THE SENSATION OF SIGHT. 211 



colours of the several primary forms of light are best seen in 

 the spectrum produced by a narrow streak of light passing 

 through a glass prism ; they are at once the fullest and the most 

 brilliant which the external world can show. 



When several of these colours are mixed together, they give 

 the impression of a new colour, which generally seems more or 

 less white. If they were all mingled in precisely the same pro- 

 portions in which thoy are combined in the sun-light, they 

 would give the impression of perfect white. According as the 

 rays of greatest, middle, or least wave-length predominate in 

 such a mixture, it appears as reddish-white, greenish-white, 

 bluish-white, and so on. 



Everyone who has watched a painter at work knows that 

 two colours mixed together give a 

 new one. Xow, although the results 

 of the mixture of coloured light 

 differ in many particulars from those 

 of the mixture of pigments, yet on 

 the whole the appearance to the eye 

 is similar in both cases. If we 

 allow two different coloured lights to 

 fall at the same time upon a white 

 screen, or upon the same part of 



our retina, we see only a single compound colour, more or less 

 different from the two original ones. 



The most striking difference between the mixture of pig- 

 ments and that of coloured light is, that while painters make 

 green by mixing blue and yellow pigments, the union of blue 

 and yellow rays of light produces white. The simplest way of 

 mixing coloured light is shown in Fig. 33. p is a small flat 

 piece of glass ; b and g are two coloured wafers. The observer 

 looks at b through the glass plate, while g is seen reflected in 

 the same ; and if g is put in a proper position, its image exactly 

 coincides with that of b. It then appears as if there was a 

 single wafer at b, with a colour produced by the mixture of the 

 two real ones. In this experiment the light from b, which 

 traverses the glass pane, actually unites with that from g, which 

 p2 



