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
object,' adducing opposing facts. 'To give you a first instance,' he
says, 'I shall need but to remind you of what I told you a little
after the beginning of this essay, touching the blue and red and
yellow that may be produced upon a piece of tempered steel; for these
colours, though they be very vivid, yet if you break the steel they
adorn, they will appear to be but superficial.' He then describes, in