USE OF THE IMAGINATION. 425 
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 
