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
phraseology which shows the delight he took in his work, the following
beautiful experiment:--

'We took a quantity of clean lead, and melted it with a strong fire,
and then immediately pouring it out into a clean vessel of convenient
shape and matter (we used one of iron, that the great and sudden heat
might not injure it), and then carefully and nimbly taking off the
scum that floated on the top, we perceived, as we expected, the smooth
and glossy surface of the melted matter to be adorned with a very
glorious colour, which, being as transitory as delightful, did almost
immediately give place to another vivid colour, and that was as
quickly succeeded by a third, and this, as it were, chased away by a
fourth; and so these wonderfully vivid colours successively appeared
and vanished till the metal ceasing to be hot enough to hold any
longer this pleasing spectacle, the colours that chanced to