the courses of so many points of intersection 

 between the radii of the wheel and the bars passing before or 

 behind it. 



A variation in the mode of observing many of these curious 

 spectra, but which still further supports the explication given, 

 is to cast the shadows of the revolving wheels, either by sun 

 or candlelight, upon a screen, and observe their appearance. 

 The way in which the cogs or radii of the wheels shut out more 

 or less of a background from the eye, as already described, 

 will enable them, to an equal degree, to intercept light, which 



