SCIENTIFIC USE OF THE IMAGINATION 119 



or the sulphide of carbon, all the waves are retarded, but 

 the smallest ones most. This furnishes a means of sepa- 

 rating the different classes of waves from each other; in 

 other words, of analyzing the light. Sent through a re- 

 fracting 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 em- 

 braces an infinity of colors; but the limits of language, 

 and of our powers of distinction, cause it to be divided 

 into seven segments: red, orange, yellow, green, blue, 

 indigo, violet. These are the seven primary or prismatic 

 colors. 



Separately, or mixed in various proportions, the solar 

 waves yield all the colors observed in nature and em- 

 ployed in art. Collectively, they give us the impression 

 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 inten- 

 sity, will still be white. The whiteness of snow with the 

 sun shining upon it is barely tolerable to the eye. The 

 same snow under an overcast firmament is still white. 

 Such a firmament enfeebles the light by reflecting it up- 

 ward; 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 m 

 each of which the proportions of wave-motion which pro- 

 duce the impression of whiteness are sensibly preserved. 



