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Very much thinner plates than those which present colours, do not reflect light, and when viewed in this position, appear black. But they still transmit light, and when viewed by transmitted light, appear white.

Wedge-shaped plates present a series of parallel bands or fringes of colour.

A plate having the form of a plano-concave lens, the thinnest part of the plate being in the centre, gives a series of concentric rings of brilliant colours. Those seen by reflected light, have a black spot in the centre, while the transmitted rings have a white spot in the centre.

These different phenomena of thin plates are brilliantly illustrated in the lecture-room by the oxyhydrogen lime-light, which, after passing through the condensers of the lantern, is polarized, then passed through films of selenite (of uniform thinness, or wedge-shaped plates, or plano-concave films) afterwards through the two lenses called the powers, and ultimately analysed by a plate of tourmaline, or a bundle of plates of thin glass. The nature of the changes will be explained hereafter.

The squares of the diameters of the reflected coloured rings are as the odd numbers, 1, 3, 5, 7, 9, &c.; while the squares of the diameters of the transmitted rings are as the even numbers, 0, 2, 4, 6, 8, 10, &c.

The brilliant colours, produced by thin plates of air between the lamina; of mica, of selenite, and of Iceland spar, and between plates of glass, are familiar illustrations of the colours caused by thin plates of a gaseous substance.

The colours caused by thin films of oil of turpentine or other essential oils, of alcohol or of water, and by soap-bubbles, are wellknown examples of the colours caused by thin plates of liquids.

The iridescent hues produced on copper or steel by heat, and which depend on the formation of a thin film of metallic oxide, are good illustrations of the colours caused by thin plates of solids. But the most brilliant are those caused by thin films of peroxide of lead, formed upon polished steel plates, by the electrolytic decomposition of acetate of lead. These splendid prismatic tints were discovered by Nobili*, and are commonly known as Nobili's colours or metallo-chromes. The mode of producing them has been described by my friend Mr. Gassiot, in a paper read before the Royal Societyf. If we place on the polished steel plate a card screen in which some device is cut out,very beautiful figures, having a splendid iridescent appearance, are produced.

In all the cases hitherto alluded to, I have supposed white or

* See Taylor's Scientific Memoirs, vol. i. part 1.

t See the Proceedings of the Royal Society, for March, 1840; also Brande's Manual of Chemistry, 5th edit., p. 836.

compound light to be used; and then the colours are iridescent or prismatic. But if monochromatic or homogeneous light be employed, the rings are of a uniform tint cr colour, and are separated by obscure bands or rings. Red light yields the broadest, violet light the narrowest rings.

Minute particles, fibres, and grooved surfaces also produce prismatic or iridescent colours by white light. Thus, minute particles of condensed vapour, obtained by breathing on glass, give rise to this effect. A familiar illustration is to be found in the halos observed around the street-lamps, when viewed at night through a coach-window, on the glass of which vapour is deposited. In this case the colours are seen by transmission. Dr. Joseph Reade's beautiful instrument, called the Irucope, brilliantly displays the colours produced by reflection from a plate covered with condensed vapour. It consists of a plate of highly-polished black glass, having its surface smeared with a solution of fine soap, and subsequently dried by rubbing it clean with apieceof chamois leather. If the surface, thus prepared,be breathed on, through a glass tube, the vapour is deposited in brilliant coloured rings. But as, in this mode of experimenting, the plate of vapouris thickest inthe middle,and thinnest in the circumference, the rings have black circumferences instead of black centres.

Minute fibres of silk, wool, and of the spider's web, also present in sunshine a most vivid iridescence.

A very minutely grooved surface also presents a prismatic or iridescent appearance in white light. Of this mother-of-pearl is a familiar instance—as also opal. Micrometer scales frequently present the same appearances; and Barton's buttons and other iris ornaments owe their resplendence to the numerous minute grooves cut in the surface of the metal. If a beam of light from the oxyhydrogen apparatus be received on one of Barton's buttons, an iridescent image may be thrown on a screen several yards distant; thus furnishing a good lecture-room illustration of the colours of grooved surfaces.

9. Double Refraction.—When a pencil of light falls in certain directions on any crystals, which do not belong to the cubical system, it is split or divided into two other pencils, which diverge and follow different paths; and when their divergence is considerable, objects viewed through them appear doubled. The change thus effected on a ray of light is denominated double refraction. The substance which is commonly used to produce this effect is that variety of transparent crystallized carbonate of lime, called Iceland spar, or sometimes calcareous spar, or, for brevity, calc-spar. In every double refracting crystal there are, however, one or more directions in which double refraction does not take place. These are called axes of double refraction: they might with more propriety be termed axes of No double refraction.

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I now pass a beam of light (produced by throwing the oxyhydrogen flame on lime) through a rhombohedron of Iceland spar, and we obtain two images on the screen. By rotating the crystal on its axis, one of the images revolves around the other, but neither disappears during the revolution. Now this fact proves that the light which falls on the crystal is unpolarized; for if it had been polarized, one image would have disappeared in certain positions, as 1 shall hereafter prove.

10. Polarization.—When submitted to certain influences, the rays of common light acquire peculiar properties, designated by the term polarization. These peculiarities are not distinguishable by the unassisted eye.

A very common question, put by persons who have not studied the subject, is, " What is polarized light f" and the philosopher feels very considerable difficulty in giving a concise and intelligible reply; so that the enquirer, perhaps after listening to a lengthened detail, frequently goes away, without obtaining as he says, a direct and short answer to his question.

