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.

Sec. 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
other work; and the efforts of Boyle and Hooke cleared the way for the
optical career of Newton. He conquered the difficulty which Hooke had
found insuperable, and determined by accurate measurements the
relation of the thickness of the film to the colour it displays. In
doing this his first care was to obtain a film of variable and
calculable depth. On a plano-convex glass lens (D B E, fig. 13) of
very feeble curvature he laid a plate of glass (A C) with a plane
surface, thus obtaining a film of air of gradually increasing depth
from the point of contact (B) outwards. On looking at the film in
monochromatic light he saw, with the delight attendant on fulfilled
prevision, surrounding the place of contact, a series of bright rings
separated from each other by dark ones, and becoming more closely
packed together as the distance from the point of contact augmented
(as in fig. 14). When he employed red light, his rings had certain
diameters; when he employed blue light, the diameters were less. In
general terms, the more refrangible the light the smaller were the
rings. Causing his glasses to pass through the spectrum from red to
blue,