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Reduction of the phenomena of the rings to a physical property of light, called Fits of easy reflexion and Transmission.
The phenomena of the rings being reduced to laws extremely exact and well adapted to calculation, Newton concentrated them all in a still simpler expression, making them depend on a physical property, which he attributed to light, and of which he defined all the particulars conformably to their laws.
Considering light as a matter composed of small molecules emitted by luminous bodies with very great velocities, he concluded, that since they were reflected within the lamina of air, at the several thicknesses t, 3t, 5t, 7t, &c., and transmitted at the intermediate thicknesses 1, 2t, 4t, 6t, &c., the molecules must have some peculiar modification, of a periodical nature, such as to incline them alternately to be reflected and refracted after passing through certain spaces. Yet this modification could not be necessary, since the intensity of the reflexion at the second surface varies with the medium contiguous to that surface, so that a given molecule arriving at it, at a given epoch of its period, may be either reflected or transmitted, according to the exterior circumstances which act on it. Newton therefore characterised this property of the luminous molecules as a simple tendency, and designated it appropriately enough by the phrase, Fit of easy reflexion or transmission.
According to this idea of the fits, their duration must evidently be proportional to the thickness t, which regulates, in each substance, the alternations of reflexion and transmission. Thus, in the first table given, we find the measure of it for a vacuum, for air, water, and glass, in the case of perpendicular incidence. In other substances, the duration of the fits must vary as the quantity t, that is, inversely as the refracting power; it will vary also, by parity of reason, with the obliquity of incidence, and the nature of the light: but the laws of these variations are exactly those which regulate the rings themselves; so that, these last being known, it remains only to apply them; this Newton did, and after having defined completely all the characters of the fits, he employed them as a simple property, not only to unite under one point of view the phenomena of the colours produced by thin plates, but also to foresee and to calculate beforehand, both as to their general tenor, and their minutest
