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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 grosser forms of matter. Indeed, Dr. Young supposed that the vibrating medium is the ether and ponderable matter conjointly.

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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 ethereal 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 state 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.

A General and Elementary View of the Undulatory Theory, as applied to the Dispersion of Light. By the Rev. 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, 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, and the blue . The absolute Jength 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, in other words, the sensibility ceases much sooner in the case of the eye than in that of the car.

Dr. Young's Course of Lectures on Natural Philosophy, vol. ii. p. 627. The above table is also taken from this work. Dr. Young calculated the velocity of light at 500,000 million feet in 8 minutes; but I have adopted Sir John Herschel's assumption of 192,000 miles per second, which makes the numbers in the fourth column of the table different to those given by Young.

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. Wollaston+ and Sir John Herschel‡, 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:

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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

* Précis Elémentaire de Physique, vol. i. p. 324.

+ Phil. Trans. 1820, p. 306.

Encyclopædia Metropolitana, art. Sound, p. 792.

linear motion. A ball, suspended by a string, describes in vibrating a curved line, or, in other words, it vibrates in the arc of a circle.

An assemblage of molecules, vibrating rectilinearly, in the same plane, and in all phases of their vibrations, constitutes a plane wave. An assemblage of molecules, vibrating curvilinearly or rotating, the rotation or vibration of every molecule being made in parallel planes, constitutes what may be termed a spiral or helicoidal wave. If the molecule revolve in a circle, the wave is circular; if in an ellipse, the wave is elliptical.

All motion being naturally rectilinear, it follows, that when we see a body moving in a curve of any kind, we conclude that it must be under the influence of at least two forces; one putting it in motion, and another drawing it off from the rectilinear course, which it would otherwise have continued to move in. The cause of these curvilinear movements of the ethereal molecules will be subsequently explained.

FIG. 8.

The Rev. Professor Baden Powell has contrived an ingenious machine, for showing in what manner rectilinear and curvilinear vibrations produce respectively plane and helicoidal (circular or elliptical) waves. It is founded upon this geometrical construction: a finite line, P Q, moves always through the point C, and with its end P always in the circumference of a given circle, whose centre is A; the end Q will describe a certain curve, which appears upon analysis to be one of a high order, but having in general some sort of oval form, which varies as the distance AC is altered. If A C be very great compared with the radius of the circle, Q will move up and down, almost in a straight line if A C be somewhat less, its path will resemble an ellipse; if still less, it will be more rounded or resemble a circle.

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Upon this the machine is constructed as follows (A and C correspond in both diagrams):

The lower part consists of a stout iron wire bent into a series of cranks, of which the two extremes are in the same position, e. g. downwards; the middle one vertical, and the intermediate ones at intermediate inclinations. Attached to each crank by a hinge or joint, is a long rod, R R', &c., which passes through an aperture in a cross-1 -bar, C C', at the top. The top of this rod is made conspicuous by an ivory ball or a ball painted white, B, the rest of the apparatus being painted black. The bar CC is attached to the supports A CA' C', by screws, and can be removed (without changing the rods) from DD to the positions D' D', or D" D". The proportions of the machine are not essential, but

only that the length of the rods should be great compared with that of the cranks. When the bar is at D D, on turning the handle a plane polarized wave is produced by the balls; when at D' D' an elliptical one; and when at D" D" a circular one-that is, what, for illustration, and to the eye, may be considered so. If the distance A D" be eighteen inches, A D' should be about twenty-four, and A D about thirty-six inches; but these are not material as to exactness.

Mr. E. M. Clarke, philosophical instrument-maker, of the Strand, has constructed this instrument without cranks. The upright rods are attached inferiorly to metallic rings, through each of which runs an axis, A A'; and on this axis the rings are fixed in a spiral or helicoidal manner.

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Professor Powell's Machine, as constructed by Mr. Clarke.

Transversal Vibrations.-I come now to a most important part of the undulatory hypothesis-that by which the phenomena of polarized, as distinguished from those of common or unpolarized, light are accounted for. I refer now to the hypothesis of transversal vibrations, first suggested, I believe, by Dr. Young, but most admirably developed and applied by Fresnel.

"The existence of an alternating motion of some kind, at minute intervals along a ray, is," says Professor Powell,* *" as real as the motion of translation by which light is propagated through space. Both must essentially be combined in any correct conception we form of light. That this alternating motion

* A General and Elementary View of the Undulatory Theory, p. 4.

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