SCIENTIFIC USE OF THE IMAGINATION. Ill 



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 spectrum.' 

 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 seg- 

 ments : red, orange, yellow, green, blue, indigo, 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 produce 

 the impression of whiteness are sensibly preserved. 



It will be understood that the condition of white- 

 ness would fail if all the waves were diminished equally, 

 or by the same absolute quantity. They must be re- 

 duced proportionately, instead of equally. If by the 

 act of reflection the waves of red light are split into 

 exact halves, then, to preserve the light white, the 



