Royal Institution of Great Britain. 335 



entirely different ; yet all these deceptive appearances result from a 

 single reflection 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 illustration, 

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

 vals 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 reflection 

 observed in the ordinary way will exhibit one uniform colour ; but 

 if the reflection be observed from between or behind the cogs, the 

 green and red immediately 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, be again developed, and it is easy in this way to 

 separate many colours from each other. The experiment in illus- 

 tration of Newton's theory of colour, by painting the head of a top 

 and spinning it, is well known ; by the means just described the 

 experiment can be still further extended, and the colours separated 

 one from another, even while the whole system remains in motion. 



The combination of other forms than wheels by the apparatus 

 described, page 208, produces very beautiful effects. The applica- 

 tion of colours here also is so evident as to need no illustration. 

 The variation of the proportion of the interval to the remaining 

 pasteboard causes many curious appearances, especially when the 

 shadows produced in sun-light are observed. 



Since the printing of the paper, a friend has referred me to the 

 article * Animalcula,' in Brewster's Encyclopedia, where an. 

 opinion on the appearance of these creatures is given, nearly the 

 same as that I have ventured. Speaking of the opinions of those 

 who suppose them to be true revolutions, it is said, * Yet notwith- 



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