216 Mr. Faraday on a 



All these effects may be simply exhibited by cutting out two 

 equal pasteboard wheels without rims, passing a pin as an axis 

 through each, spinning one upon a mahogany or dark table, and 

 then spinning the other between the fingers over it, so that the 

 two may be visually superposed. If the appearances are ob- 

 served by a lamp or candle, the wheels should be so held to 

 the light that the shadow of the upper may not fall upon the 

 lower, otherwise the effects are complicated by similar sets of 

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

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

 disposition 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 shewn by the little machine described (Fig. 4.), when 

 mounted with a single wheel carrying several equal radii at 

 equal distances. The bars of the grate should be equidistant, 

 the intervals between them being about that between the ex- 

 tremities of two contiguous spokes of the wheel. The varied 

 appearances 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 reyolved 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 opaque 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 

 immediately be seen, that all the lines produced, even when a 

 grate is used, are merely the courses of so many points of in- 



