II.—­The Colours of Natural Bodies

In examining the nature and origin of colours as the component parts of white light, the attention of Newton was directed to the explanation of the colours of natural bodies.  His earliest researches on this subject were communicated, in his “Discourse on Light and Colours,” to the Royal Society in 1675.

Dr. Hooke had succeeded in splitting a mineral substance called mica into films of such extreme thinness as to give brilliant colours.  One plate, for example, gave a yellow colour, another a blue colour, and the two together a deep purple, but as plates which produced this colour were always less than the twelve-thousandth part of an inch thick it was quite impracticable, by any contrivance yet discovered, to measure their thickness, and determine the law according to which the colours varied with the thickness of the film.  Newton surmounted this difficulty by laying a double convex lens, the radius of the curvature of each side of which was fifty feet, upon the flat surface of a plano-convex object-glass, and in the way he obtained a plate of air, or of space, varying from the thinnest possible edge at the centre of the object-glass where it touched the plane surface to a considerable thickness at the circumference of the lens.  When the light was allowed to fall upon the object-glass, every different thickness of the plate of air between the object-glasses gave different colours, so that the point where the two object-glasses touched one another was the centre of a number of concentric coloured rings.  Now, as the curvature of the object-glass was known, it was easy to calculate the thickness of the plate of air at which any particular colour appeared, and thus to determine the law of the phenomena.

By accurate measurements Newton found that the thickness of air at which the most luminous parts of the first rings were produced were, in parts of an inch, as 1, 3, 5, 7, 9, and 11 to 178,000.

If the medium or the substance of the thin plate is water, as in the case of the soap-bubble, which produces beautiful colours according to its different degrees of thinness, the thicknesses at which the most luminous parts of the ring appear are produced at 1/1.336 the thickness at which they are produced in air, and, in the case of glass or mica, at 1/1.525 at thickness, the numbers 1.336, 1.525 expressing the ratio of the sines of the angles of incidence and refraction which produce the colours.

From the phenomena thus briefly described, Newton deduced that ingenious, though hypothetical, property of light called its “fits of easy reflection and transmission.”  This property consists in supposing that every particle of light from its first discharge from a luminous body possesses, at equally distant intervals, dispositions to be reflected from, and transmitted through, the surfaces of the bodies upon which it is incident.  Hence, if a particle of light reaches a reflecting surface of glass when in its fit of easy reflection, or in its disposition to be reflected, it will yield more readily to the reflecting force of the surface; and, on the contrary, if it reaches the same surface while in a fit of easy transmission, or in a disposition to be transmitted, it will yield with more difficulty to the reflecting force.