The second point is rather more intricate, viz. that this imprisonment is going on even when friendly concord appears to be the order of the day. Let us endeavour to present this clearly to the reader. Of the pure colours, violet, green and red--the extremes and the centre--are sufficient to produce white, because each contains an element of its neighbours. Violet has a certain amount of indigo, green some yellow, red some orange; in fact every colour of the spectrum contains a greater or less degree of several of the others, but not enough to destroy its own identity. Now, suppose that we have three lanterns projecting their rays on to the same portion of a white sheet, and that in front of the first is placed a violet glass, in front of the second a green glass, in front of the third a red glass. What is the result? A white light. Why? Because they meet _on equal terms_, and as no one of them is in a point of advantage no prisoners can be made and they must work in harmony. Next, turn down the violet lantern, and green and red produce a yellow, half-way between them; turn down red and turn up violet, indigo-blue results. All the way through a compromise is effected. But supposing that the red and green glasses are put in front of the _same_ lantern and the white light sent through them--where has the yellow gone to? only a brownish-black light reaches the screen. The same thing happens with red and violet or green and violet. Prisoners have been taken, because one colour has had to _demand passage_ from the other. Red says to green, “You want your rays to pass through me, but they shall not.” Green retorts, “Very well; but I myself have already cut off all but green rays, and if they don’t pass you, nothing shall.” And the consequence of the quarrel is practical darkness. The same phenomenon may be illustrated with blue and yellow. Lights of these two colours projected simultaneously on to a sheet yield white; but white light sent through blue and yellow glass _in succession_ produces a green light. Also, blue paint mixed with yellow gives green. In neither case is there darkness or entire cutting-off of colour, as in the case of Red + Violet or Green + Red. The reason is easy to see. Blue light is a compromise of violet and green; yellow of green and red. Hence the two coloured lights falling on the screen make a combination which can be expressed as an addition sum. Blue = green + violet. Yellow = green + red. -------------------- green + violet + red = white. But when light is passed _through_ two coloured glasses in succession, or reflected from two layers of coloured paints, there are prisoners to be made. Blue passes green and violet only. Yellow passes green and red only. So violet is captured by yellow, and red by blue, green being free to pass on its way.