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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 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 whiteness. 

 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 instance, or to the top of Snowdon and 

 see, in the proper direction, the sun shining on the 

 clouds, 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 pre- 

 served. 



It will be understood that the conditions of whiteness 

 would fail if all the waves were diminished equally, or 

 by the same absolute quantity. They must be reduced 

 proportionately instead of equally. If by the act of re- 

 flection the waves of red light are split into exact halves, 

 then, to preserve the light white, the waves of yellow, 



