212 Mr. Faraday on a 



which each alone can produce is soon broken up in the super- 

 posed parts into lighter and darker portions, and when the velo- 

 cities of both are equal, the spectrum is resolved into a certain 

 number of light and dark alternations, which are perfectly fixed, 

 and which, to the mind, offer a singular contrast to the rapidly 

 moving state of the wheels, and to the variations which their 

 velocity may undergo without altering the visible result. 



This effect, strange as it at first appears, will be easily under- 

 stood by reference to Fig. 9. Suppose the eye directed to the 

 part I beyond the cogs, and the sets of cogs to be moving with 

 equal velocities in the opposite directions, indicated by the 

 arrow heads : the part I will be eclipsed by the cogs a and b 

 simultaneously, and for exactly the same time, for they begin to 

 cover it and they leave it together ; I therefore is alternately 

 open to and shut from the eye for equal times ; for what 

 these cogs have done, will be performed by all the other cogs 

 in turn, and the cogs are equal in area to the spaces between : 

 half the light, therefore, from that part of the back-ground 

 comes to the eye, and produces a corresponding impression. 

 But with respect to the point d, although the cog 6 is just 

 leaving it exposed, the cog a is just beginning to eclipse it ; 

 and by the time the latter has passed over, the edge of the 

 cog e will be upon the spot, and that cog will therefore hide 

 it until/ comes up ; so that in fact the point d is always hidden, 

 no light comes from that part of the back-ground, and it con- 

 sequently appears dark V is circumstanced just as I was, for 

 the cogs a and e cover it simultaneously, and so do all the 

 other cogs in pairs ; it is therefore a light space in the spec- 

 trum : d' is a repetition in everything of d, and is a dark space. 

 The parts intermediate between the maxima of light and dark- 

 ness will, by examination, be found to be eclipsed for inter- 

 mediate periods, and to appear more or less dark in conse- 

 quence, so that the appearance of the spectrum belonging to 

 the visually superposed parts of the two sets of cogs is as in 

 Fig. 10. 



In the case of equal wheels with radii, the fixed spectrum 

 produced when the wheels superpose each other has twice the 

 number of radii of either wheel, that being of course the num- 

 ber of times which the radii coincide with each other in one 



