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black, which is confirmed by experiment; whereas, by the emanating doctrine, they ought to be half illuminated, which is not found to be the case. M. Fresnel, whose opinion is of the first authority, thought this test conclusive. It may therefore be concluded that the rings arise entirely from the interference of the rays: the light reflected from each of the surfaces in apparent contact reaches the eye by paths of different lengths, and produces coloured and dark rings alternately, according as the reflected waves coincide or destroy one another. The breadths of the rings are unequal; they decrease in width, and the colours become more crowded, as they recede from the centre. Coloured rings are also produced by transmitting light through the same apparatus; but the colours are less vivid, and are complementary to those reflected, consequently the central spot is white.
The size of the rings increases with the obliquity of the incident light; the same colour requiring a greater thickness or space between the glasses to produce it than when the light falls perpendicularly upon them. Now if the apparatus be placed in homogeneous instead of white light, the rings will all be of the same colour with that of the light employed. That is to say, if the light be red, the rings will be red divided by black intervals. The size of the rings varies with
the colour of the light. They are largest in red, and decrease in magnitude with the succeeding prismatic colours, being smallest in violet light.
Since one of the glasses is plane and the other spherical, it is evident that, from the point of contact, the space between them gradually increases in thickness all round, so that a certain thickness of air corresponds to each colour, which, in the undulatory system, measures the length of the wave producing it. By actual measurement Sir Isaac Newton found that the squares of the diameters of the brightest parts of each ring are as the odd numbers, 1, 3, 5, 7, &c.; and that the squares of the diameters of the darkest parts are as the even numbers 0, 2, 4, 6, &c. Consequently the intervals between the glasses at these points are in the same proportion. If, then, the thickness of the air corresponding to any one colour could be found, its thickness for all the others would be known. Now, as Sir Isaac Newton knew the radius of curvature of the lens, and the actual breadth of the rings in parts of an inch, it was easy to compute that the thickness of air at the darkest part of the first ring is the
part of an inch, whence all the others have been deduced. As these intervals determine the lengths of the waves on the undulatory hypothesis, it appears that the length of a wave of the extreme
red of the solar spectrum is equal to the 0.0000266th part of an inch; that the length of a wave of the extreme violet is equal to the 0.0000167th part of an inch; and as the time of a vibration of a particle of ether producing any particular colour is directly as the length of a wave of that colour, and inversely as the velocity of light, it follows that the molecules of ether producing the extreme red of the solar spectrum perform 458 millions of millions of vibrations in a second; and that those producing the extreme violet accomplish 727 millions of millions of vibrations in the same time. The lengths of the waves of the intermediate colours and the number of their vibrations being intermediate between these two, white light, which consists of all the colours, is consequently a mixture of waves of all lengths between the limits of the extreme red and violet. The determination of these minute portions of time and space, both of which have a real existence, being the actual results of measurement, do as much honour to the genius of Newton as that of the law of gravitation.
The phenomenon of the coloured rings takes place in vacuo as well as in air; which proves that it is the distance between the lenses alone, and not the air, which produces the colours. However, if water or oil be put between them, the
rings contract, but no other change ensues, and Newton found that the thickness of different media at which a given tint is seen is in the inverse ratio of their refractive indices, so that the thickness of laminæ may be known by their colour, which could not otherwise be measured; and as the position of the colours in the rings is invariable, they form a fixed standard of comparison, well known as Newton's scale of colours; each tint being estimated according to the ring to which it belongs from the central spot inclusively. Not only the periodical colours which have been described, but the colours seen in thick plates of transparent substances, the variable hues of feathers, of insects' wings, and of striated substances, and the coloured fringes surrounding the shadows of all bodies held in an extremely small beam of light, all depend upon the same principle. Whence it appears, that material substances derive their colours from two different causes-some from the law of interference, such as iridescent metals, peacock's feathers, &c., and others from the unequal absorption of the rays of white light, such as vermilion, ultramarine, blue or green cloth, flowers, and the greater number of coloured bodies.
The ethereal medium pervading space is supposed to penetrate all material substances, occu
pying the interstices between their molecules; but in the interior of refracting media it exists in a state of less elasticity compared with its density in vacuo; and the more refractive the medium the less the elasticity of the ether within it. Hence the waves of light are transmitted with less velocity in such media as glass and water than in the external ether. As soon as a ray of light reaches the surface of a diaphanous reflecting substance, for example, a plate of glass, it communicates its undulations to the ether next in contact with the surface, which thus becomes a new centre of motion, and two hemispherical waves are propagated from each point of this surface; one of which proceeds forward into the interior of the glass, with a less velocity than the incident wave; and the other is transmitted back into the air with a velocity equal to that with which it came. Thus when refracted, the light moves with a different velocity without and within the glass; when reflected, the ray comes and goes with the same velocity. The particles of ether without the glass which communicate their motions to the particles of the dense and less elastic ether within it, are analogous to small elastic balls striking large ones; for some of the motion will be communicated to the large balls, and the small ones will be reflected. The first would cause the refracted