This mineral was first discovered during the sixteenth century, in the island of Ceylon, afterwards in Brazil, and since that period at various localities in the four quarters of the globe. In the Grevillian collection purchased many years ago by government for the British Museum, there is a fine specimen of red tourmaline valued at 500_l._ The green tourmaline is named Brazilian emerald, and the Berlin blue tourmaline is called Brazilian sapphire; the mineral chiefly consists of sand (silica) and alumina, with a small quant.i.ty of lime, or potash, or soda, boracic acid, and sometimes oxide of iron or manganese. When light is pa.s.sed through a slice of this mineral it is immediately polarized, one of the transversal vibrations being absorbed, stopped, or otherwise disposed of, the other only emerging from the tourmaline, consequently it is one of the most convenient polarizers, although the polarized light partakes of the accidental colour of the mineral. Green, blue, and yellow tourmalines are bad polarizers, but the brown and pink varieties [Page 342] are very good, and it is a most curious fact that white tourmaline does not polarize. (Fig. 330.)
[Ill.u.s.tration: Fig. 330. Crystal of tourmaline slit (parallel to the axis) into four plates, which when ground and polished, may be used for the polarization of light.]
The mineral crystallizes in long prisms, whose primitive form is the obtuse rhomboid, having the axis parallel to the axis of the prism. The term axis with reference to the earth, as shown at page 16, is an imaginary _single line_ around which the ma.s.s rotates, but in a crystal it means a _single direction_, because a crystal is made up of a number of similar crystals, each of which must have its axis, thus the whitest Carrara marble reduced to fine powder, moistened with water and placed under a microscope, is found to consist chiefly of minute rhomboids, similar to calcareous spar. The smallest crystal of this mineral is divisible again and without limit into other rhombs, each of which possesses an axis. (Fig. 331.)
[Ill.u.s.tration: Fig. 331 represents a crystal, the axis of which is the direction A B. The dotted lines show the division of the large crystal into four other and smaller ones, each of which has its axis, A C, C B, D E, F G; and every line within the large crystal parallel to A B is an axis, consequently the term is employed usually in the plural number _axes_.]
If a plate of tourmaline is held before the eye whilst looking at the sun (like the gay youth in Hogarth"s picture who is being arrested whilst absorbed with the wonders of a tourmaline, which was, in the great painter"s time, a popular curiosity,) it may be turned round in all directions without the slightest difference in the appearance of the light, which will be coloured by the accidental tint of the crystal, but if a second slice of tourmaline is placed behind the other, there will be found certain directions in which the light pa.s.ses through both the slices, whilst in other positions the light is completely cut off.
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When the axes of both plates coincide, the light polarized by one tourmaline will pa.s.s through the other, but if the axes do not coincide, and are at right angles to each other, then the polarized light is entirely stopped, and the _rationale_ of this will be appreciated at once if a tourmaline is regarded (mechanically) as if it were like a grating with perpendicular bars through which the polarized light will pa.s.s. Any number of such gratings with the bars parallel would not stop the polarized light, but if the second grating is turned round ninety degrees, the bars will be at right angles to those of the first grating, and the perpendicular wave of polarized light cannot pa.s.s. (Fig. 332.)
[Ill.u.s.tration: Fig. 332. A. Model of the first slice of tourmaline into which the transversal vibrations, B, are pa.s.sing; the horizontal wave is absorbed, and the perpendicular polarized one proceeds to the second slice of tourmaline, C, where the bars (the axes) being at right angles to those of A, it is stopped, and cannot pa.s.s through until the bars of C are parallel with A.]
