THE SENSATION OF SIGHT. 239 



Lave the smallest wave-length. This series of colours is 

 universally known in the rainbow. We also see it if we 

 look towards the light through a glass prism, and a dia- 

 mond sparkles with hues which follow in the same order. 

 In passing through transparent prisms, the primitive 

 beam of white light, which consists of a multitude of 

 rays of various colour and various wave-length, is de- 

 composed by the different degree of refraction of its 

 several parts, referred to in the last essay ; and thus 

 each of its component hues appears separately. These 

 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 proportions in which they are com- 

 bined 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. Now, 

 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 

 pigments and that of coloured light is, that while 



