OPTICAL TOYS. 317 



ently by means of a wheel and band) ; on looking at the disc all 

 the colours will be seen blended together, producing the appearance 

 of a greyish white disc, if the shades of colour are properly chosen 

 and the sizes of the sectors properly apportioned. 



In this experiment, as in that indicated by fig. 158, the result 

 is mainly due to the fact that an impression conveyed to the eye 

 lasts for an appreciable time before it fades ; so that if a red sector 

 is before the eye at any given moment, the sensation of redness 

 lasts during the period whilst the other colours are successively 

 presented to the eye by the revolution of the disc ; and similarly 

 with these, so that all are practically viewed simultaneously. 



As in Expt. 350, if any one of the sectors be rendered invisible 

 (e.g., by pasting over it a piece of black cloth, &c.), the effect 

 produced on rotating the disc will be that of a disc tinted 

 with some kind of colour or other, the combination of sen- 

 sations now produced by the light from the disc on reaching 

 the observer's eye being no longer that producing the sensation of 

 whiteness. 



Expt. 352. Chromatropes and Colour Tops. A variety of 

 optical colour toys are in the market, depending on much the same 

 principles as Newton's disc. Usually a wheel or top, or some 

 similar rotating contrivance, is set spinning by means of a piece of 

 string wound round the axle and then pulled away rapidly. 

 Coloured discs, tinted plates shaped as sectors or otherwise, &c., are 

 applied in fashions variable to some extent at will, so that as the 

 apparatus revolves various combinations of colour sensations can 

 be produced, according to the particular colours employed on the 

 tinted surface rendered visible and the relative proportions of 

 these surfaces as regards area. In all cases the ultimate colour 

 effect is due to the combination of a number of separate colour 

 effects, viewed one after the other so rapidly that the whole series 

 is rendered visible before the effect of the first has subsided ; just 

 as a stick with a glowing ember at the end, when whirled through 

 the air sufficiently rapidly, produces the sensation of a complete 

 circle of fire. 



Expt. 353. Thaumatropes and Zoetropes. Several ingenious 

 optical toys are also known, based on the above principle of per- 

 sistence of visual impressions, although the phenomena of colour 

 are not necessarily here involved. One of the oldest of these 

 thaumatropes consists of a disc, capable of spinning about one of 

 its diameters as an axis (fig. 159). On one side is painted part 

 of a device or picture, &c., such as a bird or mouse, and on the 

 other side the other half, e.g., a cage or trap. On setting the disc 

 spinning and looking at it, either directly or in a mirror, both sides 



