the prism and the cylindrical lens may be seen the colours, tracking themselves through the dust of the room.  Cutting off the more refrangible fringe by a card, the rectangle is seen red:  cutting off the less refrangible fringe, the rectangle is seen blue.  By means of a thin glass prism (W), I deflect one portion of the colours, and leave the residual portion.  On the screen are now two coloured rectangles produced in this way.  These are complementary colours—­colours which, by their union, produce white.  Note, that by judicious management, one of these colours is rendered yellow, and the other blue.  I withdraw the thin prism; yellow and blue immediately commingle, and we have white as the result of their union.  On our way, then, we remove the fallacy, first exposed by Wuensch, and afterwards independently by Helmholtz, that the mixture of blue and yellow lights produces green.

Restoring the circular aperture, we obtain once more a spectrum like that of Newton.  By means of a lens, we can gather up these colours, and build them together, not to an image of the aperture, but to an image of the carbon-points themselves.

Finally, by means of a rotating disk, on which are spread in sectors the colours of the spectrum, we blend together the prismatic colours in the eye itself, and thus produce the impression of whiteness.

Having unravelled the interwoven constituents of white light, we have next to inquire, What part the constitution so revealed enables this agent to play in Nature?  To it we owe all the phenomena of colour, and yet not to it alone; for there must be a certain relationship between the ultimate particles of natural bodies and white light, to enable them to extract from it the luxury of colour.  But the function of natural bodies is here selective, not creative.  There is no colour generated by any natural body whatever.  Natural bodies have showered upon them, in the white light of the sun, the sum total of all possible colours; and their action is limited to the sifting of that total—­the appropriating or absorbing of some of its constituents, and the rejecting of others.  It will fix this subject in your minds if I say, that it is the portion of light which they reject, and not that which they appropriate or absorb, that gives bodies their colours.

Let us begin our experimental inquiries here by asking, What is the meaning of blackness?  Pass a black ribbon through the colours of the spectrum; it quenches all of them.  The meaning of blackness is thus revealed—­it is the result of the absorption of all the constituents of solar light.  Pass a red ribbon through the spectrum.  In the red light the ribbon is a vivid red.  Why?  Because the light that enters the ribbon is not quenched or absorbed, but in great part sent back to the eye.  Place the same ribbon in the green of the spectrum; it is black as jet.  It absorbs the green light, and renders the space on which that light falls a space of intense darkness.  Place a green ribbon in the green of the spectrum.  It shines vividly with its proper colour; transfer it to the red, it is black as jet.  Here it absorbs all the light that falls upon it, and offers mere darkness to the eye.