e.' He then describes fully and clearly the
experiment with pressed glasses already referred to:--

'Take two small pieces of ground and polished looking-glass plate,
each about the bigness of a shilling: take these two dry, and with
your forefingers and thumbs press them very hard and close together,
and you shall find that when they approach each other very near there
will appear several irises or coloured lines, in the same manner
almost as in the Muscovy glass; and you may very easily change any of
the colours of any part of the interposed body by pressing the plates
closer and harder together, or leaving them more lax--that is, a part
which appeared coloured with a red, may presently be tinged with a
yellow, blue, green, purple, or the like. 'Any substance,' he says,
'provided it be thin and transparent, will show these colours.' Like
Boyle, he obtained them with glass films; he also procured them with
bubbles of pitch, rosin, colophony, turpentine, solutions of several
gums, as gum arabic in water, any glutinous liquor, as wort, wine,
spirit of wine, oyl of turpentine, glare of snails, &c.

Hooke's writings show that even in his day the idea that both light
and heat are modes of motion had taken possession of many minds.
'First,' he says, 'that all kind _of fiery burning bodies_ have their
parts in motion I think will be easily granted me. That the spark
struck from a flint and steel is in rapid agitation I have elsewhere
made probable;... that heat argues a motion of the internal parts is
(as I said before) generally granted;... and that in all extremely hot
shining bodies there is a very quick motion that causes light, as well
as a more robust that causes heat, may be argued from the celerity
wherewith the bodies are dissolved. Next, it must be _a vibrative
motion.'_ His reference to the quick motion of light and the more
robust motion of heat is a remarkable stroke of sagacity; but Hooke's
direct insight is better than his reasoning; for the proofs he adduces
that light is 'a vibrating motion' have no particular bearing upon the
question.

Still the Undulatory Theory had undoubtedly dawned upon the mind of
this remarkable man. In endeavouring to account for the colours of
thin plates, he again refers to the relation of colour to thickness:
he dwells upon the fact that the film which shows these colours must
be transparent, proving this by showing that however thin an opaque
body was rendered no colours were