SCIENTIFIC USE OF THE IMAGINATION, m 



nishes a means of separating the different classes of 

 waves from each other; in other words, of analysing 

 the light. Sent through a refracting prism, the waves 

 of the sun are turned aside in different degrees from 

 their direct course, the red least, the violet most. They 

 are virtually pulled asunder, and they paint upon a 

 white screen placed to receive them 'the solar spec- 

 trum/ Strictly speaking, the spectrum embraces an 

 infinity of colours; but the limits of language, and of 

 our powers of distinction, cause it to be divided into 

 seven segments: red, orange, yellow, green, blue, in- 

 digo, violet. These are the seven primary or prismatic 

 colours. 



Separately, or mixed in various proportions, the 

 solar waves yield all the colours observed in nature and 

 employed in art. Collectively, they give us the im- 

 pression of whiteness. Pure unsifted solar light is 

 white; and, if all the wave-constituents of such light 

 be reduced in the same proportion, the light, though 

 diminished in intensity, will still be white. The white- 

 ness of snow with the sun shining upon it, is barely 

 tolerable to the eye. The same snow under an over- 

 cast firmament is still white. Such a firmament en- 

 feebles the light by reflecting it upwards; and when 

 we stand above a cloud-field on an Alpine summit, 

 for instance, or on the top of Snowdon and see, in the 

 proper direction, the sun shining on the clouds below 

 us, they appear dazzlingly white. Ordinary clouds, in 

 fact, divide the solar light impinging on them into two 

 parts a reflected part and a transmitted part, in each 

 of which the proportions of wave-motion which pro- 

 duce the impression of whiteness are sensibly pre- 

 served. 



It will be understood that the condition of white- 

 ness would fail if all the waves were diminished equally, 



