By accurate measurements, Sir Isaac found that the thicknesses of air at which the most luminous parts of the first rings were produced, were in parts of an inch 1/178000, 3/178000, 5/178000, 7/178000, 9/178000, 11/178000. 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 rings appear are produced at 1/1·336 of the thickness at which they are produced in air, and in the case of glass or mica at 1/1·525 of that thickness; the numbers 1.336, 1.525 expressing the ratio of the sines of the angles of incidence and refraction in the substances which produce the colours.
From the phenomena thus briefly described, Sir Isaac Newton deduces 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 bodies upon which it is incident. Hence, if a particle of light reaches a reflecting surface of glass when it is in its fit of 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. Sir Isaac has not ventured to inquire into the cause of this property; but we may form a very intelligible idea of it by supposing, that the particles of light have two attractive and two repulsive poles at the extremities of two axes at right angles to each other, and that the particles revolve round their axes, and at equidistant intervals bring one or other of these axes into the line of the direction in which the particle is moving. If the attractive axis is in the line of the direction in which the particle moves when it reaches the refracting surface, the particle will yield to the attractive force of the medium, and be refracted and transmitted; but if the repulsive axis is in the direction of the particle’s motion when it reaches the surface, it will yield to the repulsive force of the medium, and be reflected from it.
The application of the theory of alternate fits of reflection and transmission to explain the colours of thin plates is very simple. When the light falls upon the first surface AB, Fig. 8 of the plate of air between AB and CED, the rays that are in a fit of reflection are reflected, and those that are in a fit of transmission are transmitted. Let us call F the length of a fit, or the distance through which the particle of light moves while it passes from the state of being in a fit of reflection to the state of being in a fit of transmission. Now, as all the particles of light transmitted through AB were in a state of easy transmission when they entered AB, it is obvious, that, if the plate of air at E is so thin as to be less than one-half of F, the particles of light will still be in their disposition to be transmitted, and consequently the light will be all transmitted, and none reflected at the curve surface at E. When the plate becomes thicker towards a, so that its thickness exceeds half of F, the light will not reach the surface CE till it has come under its fit of reflection, and consequently at a the light will be all reflected, and none transmitted. As the thickness increases towards m, the light will have come under its fit of transmission, and so on, the light being reflected at a, l, and transmitted at E, m. This will perhaps be still more easily understood from [fig. 9], where we may suppose AEC to be a thin wedge of glass or any other transparent body. When light is incident on the first surface AE, all the particles of it that are in a fit of easy reflection will be reflected, and all those in a fit of easy transmission will be transmitted. As the fits of transmission all commence at AE, let the first fit of transmission end when the particles of light have reached ab, and the second when they have reached ef; and let the fits of reflection commence at cd and gh. Then, as the fit of transmission continues from AE to ab, all the light that falls upon the portion mE of the second surface will be transmitted and none reflected, so that to an eye above E the space mE will appear black. As the fit of reflection commences at ab, and continues to cd, all the light which falls upon the portion nm will be reflected, and none transmitted; and so on, the light being transmitted at mE and pn, and reflected at nm and qp. Hence to an eye above E the wedge-shaped film of which AEC is a section will be covered with parallel bands or fringes of light separated by dark fringes of the same breadth, and they will be all parallel to the thin edge of the plate, a dark fringe corresponding to the thinnest edge. To an eye placed below CE, similar fringes will be seen, but the one corresponding to the thinnest edge mE will be luminous.
Fig. 9.
If the thickness of the plate does not vary according to a regular law as in [fig. 9], but if, like a film of blown glass, it has numerous inequalities, then the alternate fringes of light and darkness will vary with the thickness of the film, and throughout the whole length of each fringe the thickness of the film will be the same.
We have supposed in the preceding illustration that the light employed is homogeneous. If it is white, then the differently coloured fringes will form by their superposition a system of fringes analogous to those seen between two object-glasses, as already explained.
The same periodical colours which we have now described as exhibited by thin plates were discovered by Newton in thick plates, and he has explained them by means of the theory of fits; but it would lead us beyond the limits of a popular work like this to enter into any details of his observations, or to give an account of the numerous and important additions which this branch of optics has received from the discoveries of succeeding authors.