SCIENTIFIC USE OF THE IMAGINATION. 137 



carbon, all the waves are retarded, but the smallest ones 

 most. This furnishes a means of separating the different 

 classes of waves from each other ; in other words, of ana- 

 lyzing 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 vir- 

 tually pulled asunder, and they paint upon a white screen 

 placed to receive them "the solar spectrum." Strictly 

 speaking, the spectrum embraces 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 employed 

 in art. Collectively, they give us the impression of white- 

 ness. Pure unsifted solar light is white ; and if all the 

 wave-constituents of such light be reduced in the same pro- 

 portion the light, though diminished in intensity, will still 

 be white. The whiteness of Alpine 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 reflection, and when we lift 

 ourselves above a cloud-field to an Alpine summit, for in- 

 stance, or to the top of Snowdon and see, in the proper 

 direction, the sun shining on the clouds, they appear daz- 

 zlingly 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 conditions of whiteness 

 would fail if all the waves were diminished equally r , or by 

 the same absolute quantity. They must be reduced pro- 

 portionately, instead of equally. If by the act of reflection 



