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 produced. 'This,' he says, 'I have
often tried by pressing a small globule of mercury between two smooth
plates of glass, whereby I have reduced that body to a much greater
thinness than was requisite to exhibit the colours with a transparent
body.' Then follows the sagacious remark that to produce the colours
'there must be a considerable reflecting body adjacent to the under or
further side of the lamina or plate: for this I always found, that the
greater that reflection was the more vivid were the appearing colours.
From which observation,' he continues, 'it is most evident, _that the
reflection from the further or under side of the body is the principal
cause of the production of these colours._'

He draws a diagram, correctly representing the reflection at the two
surfaces of the film; but here his clearness ends. He ascribes the
colours to a coalescence or confusion of the two reflecting pulses;
the principal of interference being unknown to him, he could not go
further in the way of explanation.


§ 8. _Newton's Rings. Relation of Colour to Thickness of Film_.

[Illustration: Fig. 13]

In this way, then, by the active operation of different minds, facts
are observed, examined, and the precise conditions of their
appearance determined. All such work in science is the prelude to