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This light,which I shall, for brevity,call thelime-light,\s condensed by two crossed lenses (called condensers), and thrown on a plate of glass, blackened at the back, and placed at an angle of 56° 45'. The light is polarized by this plate, and being then refracted by two plano-convex glasses (termed the power), is afterwards received on a semi-transparent calico screen, strained on a wooden frame, and moistened with water. A bundle of glass plates is sometimes used as a reflecting polarizer: it has the advantage of reflecting more light, but a portion of it is unpolarized.

The light thus polarized is not distinguishable by the eye from common light, but to prove its nature, I proceed to test it. For this purpose, I place a plate of tourmaline on the front of the polariscope, and on revolving it, you observe the light on the screen is alternately cut off and admitted. At 0 the tourmaline transmits the light, at 90° it absorbs it, at 180° it transmits it, at 270° it absorbs it. I remove the tourmaline, and substitute a bundle of glass plates, placed at an angle of 56° 45'. On revolving this, the light is observed to be alternately cut off and transmitted, as in the case of the tourmaline. I now substitute a double refracting prism for the bundle of glass, and on revolving this, it is seen that in two positions double refraction takes place, and two images are produced on the screen, while in two intermediate positions, one image is cut off. Thus, then, the light reflected from the blackened glass plate of the polariscope, is polarized, since it possesses the characters assigned to this kind of light.

2. Polarization by single Refraction.—If light be transmitted obliquely through a bundle of diaphanous laminae, it suffers polarization. The very thin parallel glass plates used for microscopes, and sold by Bromley and Drake, at 315, Oxford Street, form the best medium. Sixteen or more of Polarization by single Refraction. these are to be placed parallel, V. SSdU^"ate"!"TM U8M- and the bundle then placed at

c. Reflected polarized ray. an angle of 56° 45' tO the rav

d. Refracted polarized ray. This ray is , ° , . , .| oppositely polarized to c. . to be polarized.

Common crown or window glass serves well enough for ordinary purposes. The flattest, thinnest, most colourless, and perfect pieces are to be selected. A very convenient mode of using them is the following:—Take two one ounce paper pillboxes, remove the lids and the bottoms, and then paste together the two cylinders of the boxes, so as to form a tube. Into this, place obliquely sixteen pieces of window glass.

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Having cut in each of the lids a circular hole, of about the Tht J KXpe"Ce' P'a,C? a lid at each end of the cylinder

A bundle of mica plates may be used for polarizing but it is .nfer.or to the bundle of microscope glass above referred to

3. Polarization by Double Refraction.—I have alreidv demonstrated the double refractive power of IcelandI SP

nZt1 Jta* hithert0,said anything of the nature Ke light of which the two pencils are made up

r,rnVoW/uoCe^ *o deraonstrate that the two pencils or ravs produced by this process are polarized; but the polarized Z of the one ray ,s of an opposite kind to that of theothe • 80 that

hV::jT„Ze7lt0 b? ****''***** That thLi «nnlv T fP f f y fPP'yng our tests to them. Thus, if J apply a plate of tourmaline, you observe that, as this is rotated one of the lum.nous rays is alternately cut off, which would not be the case were the rays composed of common light; whHelf both were similarly polarized they would be *im,ilt=n»U i . not alternately absorbed. If I fflSte^KStoS^ p ates for the tourmaline, one of the rays is reflec ed and th* other transmuted; and by revolving L bundle 90°! "e ay wh.ch was reflected is now transmitted, and that which was transmitted is now reflected. as

Nichols prism, or Nichols eye-piece*, is a most valuable and convenient polarizer. It is an oblique rhombic prism, whose terminal planes form an angle of 68° with the adjoining obtuse lateral edges. It is formed by ,* joining, by means of Canada i balsam> tw« wedge-shaped pieces

of Iceland spar. It is a double DnbURefractimbyarhombohedronof refractor, but the two rays are

a. b. The obtut-cLglJr'Aline drawn made. t0 deviate SO far, that Only

cr^ta? to re',resents.the «*» of the one image is seen in its usual r r„„irf„„t , . position. The cause of this is

Fig. 5

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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 (\vyicvpiov). 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 unsyrnmetrical crystals, the tourmaline becomes electrical while changing its temperature, one extremity becoming positive, the other negative.

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-Elect." .. . ..

Tourmaline.

a. Brazilian tourmaline. By cooling, the upper end becomes positively ( + )» the lower negatively (—) electrical. /'. Another tourmaline.

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

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alumina. Boracic acid is always present, as also magnesia. Iron, potash, soda, fyc., 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 -fo 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 (6), if held in the same position, transmits the light polarized by the first plate; but if the second plate (U) 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 Plata on Light, a. b. Two plates of tourmaline (cat parallel to the axis of the crystal), with their axes coincident; a. is called the polarizer, and 6. 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.

of. V. 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.

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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.

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