_Splendid Chromatic effects produced by Polarized Light._
Having discussed the various modes of obtaining polarized light, the next step is to arrange an apparatus by which certain double refracting crystals, and other bodies, shall divide a ray of polarized light, and then by subsequent treatment with another polarizing surface, the divided rays are caused to _interfere_ with each other, and afford the phenomena of colour. Bodies that refract light singly, such as gases, vapours or liquids, annealed gla.s.s, jelly, gums, resins, crystallized bodies of the tessular system, such as the cube and octohedron, do not afford any of the results which will be explained presently, except by the influence of pressure, as in unannealed gla.s.s, or a bent cold gla.s.s bar. By compression or dilatation, they are changed to double refractors of light. The bodies that possess the property of double refraction (though not to the visible extent of Iceland spar), are all other bodies such as crystallized chemicals, salts, crystallized minerals, animal and vegetable substances possessing a uniform structure, such as horn and quill; all these substances divide the ray of polarized light into two parts, and by placing a thin film of a crystal of selenite (which is one of the best minerals that can be used for the purpose) in the path of the beam of polarized light, coming either from the gla.s.s plates, as in No. 2, (Fig. 325), page 338, or from a slice of tourmaline, and then receiving it through the ordinary focusing lenses or object-gla.s.ses of the oxy-hydrogen microscope, no colour is yet apparent in the image of the selenite on the screen, until [Page 344] another tourmaline, or a bundle of gla.s.s plates, is placed at an angle of 56 45", and at right angles to the plane of reflection of the first set of plates; then the most gorgeous colours suddenly appear over all parts of the film of selenite as depicted on the screen, like other objects shown by the oxy-hydrogen microscope. (Fig. 333.)
[Ill.u.s.tration: Fig. 333. Duboscq"s polarizing apparatus, A. The light and the condenser lens. B. The plates of gla.s.s at the proper angle, C.
The selenite object, D. The focusing lens. E. The second bundle of plates of gla.s.s called the a.n.a.lyser, F. A stop for extraneous rays of light, G. The image of the film of selenite most beautifully coloured.
G.o.ddard"s oxy-hydrogen polariscope is one of the most convenient, because either the reflected or refracted polarized rays can be rendered available; it consists of the apparatus shown at Fig. 325, and to this is added a low microscope power, and stage to hold the selenite or other objects, with another bundle of sixteen plates of the thin microscopic gla.s.s or mica, called the a.n.a.lyser. A slice of tourmaline, or a Nicol"s prism may be employed, instead of the second bundle of reflecting plates. When the ray of polarized light reflected from the first set of gla.s.s plates enters the doubly refracting film of selenite, which is about the fortieth or fiftieth part of an inch in thickness, it is split into the ordinary and extraordinary rays, and is said to be _dipolarized_, and forms two planes of polarized light, vibrating at right angles to each other. When the latter are received on another bundle of plates of gla.s.s called the a.n.a.lyser, at an angle of 56 45", but at right angles to the first set of gla.s.s plates, they interfere, because in the pa.s.sage of the two rays from the selenite they have traversed it in different directions, with different velocities; one of these sets of waves will therefore, on emerging from the opposite face of the selenite be r.e.t.a.r.ded, and lie [Page 345] behind the other; but being polarized in different planes, they cannot _interfere_ until their planes of polarization are made to coincide, which is [Page 346]
effected by means of the second bundle of gla.s.s plates called the a.n.a.lyser; and when this is brought into a position at right angles to the first set of reflecting gla.s.s plates, half the ordinary wave interferes with half the extraordinary wave; and being transmitted through the a.n.a.lyser, produces, say red and orange, whilst the remaining halves also interfere, and being reflected, afford the complementary colours green and blue. (Fig. 334.) The term _complementary_ is intended to define any two colours containing red, yellow, and blue, because the three combined together produce white light; for example, the complementary colour to red would be green, because the latter contains yellow and blue; the complementary colour to orange would be blue, because the former contains red and yellow. Any two colours, therefore, which together contain red, yellow, and blue are said to be _complementary_; and if this principle was better understood, ladies would never commit such egregious blunders as they occasionally do in the choice of colours for bonnets and dresses, and select a blue bonnet to be worn with a green dress, or _vice versa_. By rotating the a.n.a.lyser, the reflected and refracted rays change colours, and if the former is red and the latter green, by moving the a.n.a.lyser round 90, the reflected rays change to green and the refracted to red; at 180 the colours again change places; at 270 the reflected ray will be again green, and the refracted red; to be once more brought back at 360 to the original position, viz., reflected rays red, refracted green. The thickness of the films of selenite determines the particular colour produced.
