COMMON THINGS COLOUR. 



spectrum. If the centre of this card be placed upon a spindle, 

 and a very rapid motion of rotation be imparted to it, the ring on 



Fig. 11. 



which the seven colours are painted will present the appearance of 

 a greyish white. In this case, if all the colours except one were 

 covered with black, the revolving card would present the appear- 

 ance of a continuous ring of that colour ; and, consequently, if 

 all the coloured spaces be uncovered, seven continuous rings of 

 the several colours would be produced ; but these rings being 

 superposed and mingled together will produce the same effect on 

 the sight as if all the seven colours were mixed together in the 

 proportion which they occupy on the card. If the colours were as 

 intense and as pure as they are in the spectrum, the revolving 

 card would exhibit a perfectly white ring ; but as the colours of 

 natural bodies are never perfectly pure, the colour produced in 

 4his case is greyish. 



This experiment may be further varied by leaving uncovered 

 any two, three, or more combinations of the colours depicted on 

 the card. In such case the rotation of the card produces the 

 appearance of a ring of that colour which would result from the 

 mixture of the colours left uncovered ; thus, if the red and 

 yellow spaces remain uncovered, the card will produce the appear- 

 ance of an orange ring ; if the yellow and blue remain uncovered, 

 it will produce the appearance of a green ring; and so on. 



The following pretty experiment, illustrating the recomposition 

 of ligb.t, was suggested by Newton. 



The spectrum is received upon seven plane reflectors, as shown 

 in fig. 10, p. 57, which are so suspended as to be capable of 

 shifting their planes at pleasure. They are so adjusted as to receive 

 the light proceeding from the prism, which correspond to the seven 

 different colours, and to reflect it to the same point upon a screen 

 72 



