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BenbanVe '' artificial Spectrum Zo\?:' 



By G. H. Bryan, M.A. 



IT is well known that if a circular disc of cardboard is painted 

 in sectors with the different colours of the spectrum, and is 

 made to spin rapidly as a top or " tee-to-tum," the disc 

 appears white in consequence of the impressions produced by the 

 various colours on the retina remaining behind for a short time 

 and so mixing together. But that the opposite effect can be pro- 

 duced — namely, that of seeing colours on a rapidly revolving top 

 where no colours really exist — is a discovery that one would hardly 

 have expected. Yet such is the result accomplished in the latest 

 optical novelty, invented by Mr. C. E. Benham, and sold by 

 Messrs. Newton and Co. under the name of "The Artificial 

 Spectrum Top." 



Fig. 5. 

 The top consists of a cardboard disc, one half of which is 

 black, while the other is white, with a number of circular arcs of 

 black projecting into it on either side, and subtending an angle of 

 about 45 '^ at the centre. The arrangement of these arcs is imma- 

 terial, provided that the arcs on one side of the centre are arranged 

 not to clash with those on the other, and a possible arrangement 

 is shown in Fig. 5. -When such a disc is mounted on a peg as a 

 top and spun, or carefully centred on a microscopist^s turn-table 

 and rotated in a bright light, the various arcs will produce the 

 appearance of rings, and these rings will appear coloured. If the 

 disc is rotated in the direction of the hands of a clock (as will be 



