But, inasmuch as the waves of light are of different lengths, it is plain that, to produce extinction in the case of the longer waves, a greater thickness of film is necessary than in the case of the shorter ones.  Different colours, therefore, must appear at different thicknesses of the film.

Take with you a little bottle of spirit of turpentine, and pour it into one of your country ponds.  You will then see the glowing of those colours over the surface of the water.  On a small scale we produce them thus:  A common tea-tray is filled with water, beneath the surface of which dips the end of a pipette.  A beam of light falls upon the water, and is reflected by it to the screen.  Spirit of turpentine is poured into the pipette; it descends, issues from the end in minute drops, which rise in succession to the surface.  On reaching it, each drop spreads suddenly out as a film, and glowing colours immediately flash forth upon the screen.  The colours change as the thickness of the film changes by evaporation.  They are also arranged in zones, in consequence of the gradual diminution of thickness from the centre outwards.

Any film whatever will produce these colours.  The film of air between two plates of glass squeezed together, exhibits, as shown by Hooke, rich fringes of colour.  A particularly fine example of these fringes is now before you.  Nor is even air necessary; the rupture of optical continuity suffices.  Smite with an axe the black, transparent ice—­black, because it is pure and of great depth—­under the moraine of a glacier; you readily produce in the interior flaws which no air can reach, and from these flaws the colours of thin plates sometimes break like fire.  But the source of most historic interest is, as already stated, the soap-bubble.  With one of the mixtures employed by the eminent blind philosopher, Plateau, in his researches on the cohesion figures of thin films, we obtain in still air a bubble ten or twelve inches in diameter.  You may look at the bubble itself, or you may look at its projection upon the screen; rich colours arranged in zones are, in both cases, exhibited.  Rendering the beam parallel, and permitting it to impinge upon the sides, bottom, and top of the bubble, gorgeous fans of colour, reflected from the bubble, overspread the screen, rotating as the beam is carried round.  By this experiment the internal motions of the film are also strikingly displayed.

Not in a moment are great theories elaborated:  the facts which demand them become first prominent; then, to the period of observation succeeds a period of pondering and of tentative explanation.  By such efforts the human mind is gradually prepared for the final theoretic illumination.  The colours of thin plates, for example, occupied the attention of Robert Boyle.  In his ‘Experimental History of Colours’ he contends against the schools which affirmed that colour was ’a penetrative quality that reaches to the innermost parts of the