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ether. It is supposed to be a rare, highly elastic, subtile fluid, which occupies all space and pervades all bodies. As the sensation of light is supposed to be excited by the undulations of this medium, so, where light exists, there ether must be. Hence it fills all space. It is between the sun and the earth, the earth and the stars, and so on. If it did not exist in water, diamonds, glass, &c., these bodies would not be diaphanous. So that it must pervade all bodies. Even opake substances must contain it, since, as in the case of gold, these become transparent when excessively thin.

Existence of an Ether.—We have no independent evidence to adduce of the existence of this medium. It is, therefore, an assumption; but one which is sanctioned by the high authority of Descartes, Huyghens, Euler, Hooke, Newton, Young, Fresnel, and some of the most distinguished philosophers of the present day, among whom are Sir John Herschel and Arago. These eminent men have seen in this assumption nothing inconsistent with their knowledge of the constitution of the universe. The electrician and the magnetician have assumed, respectively, an electric and a magnetic fluid, and there can be no impropriety, therefore, in the optician assuming a luminiferous ether, provided, however, that it be compatible with well ascertained facts, and do not violate known laws. Moreover, it is by no means improbable that the fluids which have been respectively assumed as the causes of electrical, magnetical, calorific, and luminous phenomena, may be one and the same.

Even gravity, perhaps, may be referable to the same cause. Newton* himself has thrown out a speculation of this kind. Alluding to the ether, he says, "Is not this medium much rarer within the dense bodies of the sun, stars, planets, and comets, than in the empty celestial spaces between them? And in passing from them to great distances, doth it not grow denser and denser perpetually, and thereby cause the gravity of those great bodies towards one another, and of their parts towards the bodies; every body endeavouring to go from the denser parts of the medium towards the rarer?" Very recently, Dr. Rogetf and Mosotti J have shown how, on the assumption of an ethereal medium, the phenomena of gravitation and electricity, may be included in the same law.

It has been said, that if the universe contained a fluid of the kind here referred to, the planets must experience some resistance to their motions, and, therefore, that as no resistance can be detected, there can be no ethereal medium. This conclusion,

* Opticks, p. 325. Query 21.

t Electricity. Published in the Library of Useful Knowledge. % On the Forces which regulate the Internal Constitution of Bodies, in Taylor's Scientific Memoirs, part iii.

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however, is by no means a necessary one, for " if this ether," says Newton*, "should be supposed 700,0C0 times more elastic than our air, and above 700,000 times more rare, its resistance would be above 600,000,000 times less than that of water. And so small a resistance would scarce make any sensible alteration in the motions of the planets in ten thousand years." The most satisfactory evidence of this resistance, if indeed it exist, might be expected to be found in the case of the comets, bodies made up of the lightest materials, in fact, masses of vapour, and, therefore, from their less momentum, more likely to suffer retardation. In the case of Encke's comet, evidence of this resistance is believed to have been obtained. The mean duration of one entire revolution of this comet is about 1207 days, and the " magnitude of the resistance is such as to diminish the periodic time about Taivv of tne whole at each revolution; a quantity so large that there can be no mistake about its existence.!"

The following table of the mean duration of one entire revolution of this comet, allowance being made for perturbations occasioned by the action of neighbouring planets, is taken from a memoir by EnckeJ.

Daj-s.

From 1786 to 1795 1208.112

"1795 to 1805 1207.879

"1805 to 1819 1207.424

Sir John Herschel § observes, that "on comparing the intervals between the successive perihelion passages of this comet, after allowing, in the most careful and exact manner, for all the disturbances due to the actions of the planets, a very singular fact has come to light, viz., that the periods are continually diminishing, or, in other words, the mean distance from the sun, or the major axis of the ellipse, dwindling by slow but regular degrees. This is evidently the effect which would be produced by a resistance experienced by the comet from a very rare ethereal medium pervading the regions in which it moves, for such resistance, by diminishing its actual velocity, would diminish also its centrifugal force, and thus give the sun more power over it to draw it nearer. Accordingly (no other mode of accounting for the phenomenon in question appearing) this is the solution proposed by Encke, and generally received. It will, therefore, probably fall ultimately into the sun, should it not first be dissipated altogether, a thing no way improbable, when the lightness of its materials is considered, and which seems authorized by the observed fact of its having been less and less conspicuous at each reappearance."

* Opticks, p. 327. Query 22.

t Airy, Report on the Progress of Astronomy, in the Report of the British Association for 1833.

J Astrcnomische Nachrichten, Nos. 210, 211.

§ Treatise on Astronomy (in Lardner's Cyclopaedia), p. 309.

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Leaving these speculations, and assuming, then, the existence of a luminiferous ether, I proceed to point out the properties such a fluid must be supposed to possess.

