IJS'E OP THE IMAGINATION. 



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 

 proportion, the light, though diminished in intensity, 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 upward; 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 whiteness 

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

 the same absolute quantity. They must be reduced pro- 

 portionately, 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 waves of yellow, orange, 

 green, and blue, must also be split into exact halves. In 

 short, the reduction must take place, not by absolutely 

 equal quantities, but by equal fractional parts. In white 

 light the preponderance, as regards energy, of the larger- 

 over the smaller waves must always be immense. Were the 

 case otherwise, the visual correlative, blue, of the smaller 

 waves would have the upper hand in our sensations. 



Not only are the waves of ether reflected by clouds, by 

 solids, and by liquids, but when they pass from light air to 

 dense, or from dense air to light, a portion of the wave- 

 motion is always reflected. Now our atmosphere changes 

 continually in density from top to bottom. It will help 

 our conceptions if we regard it as made up of a series of 

 thin concentric layers, or shells of air, each shell being of 

 the same density throughout, a small and sudden change 

 of density occurring in passing from shell to shell. Light 

 would be reflected at the limiting surfaces of all these 

 shells, and their action would be practically the same as 

 that of the real atmosphere. And now I would ask your 

 imagination to picture this act of reflection. What must 



