NEWTON'S DISC. 541 



easy it is to observe the effects resulting from this 

 duration of impressions. If a circular piece of card- 

 board, of which the four quarters are alternately 

 covered by black and white paper, as in fig. 296, is 

 fixed on the whirling- table and rapidly turned, the 

 disc will appear uniformly grey. The reason of this 

 is that we never really lose the impression of the 

 white, nor that of the black. The im- 

 pressions of white and black continue 

 together; hence the disc appears grey, 

 which is nothing else but a mixture of 

 white and black. 



If instead of a disc containing only FIG. 296 (i real size}. 

 white and black, one containing the seven principal 

 colours of the spectrum, as fig. III. on the frontis- 

 piece, is fixed to the whirling table, it will also 

 appear grey if rapidly turned. Since white light may 

 be revolved into coloured light, coloured light should 

 be capable of being compounded so as to produce 

 again white light ; and apparatus of various kinds are 

 constructed, by means of which the colours of the 

 spectrum may be again reunited into a pure white 

 light. A disc coloured similarly to that in fig. III. 

 is called a 'Newton's disc,' because Sir Isaac Newton 

 demonstrated by it the recomposition of decomposed 

 light. But for two reasons our disc does not repro- 

 duce white light when turned rapidly. In the first 

 place, consider one of the coloured sectors for ex- 

 ample, the red one. If this strip is to appear white, it 

 must give out at the same time the light of the 

 other colours, viz., orange, yellow, green, &c. ; in 

 other words, the quantity of light given out by the 