Ethereal Molecules The ether consists, or is made up of minute parts, which we call molecules, between which there must exist attractive and repulsive forces *, in virtue of which the ether possesses extreme elasticity. Moreover, there appears to exist some attractive force between the ethereal molecules and the particles of the grosserforms of matter. Indeed, Dr. Young supposed that the vibrating medium is the ether and ponderable matter conjointly.

But instead of insisting on the actual existence of an ethereal medium composed of molecules, we " may be content to look at the theory simply as a mathematical system, which faithfully represents, at least, a wide range of phenomena, and to some extent connects the laws so made out with dynamical principles regulating the motions of a system of points, combined to form an elastic system, which, for brevity and illustration, we call molecules, constituting an cethereal medium^,"

Ethereal Waves.—If we suppose the existence of attractive and repulsive forces between the ethereal molecules, it follows, that when these molecules are at rest or have attained a slate of equilibrium, any attempt to move one molecule must be attended with the displacement of several; for the motion is extended to adjacent molecules. So that if a vibratory movement be communicated to one, it is extended to several. Now, an assemblage of vibrating molecules, in all phases of vibration, constitutes an ethereal wave. These vibrations being communicated through successive portions of the ethereal medium, reach the retina or expanded optic nerve, and are propagated along the optic nerve to the brain, where they excite in us the sensation of light, just as the vibrations of the air communicated to the auditory nerve, and from thence to the brain, produce the sensation of sound.

The number of impulses made by the ethereal molecules on the retina in a given time, determines the colour of the light, just as the number of impulses by the aerial molecules on the auditory nerves determines the pitch, note, or tone of sound. Hence colours are to the eye what tones are to the ear.

"From Newton's measures of the thicknesses reflecting the different colours, the breadth and duration of their respective

* See a paper by Mr. Earnshaw, On the Nature of the Molecular Forces which regulate the Constitution of the Luminiferous Ether, in the Transactions of the Cambridge Philosophical Society, vol. vii., part 1.

t A General and Elementary View of the Undidatory Theory, as applied to the Dispersion of Light. By the Eev. Baden Powell, 1841, pp. 4 and 5.

undulations may be very accurately determined. The whole visible spectrum appears to be comprised within the ratio of three to five, which is that of a major sixth in music; and the undulations of red, yellow, and blue, to be related in magnitude as the numbers 8, 7, and 6; so that the interval from red to blue is a fourth. The absolute frequency expressed in numbers, is too great to be distinctly conceived, but it may be better imagined by a comparison with sound. If a chord sounding the tenor c, could be continually bisected forty times, and should then vibrate, it would afford a yellow green light: this being denoted by ", the extreme red would be f, and the blue ". The absolute length and frequency of each vibration is expressed in the table*;" supposing light to travel at the rate of 192,000 miles per second.

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There is a limit to the sensibility of both ear and eye; that is, a certain number of impulses must be made in a given time on these organs before we become sensible of them; and if we go on augmenting the number, we cease to be sensible of them after a certain time. Now, the limits of sensibility of the eye are much more confined than those of the ear; or, iti other words, the sensibility ceases much sooner in the case of the eye than in that of the oar.

* Or. Younu'i t\wr«n (;/' J,iWtm*» ,m .Wttuiul l%l,w/'hi/, vol. ii. p. 627. The above table In aha taken front ll,U work, Dr. Young calculated the velocity of Unlit at 800,000 mill,,.,, n*.t ,,, Hj mlmttc« i tall I have adopted Sir John Hcrai'licl'* u»»um|ition of |.,',„„, ,,,,i,,., ii,,r mvond, which makes the numbers In the fuut'tli column of ihu , .u, ditUwut to those given b\Young.

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The following is the range of the human hearing according to Biot*:—

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But the actual range varies in different individuals, and we shall not be far from the truth if we assume, with Dr. Wollastonf and Sir John Herschel J, that the whole range of human hearing includes about nine octaves.

Now on comparing the range of human hearing with that of vision, we find the relative limits of the two senses to be as follows:—

Eye. Ear.

Commencement of sensibility 1 1

Cessation of ditto 1-nny

1024

It is highly probable, however, that the range of human vision, like that of hearing, is subject to variation in different individuals.

From these observations, then, it will be understood, that, according to the undulatory theory, the colour of the light depends on the lengths of the waves, or on their number in a given time. Thus red has the largest waves, and, therefore, the smallest number in a given time; while violet has the Shortest waves, and, therefore, the greatest number in a given time.

The intensity of the light depends on the amplitude or extent of excursion of the ethereal molecules from their points of rest; or in other words, on the height of the wave. Just as when we make a cord sound, we find that the sound diminishes in proportion to the diminution of the amplitude of the oscillations.

Vibrations.—The vibrations of the ethereal molecules may be rectilinear or curvilinear. It is not easy to give a popular illustration of the first, which, however, may be easily conceived; but the motion of a pendulum is an excellent example of curvi

* Precis Elementaire de Physique, vol. i. p. 324.

t Phil. Trans. 1820, p. 306.

X Encyclopedia Metropolitana, art. Sound, p. 792.

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