432 ELEMENTARY PHYSIOLOGY less. 



white and as being "deep," "rich," or "full " if highly 

 saturated, i.e. unmixed with white. 



The colours of objects depend on the power they possess 

 of absorbing some of the constituents of ordinary white 

 light and allowing others to ])ass or to be reflected. Thus 

 a piece of glass is red if it allows the red rays to pass to 

 the eye and stops the others. Similarly the colour of an 

 oparpie red object is due to an absorption of the spectral 

 colours other than red by the superficial layer of the 

 object and the reflection of the unabsorbed red rays from 

 its internal parts. 



When white light has been split up into its coloured 

 constituents by means of a prism, these may be gathered 

 up again by a second prism, suitably placed, and 

 recombined to make white light. In this experiment the 

 several colours of the spectrum are mixed once more after 

 having been sorted out or separated, and the mixing is a 

 physical process. But colours may also be mixed physio- 

 ^^gficrtiii/ by taking advantage of that2)ersistence of luminous 

 impressions to which we have already drawn attention 

 (p. 423). Thus if the several colours of the sj)ectrum are 

 painted in sectors on a circular disc and the disc is made 

 to spin rapidly round its centre, the sensations due to 

 each colour are blended together and the disc appears 

 white. The instrument used in this mode of mixing 

 colours is called a '•^colour top" and the principle it 

 embodies will be not unfamiliar to most people in the 

 form of a common toy. 



By the use of a colour top it is at once possible to mix 

 not merely all the spectral colours but any two or three 

 of them. Experimenting in this way with pairs of 

 colours we find that there are several pairs which 

 when mixed, in the right proportions, give rise to the 

 sensation of white : thus red and green, orange and 

 blue, yellow and indigo-blue, greenish-yellow and violet. 

 Colours which when mixed in this way in pairs give 

 white are known as complementary colours, and 



