CHAPTER XXI. 



COLOUR VISION. 



It is a familiar fact that when ordinary white light is passed 

 through a prism the waves of which it is composed are refracted 

 to a greater or less extent according to their length. As a result 

 the single beam which was made up of waves of all lengths is spread 

 out into a band of light, one end of which is formed by the shortest 

 of all the visible rays, the part immediately next to it by those a 

 little longer, and so on throughout the whole band until the other 

 end is reached, which contains the longest waves that the eye can 

 perceive as light. The physiological effect of this re-arrangement 

 of the waves is to make the light, which when mixed was uncoloured 

 or white, appear to be made up of a whole series of colours, known 

 as the spectrum. The colours are arranged in a definite order 

 according to the length of the waves of each part of the band of 

 light. The shortest of all look violet, those next them blue, the 

 waves next these again green, then yellow, then orange, and then 

 red, first scarlet and then, at the end of the band, crimson. Each 

 colour shades inperceptibly into the one next it, so that as well as 

 those which we have mentioned we can find in the spectrum 

 numbers of others, such as greenish-blue, violet-blue, greenish- 

 yellow, and so on.* 



There are two other ways in which lights may differ from one 

 another. One is in BRIGHTNESS. Of two rays of the same colour 

 if one is more intense than the other it looks brighter. 

 Experiment 66. Take two sheets of paper of the same colour. 



Place one near an electric light of low, and the other near a light of 



high, candle-power. The second looks brighter than the first. 



*Each of the spectral colours may also be produced by the use of pigments. 

 The colours of these depend on the fact that, from the waves of different lengths 

 in a beam of white light falling on them, they absorb all except those which 

 belong to a very limited area of the spectrum. Blue paint, for instance, reflects 

 only the blue waves and a few of the green and absorbs all the others. Yellow 

 glass absorbs most of the wave lengths falling on it except the yellow ones, and 

 light which has passed through it consequently has that colour. 



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