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opposite manner, before they enter the posterior half of the prism. Over the tourmaline this prism has the advantage of being perfectly free from colour, but it has the great objection of giving a very limited field of vision.

4. Polarization by the Tourmaline. The last mode of polarizing light to which I shall have occasion to allude, is by transmission through a plate of tourmaline, cut parallel to the axis of the crystal.

The substance called tourmaline, and to which I have already referred, is a precious stone, which is occasionally cut and polished, and worn as a jewel. There is good reason for supposing that it is the substance to which Theophrastus alludes under the name of lyncurium (Xvykupiov). It is found in various parts of Europe, Asia, Africa, and America. Much of that found in commerce comes from the Brazils. It occurs in thick and short, as well as in acicular prismatic crystals, belonging to the rhombohedric system, and which have three, six, or more sides and dissimilar summits. Thus in most tourmalines the extremities or summits of the crystal differ from each other in the number or situation of the planes; and like other unsymmetrical crystals, the tourmaline becomes electrical while changing its temperature, one extremity becoming positive, the other negative. +Elect.

FIG. 6.

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c. Tourmaline slit, parallel to the axis, into four plates, which, when ground and polished, are used as either polarizers or analysers.

d. Tourmaline cut at right angles to its axis. The plates, thus obtained, are ground and polished, and then used in the polariscope for producing coloured rings.

The colour of the tourmaline is various, but green and brown are the prevailing tints. Curiously enough, there appears to be a remarkable connection between the colour and the other optical, as well as the electrical properties of the tourmaline. Green, blue, and yellow colours are, in general, imperfect polarizers. The brown and pinkish tints are the best. White colourless tourmalines do not polarize.

The principal constituents of the tourmaline, are silica and

alumina. Boracic acid is always present, as also magnesia. Iron, potash, soda, &c., are not constant ingredients.

For optical purposes, the tourmaline is cut in two directions, viz. parallel, and likewise at right angles to the crystallographical axis. Tourmaline plates for polarizing or analyzing, are cut parallel to the axis about of an inch thick; but for depolarizing, or showing coloured rings, at right angles to the axis. Considerable care and experience are required to prepare good plates.* If they be not cut perfectly parallel to the axis, their polarizing and analyzing powers are greatly impaired. In buying plates, avoid cracks, flaws, and deep colours, and select those which by experiment you find to be good polarizers, for as the polarizing powers are very unequal in different crystals, nothing but a trial of each plate can determine its goodness.

The light which is transmitted by a plate of tourmaline (a or a') (cut parallel to the axis), is plane-polarized. A second plate of tourmaline (b), if held in the same position, transmits the light polarized by the first plate; but if the second plate (b) be turned round, so that its axis is at right angles with the axis of the first plate, no light is transmitted.

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Action of Tourmaline Plates on Light.

a. b. Two plates of tourmaline (cut parallel to the axis of the crystal), with their axes coincident; a. is called the polarizer, and b. the analyser.

c. Incident ray of unpolarized light.

d. Transmitted ray of light polarized by a.

e. Ray polarized by a, and transmitted by b.

a. b. Two plates of tourmaline with their axes opposed, so that the light, polarized by a', is intercepted by b'.

The great objection to the tourmaline, as a polarizer, is, that the transmitted polarized beam is more or less coloured. If large, transparent, and colourless polarizing tourmalines could be obtained, they would be invaluable to the optician.

That common light is polarized by transmission through a plate of tourmaline, as above described, is proved thus: - A second tourmaline, placed with its axis at right angles to the first, does not permit light to pass. But when the axes of the plates coincide, the light polarized by the first plate is trans

* Mr. Darker, of Paradise Street, Lambeth, prepares tourmaline plates for most of the opticians.

mitted by the second. Moreover, if the light transmitted through the first tourmaline be received at an oblique angle on a plate of glass, blackened at the back, it is reflected only on two sides of the ray, and at an angle of 56° 45'. Lastly, if it be tested by a double refracting prism, it is found to produce double refraction in two positions only of the ray, for on rotating the double refracting prism on its axis, we find that one of the images is alternately cut off, and in intermediate positions, two faint images only are produced.

2. WAVE HYPOTHESIS OF LIGHT.

There are two hypotheses or theories which have been formed to account for the phenomena of light; one of these is called, the projectile theory, or the theory of emission;-while the second is denominated the wave, or undulatory theory of light.

The first is sometimes called the material or Newtonian theory of light. But as on both hypotheses a fine subtile form of matter is required to account for luminous phenomena, the one hypothesis equally deserves the name of material with the other. Moreover, I cannot understand why the projectile theory is to be exclusively honoured with the name of the Newtonian; for though on some occasions Newton certainly adopts it, yet on others he appears to support the theory of waves.

On the present occasion it is not my intention to enter into any details respecting the projectile theory; for however ably and plausibly it accounts for some optical phenomena, it is manifestly incompetent to explain those which it is the object of this course of lectures to describe.

Light, a Property or Motion.-The wave theory supposes that light is a property-a motion-a vibration of something. But of what? Euler imagined that the vibrating medium, in dense bodies, was the body itself; through the gross particles of which he supposed the light to be propagated in the same manner as sound. This hypothesis, Dr. Young* declares to be "liable to strong objections;" and he adds, that "on this supposition, the refraction of the rays of light, on entering the atmosphere from the pure ether which he describes, ought to be a million times greater than it is."

Ether. To account for the phenomena of light, philosophers have assumed the existence of a vibrating medium, which has been called the ethereal medium, the luminiferous ether, or simply

A Course of Lectures on Natural Philosophy, vol. ii., p. 542. Also Phil. Trans, for 1800.

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. Roget + and Mosotti 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.

+ 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.

however, is by no means a necessary one, for "if this ether," says Newton*, "should be supposed 700,000 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 Toooo of the 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 Encke‡.

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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.

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

Astronomische Nachrichten, Nos. 210, 211.

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

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