There are two modes of reply: one is to describe, independent of all hypotheses, the properties by which polarized light is distinguished from common light; the. other, is to adopt some hypothesis of the nature of light, and, therefore, to give an hypothetical explanation of the nature of polarized light. Whichever method we adopt—and I shall give both—lengthened details are necessary to enable the uninitiated to comprehend the subject.

There are four methods of polarizing light, viz.

a. Reflection.

b. Simple refraction.

c. Double refraction.

d. Transmission through a plate of tourmaline.

In the following table I have contrasted some of the distinguishing characteristics of common and polarized light:

A Ray of Common Light, A Ray of Polarized Light,

1. Is capable of reflection, at oblique 1. Is capable of reflection, at oblique angles of incidence, in every po- angles of incidence, in certain posisition of the reflector. tions only of the reflector.

2. Penetrates a plate of tourmaline 2. Penetrates a plate of tourmaline (cut parallel to the axis of the (cut parallel to the axis of the crystal) in every position of the crystal) in certain positions of the plate. plate, but in others is wholly intercepted.

3. Penetrates a bundle of parallel 3. Penetrates a bundle of parallel glass plates, in every position of glass plates, in certain positions of the bundle. the bundle, but not in others.

4. Suffers double refraction by Ice- 4. Does not suffer double refraction land spar in every direction, ex- by Iceland spar in every direction, cept that of the axis of the except that of the axis of the crystal. crystal. In certain positions, it

suffers single refraction only.

Thus, then, one mode of replying to the before-mentioned question would be, by recapitulating the facts stated in this second column. This reply would form what I may term a matterof-fact answer, being independent of all hypothesis.

The naked or unassisted eye cannot then distinguish common from polarized light. Every person must have repeatedly seen polarized light, but not knowing how to recognize it, has failed to distinguish it from common light. If you look at a polished mahogany table, placed between you and the window, part of the light reflected from the table is polarized. When you look obliquely at the goods in a linendraper's shop, through the plate-glass window, part of the light by which you see the articles is polarized. When you see two images by a crystal of Iceland spar the transmitted light is polarized. The atmospheric light is frequently polarized, especially in the earlier and later periods of the day when the solar rays fall very obliquely on the atmosphere. At the present season, the effect may be perceived at eight or nine o'clock in the morning and five or six o'clock in the afternoon, the observer standing with his back to the sun, or with his face north or south. I have found that the effect is best perceived when the sun is shining, and the atmosphere more or less misty.

It is obvious, therefore, that after we have polarized a ray of light, we must employ some agent to detect its peculiar properties. This agent is called the analyser. It would be better understood if it were termed the test. It may be a reflecting plate, a plate of tourmaline, a bundle of glass plates, a Nichol's prism, or a double refracting prism; in fact, the analyser or test must be a polarizer.

Thus, then, apolariscope consists of two parts : one for polarizing, the other for analysing or testing the light. There is no essential difference between the two parts, except what convenience or economy may lead us to adopt; and either part, therefore, may be used as polarizer or analyser; but whichever we use as the polarizer, the other then becomes the analyser.

a. Polarization by reflection.—This method of polarizing light was discovered by Malus, in 1808. He was viewing, through a double refracting prism, the light of the setting sun reflected from the glass windows of the Luxemburgh palace in Paris; and, on turning round the prism, he was surprised to observe a remarkable difference in the intensity of the two images: the most refracted alternately surpassing and falling short of the least refracted in brightness.

Polarizing reflectors are usually glass. This should be either ground or blackened at the back to prevent posterior reflection. Water is seldom made use of. Mica may be employed instead of glass. A well-polished or varnished piece of wood (as a table, top of a pianoforte, or a counter) is very convenient. Marble also answers tolerably well. The shining back of a book is oftentimes serviceable. Metallic plates are objectionable; since by one reflection only from them, the light is found to be ellipti

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cally polarized; though by successive reflections it becomes plane polarized.

The polarizing angle varies for different substances, as the following table shows:

Angles of Polarization by reflection.

Polarisation by Reflection.

a. Incident ray of common or unpo larized light.

b. Plate of glass (polarizing plate).

c. Reflected ray of polarized light.

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From a very extensive series of experiments, made to determine the maximum polarizing angles of various bodies, Dr. Brewster arrived at the following law: the index of refraction is the tangent of the angle of polarization. It follows, therefore, that the reflected polarized ray forms a right angle with the i efracted ray.

Here, perhaps, is the most convenient place for referring to a suggested application of polarized light. I have stated that light is polarized by water, at an angle of 52° 45'. By the analyser (as a tourmaline, or Nichol's prism, or a bundle of glass plates) the whole of this reflected polarized light may be intercepted without offering any impediment to the unpolarized but refracted light which has traversed the water; so that objects may be more readily seen at the bottom of ponds, rivers, and the sea, by this expedient than otherwise, since the glare of the reflected light is prevented. Hence anglers, and those fond offish-spearing, may employ this property of polarized light in the discovery of the objects of their sport; and commanders of vessels may avail themselves of it to detect rocks and shoals in the bottom of the ocean, which are not otherwise visible except by viewing them from the mast-head, by which the angle of reflexion is diminished, and consequently the quantity of light reflected is thereby lessened.

I proceed now to demonstrate the polarization of light by reflection, and the essential properties of the polarized ray. For this purpose, I obtain an intense light by throwing the flame of a jet of mixed oxygen and hydrogen gases on a cylinder of lime.

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