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 de

rive 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

wave, and the last the reflected.

Conversely, when the light passes from glass to air, the action is similar to large balls striking small ones. The small balls receive a motion which would cause the refracted ray, and the part of the motion retained by the large ones would occasion the reflected wave; so that when light passes through a plate of glass or of any other medium differing in density from the air, there is a reflection at both surfaces. But this difference exists between the two reflections, that one is caused by a vibration in the same direction with that of the incident ray, and the other by a vibration in the opposite direction.

A single wave of air or ether would not produce the sensation of sound or light. In order to excite vision, the vibrations of the molecules of ether must be regular, periodical, and very often repeated; and as the ear continues to be agitated for a short time after the impulse, by which alone a sound becomes continuous, so also the fibres of the retina, according to M. d'Arcet, continue to vibrate for about the eighth part of a second, after the exciting cause has ceased. Every one must have observed when a strong impression is made by a bright light, that the object remains visible for a short time after shutting the eyes, which is supposed to be in consequence of the continued vibra

tions of the fibres of the retina. It is quite possible that many vibrations may be excited in the ethereal medium incapable of producing undulations in the fibres of the human retina, which yet have a powerful effect on those of other animals or of insects. Such may receive luminous impressions of which we are totally unconscious, and at the same time they may be insensible to the light and colours which affect our eyes, their perceptions beginning where ours end.

SECTION XXII.

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IN giving a sketch of the constitution of light, it is impossible to omit the extraordinary property of its polarization, 'the phenomena of which,' Sir John Herschel says, are so singular and various, that to one who has only studied the common branches of physical optics, it is like entering into a new world, so splendid as to render it one of the most delightful branches of experimental inquiry, and so fertile in the views it lays open of the constitution of natural bodies, and the minuter mechanism of the universe, as to place it in the very first rank of the physico-mathematical sciences, which it maintains by the rigorous application of geometrical reasoning its nature admits and requires.'