310 On a Peculiar Class of Optical Deceptions. [1881. 



but one or two varieties in the appearances, which have occurred 

 to me since, are so striking, that I am glad of the opportunity 

 of noticing them briefly in the same Number with the paper. 

 At page 304 I have described the singular appearance pro- 

 duced when the reflected image of a revolving cog-wheel, held 

 before a glass, is observed through the cog-wheel itself. If, 

 in such a wheel, a little nearer the centre, a series of regular 

 apertures be cut, so as to represent cogs and their intervals, 

 but the number different by 1.2.3, or any small quantity, from 

 the number of the cogs, then, upon making the experiment as 

 before, that series of cogs in the revolving wheel through which 

 the eye looks will appear to stand still, but the other series 

 will travel in the spectrum : upon changing the eye to the other 

 series of apertures, then the quiescent part of the spectrum 

 will move, and the moving part become quiescent. If two or 

 three series more of such apertures be cut in the wheel, con- 

 centric one to another, but the number of intervals varying in 

 each, then a great variety of changes are produced, as the eye 

 looks through one part or another of the wheel. The series of 

 cogs in the spectrum move with different velocities, or in oppo- 

 site directions, changing with the slightest motion of the eye. 

 Two or three persons looking through different parts of the 

 wheel see appearances entirely different ; yet all these decep- 

 tive appearances result from a single reflexion of a single wheel, 

 moving in a constant direction and with uniform velocity. 



By the application of colours and coloured foils, very curious 

 effects occur, which are endless in their variety. As an illus- 

 tration, let a wheel with a single series of cogs at the edge, and 

 with intervals equal to the cogs, have a circle of colour applied 

 between the cogs and the centre of the wheel ; let the part below 

 the cogs be green, and the part below the spaces red; the 

 coloured circle will consist of green and red alternately. If 

 this wheel be revolved before the glass, the green and red 

 mingle, and the reflexion observed in the ordinary way will 

 exhibit one uniform colour ; but if the reflexion be observed 

 from between or behind the cogs, the green and red imme- 

 diately separate, and besides having the appearance of fixed 

 cogs, there is also the appearance of fixed unmingled colours. 

 If the interval be equal to only half a cog, and three colours be 

 applied, the three colours may, after being mingled by rotation, 