[Ill.u.s.tration: Fig. 334. The electric lamp and lantern of Duboscq, showing the projection of the carbon poles on the disc. This experiment is performed with the help of the plano-convex lens, A, and the rays pa.s.s through a very narrow aperture at B.]
[Ill.u.s.tration: Fig. 335. A A. Card model of a beam of polarized light coming from the first bundle of plates of gla.s.s, shown at Fig. 326, p.
339. B. Model of the film of selenite, which divides or dipolarizes the ray A A into C and D, which, interfering by means of the second bundle of plates of gla.s.s called the a.n.a.lyser Z, produce reflected chromatic effects by interference at E, and refracted effects at F.]
If the selenite is of a uniform thickness, one colour only is obtained, and by ingeniously connecting pieces of various thicknesses (in the same forms as stained gla.s.s for cathedral windows), the most beautiful designs were made by the late Mr. J. T. Cooper, jun., which have since been manufactured in great quant.i.ty and variety by Mr. Darker, of Paradise-street, Lambeth. The colours of these selenite objects are seen by placing them in front of a piece of black gla.s.s, fixed at the polarizing angle, and then examining the design with a slice of tourmaline, or still better with a single-image Nicol prism, when the most brilliant colours are obtained, and varied at every change of the angle of the a.n.a.lyser.
Selenite, or sparry-gypsum, is the native crystallized sulphate of lime, which contains water of crystallization (CaO, SO_{3}, 2H_{2}O). It frequently occurs imbedded in London clay, and is called _quarry gla.s.s_ by the labourers who find it at Shotover Hill, near Oxford, and also in the Isle of Sheppey.
At a very early period, before the discovery of gla.s.s, selenite was used for windows; and we are told that in the time of Seneca, it was imported into Rome from Spain, Cyprus, Cappadocia, and even from Africa. It continued to be used for this purpose until the middle ages, for Albinus informs us, that in his time, the windows of the dome of Merseburg were of this mineral. The first greenhouses, those invented by Tiberius, were covered with selenite. According to Pliny, beehives were encased in selenite, in order that the bees might be seen at work.
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The late Dr. Pereira has placed the phenomena already described in the form of a most instructive diagram, which we borrow from his elaborate work on "Polarized Light." (Fig. 336.)
[Ill.u.s.tration: Fig. 336. A. A ray of common or unpolarized light, incident on B. B. The polarizer (a plate of tourmaline). C. A ray of plane polarized light, incident on D. D. The doubly-refracting film of selenite. E. The extraordinary ray. O. The ordinary ray, produced by the double refraction of the ray C. G. The a.n.a.lyser (or doubly-refracting or Nicol"s prism). E O. The ordinary ray. E E. The extraordinary ray, produced by the double refraction of the extraordinary ray, E. O O. The ordinary ray. O E. The extraordinary ray, produced by the double refraction of the ordinary ray, O.]
[Ill.u.s.tration: Fig. 337. No. 1. Unannealed gla.s.s for the polariscope.
Nos. 2 and 3. Appearance of the black cross and coloured circles in a square and circular piece of unannealed gla.s.s in the polariscope.]
The chromatic effects described are not confined to selenite objects only, but are obtained from gla.s.s, provided the particles are in a state of unequal tension, as in ma.s.ses of unannealed gla.s.s of various forms.
(Fig. 337.) Consequently, polarized light becomes a most valuable means for ascertaining the condition of particles otherwise invisible and inappreciable. One of the most beautiful experiments can be made [Page 348] with a bar of plate-gla.s.s, which refracts light singly until pressure is applied to the centre, in order to bend it into an arch or curve, when the appearance presented in Fig. 338 is apparent.
