Thus, when white light is employed, the red sifts it by quenching the green, and the green sifts it by quenching the red, both exhibiting the residual colour.  The process through which natural bodies acquire their colours is therefore a negative one.  The colours are produced by subtraction, not by addition.  This red glass is red because it destroys all the more refrangible rays of the spectrum.  This blue liquid is blue because it destroys all the less refrangible rays.  Both together are opaque because the light transmitted by the one is quenched by the other.  In this way, by the union of two transparent substances, we obtain a combination as dark as pitch to solar light.  This other liquid, finally, is purple because it destroys the green and the yellow, and allows the terminal colours of the spectrum to pass unimpeded.  From the blending of the blue and the red this gorgeous purple is produced.

One step further for the sake of exactness.  The light which falls upon a body is divided into two portions, one of which is reflected from the surface of the body; and this is of the same colour as the incident light.  If the incident light be white, the superficially reflected light will also be white.  Solar light, for example, reflected from the surface of even a black body, is white.  The blackest camphine smoke in a dark room, through which a sunbeam passes from an aperture in the window-shutter, renders the track of the beam white, by the light scattered from the surfaces of the soot particles.  The moon appears to us as if

    ‘Clothed in white samite, mystic, wonderful;’

but were it covered with the blackest velvet it would still hang as a white orb in the heavens, shining upon our world substantially as it does now.

Sec. 8. Colours of Pigments as distinguished from Colours of Light.

The second portion of the incident light enters the body, and upon its treatment there the colour of the body depends.  And here a moment may properly be given to the analysis of the action of pigments upon light.  They are composed of fine particles mixed with a vehicle; but how intimately soever the particles may be blended, they still remain particles, separated, it may be, by exceedingly minute distances, but still separated.  To use the scientific phrase, they are not optically continuous.  Now, wherever optical continuity is ruptured we have reflection of the incident light.  It is the multitude of reflections at the limiting surfaces of the particles that prevents light from passing through snow, powdered glass, or common salt.  The light here is exhausted in echoes, not extinguished by true absorption.  It is the same kind of reflection that renders the thunder-cloud so impervious to light.  Such a cloud is composed of particles of water, mixed with particles of air, both separately transparent, but practically opaque when thus mixed together.