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 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, the rings gradually contracted; when the passage was from blue to red, the rings expanded.  This