The order and the dimensions of the coloured ring are the rune, whether air occupy the space between the lenses or whether the latter be in the exhausted receiver of an air.pump ; but when a transparent medium, as water, of greater refractive power than air, is interposed between them, the tints are fainter and the diameters of the rings are loss ; or smaller distances between the lenses are requisito in order to produce the same tints: it Is ascertained that, with different media, these distances, in the case of a perpendicular incident ray, are inversely proportional to the refractive indices.
Corresponding rings of colour were observed by Newton in thin plates surrounded by media less dense than the plates ; thus a bubble of soap-water exhibits, by the gradual subsidence of the fluid, rings of colour exactly conformable to those between glass lenses; and, before the bubble bursts, a dark spot. about half an inch in diameter, is formed at its upper part. The like phenomena have been observed in thin plates of mica and in bubbles of glass blown so thin as to burst.
Newton also examined and described the phenomena of the coloured rings or bands between lenses when the light is transmitted through the latter. These rings are lese'bright than those which are formed by reflection ; but when the obliquity of the transmitted rays to the plane of the rings is considerable they are sufficiently distinct : in the centre is a white spot, and the colours of all the rings are exactly com plementary to those of the corresponding rings which are seen by the reflected rays. Newton's arrangement of the coloured tints in the first and second rings or bands, reckoning from the centre, is given in the following table, with the thicknesses of the plates of air, water, and glass, at the places where the tints are produced. The unit of measure is one millionth part of an inch.
This table, extended so as to include the seventh band of reflected tints, constitutes that which is called Newton's Scale of Colours.
It was to explain the phenomena of these coloured rings that Newton proposed the hypothesis of ' Fits of easy lteflection and Transmission,' which in many respect° explains the laws of the phenomenon. But besides giving no indication beforehand of what ought to be the variation of the diameter of a ring with the refractive index of the Interposed medium, or with the obliquity of incidence, it tends to at least one result at variance with observation. According to the hypo
thesis of fits, the central black spot, as well as the dark parts of the rings seen with homogeneous light, ought to be of half the brightness of the brightest parts; according to the theory of undulations, they ought to be perfectly black. Observation shows, that at least the ceutral spot (which is most easily observed, and does not require homo geneous light) is perfectly black.
A general explanation of the rings according to the principles of the undulatory theory may readily be given. Let MN e Q be two plates or spherical lenses, which we may suppose plano-convex (though all that is essential is, that the more curved of the adjacent surfaces should be convex), in contact with each other at z; and let A B bo the direction of a pencil, or of a wave of light incident upon the first or upper plate this will be refracted in some direction as 13 c, and at the point o part of the pencil will emerge and fall on the other plate in some point E, where it will be partly transmitted through that plate in the direction II, and partly reflected in the direction E F. Another part of the refracted pencil tic will be reflected at c In the direction 0 13, some of it emerging at n, and the rest being reflected back into the plate. The reflected pencil E P will also be, in part, transmitted through the uppci plate in some direction as P o, and, in part, reflected in the directior P rat : at the point 11 a portion will be transmitted in the direction it s while another is reflected in the direction it K, and so on. The twc principal reflected pencils are those of which the courses are Asap anc ABCEPO; and these being each once reflected will be of nearly equal intensity. The other pencils in the general reflected beam, having beer reflected 3, 5, 7, &c. times, will be comparatively weak, except at ver3 Teat incidences. The two principal transmitted pencils, A ti c E R and LBOEFIls will be of very unequal intensity, the latter having been ,wise reflected, and the former not reflected at all. These will be iccompanied by pencils reflected 4, 6, 8, eke. times, which will lie cons mratively insignificant.