before it broke it suffered itself to be reduced into films so
extremely thin that they constantly showed upon their surface the
varying colours of the rainbow.'[14]

Subsequent to Boyle the colours of thin plates occupied the attention
of Robert Hooke, in whose writings we find a dawning of the undulatory
theory of light. He describes with great distinctness the colours
obtained with thin flakes of 'Muscovy glass' (talc), also those
surrounding flaws in crystals where optical continuity is destroyed.
He shows very clearly the dependence of the colour upon the thickness
of the film, and proves by microscopic observation that plates of a
uniform thickness yield uniform colours. 'If,' he says, 'you take any
small piece of the Muscovy glass, and with a needle, or some other
convenient instrument, cleave it oftentimes into thinner and thinner
laminæ, you shall find that until you come to a determinate thinness
of them they shall appear transparent and colourless; but if you
continue to split and divide them further, you shall find at last that
each plate shall appear most lovely tinged or imbued with a
determinate colour. If, further, by any means you so flaw a pretty
thick piece that one part begins to cleave a little from the other,
and between these two there be gotten some pellucid medium, those
laminated or pellucid bodies that fill that space shall exhibit
several rainbows or coloured lines, the colours of which will be
disposed and ranged according to the various thicknesses of the
several parts of the plate.' 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,