[Ill.u.s.tration: Fig. 338. A B. Bar of gla.s.s under the pressure of the screw C, and appearance of bands or fringes of coloured light, which entirely disappear on the removal of the screw. An effect, of course, only visible by polarized light.]
A quill placed in the polarizing apparatus is also discovered to be in a state of unequal tension by the appearance of coloured fringes within it, which change colour at every movement of the a.n.a.lyser.
Another series of beautiful appearances present themselves when a ray of white polarized light is made to pa.s.s perpendicularly through a slice of any crystallized substance with a single axis; if the a.n.a.lyser consist of a slice of tourmaline, a number of concentric coloured rings are rendered visible with a black cross in the centre, which is replaced with a white one on moving the tourmaline through each quadrant of the circle.
Crystals of Iceland spar present this phenomenon in great beauty; and if the crystal (such as nitre) has two axes of double-refraction, a double-system of coloured rings is apparent, with the most curious changes and combinations of the black and white crosses with them. (Fig.
339.)
[Ill.u.s.tration: Fig. 339. Crystal of nitre with two axes, as seen in polarized light.]
Mr. G.o.ddard has recommended the optical arrangement (Fig. 340) for showing the rings with great perfection, as also the number of rings that increase in some crystals (the topaz, for example), with the divergence of the rays of polarized light pa.s.sing through them.
Mr. Woodward"s table and oxy-hydrogen polariscope and microscope, made by Smith and Beck, of Coleman-street, is well adapted, from its [Page 349] simplicity and perfection, to exhibit all the varied and beautiful effects of polarized light; and we only regret that want of s.p.a.ce prevents us describing it in detail, although the reader may see the body of the apparatus at page 123, where the modifications of the oxy-hydrogen light are described and figured; and the polarizing apparatus would be placed, of course, in front of the light issuing from the lantern.
[Ill.u.s.tration: Fig. 340. A A A. Polarized light. B B. A lens of short focus, transmitting a cone of light with an angle of divergence for its rays, C C, of 45. D D. The crystal of topaz, Iceland spar, or nitre. E E. The slice of blue tourmaline for a.n.a.lysing.]
Finally, the question of utility (the _cui bono_) may be considered in answer to the query, What is the use of polarized light?
The value to scientific men of a knowledge of the nature of this modification of common light cannot be overrated. It has given the philosopher a new kind of test, by which he discovers the structure of things that would otherwise be perfectly unknown; it has given the astronomer increased data for the exercise of his reasoning powers; whilst to the microscopist the beauty of objects displayed by polarized light has long been a theme of admiration and delight, and has served as a guide for the identification of certain varieties of any given substance, such as starch.
A tube provided with a polarizer of tourmaline, or a single-image Nicol prism, is invaluable to the look-out at the mast-head in cases where vessels are navigating either inland or sea water, where the presence of hidden rocks is suspected, because the polarizer rejects all the glare of light arising from unequal reflection at the surface of water, and enables the observer to gaze into the depths of the sea and to examine the rocks, which can only be perfectly visible by the refracted light coming from their surfaces through the water.
Professor Wheatstone has invented an ingenious polarizing clock for showing the hour of the day by the polarizing power of the atmosphere.
Birt, Powell, and Leeson have each invented instruments for examining the circular polarization of fluids, by which a more intimate knowledge of the relative values of saccharine solutions may be obtained, besides unfolding other truths important to investigators in this branch of science.
And last, but not least, it was with the a.s.sistance of polarized light [Page 350] that Dr. Faraday established the relation that exists between light and magnetism, and through the latter, with the force of electricity; and the next figure indicates the necessary apparatus required to repeat this highly important physical truth--viz., the deviation of the plane of polarization of light by the influence of the magnetic force from a powerful electro-magnet. (Fig. 341.)
[Ill.u.s.tration: Fig. 341. A. The light and condenser lens. B.
Single-image Nicol prism. C. Rock crystal of two rotations. D. A double-convex lens. E E. Faraday"s heavy gla.s.s. F F. The powerful electro-magnet connected with battery. G. Double-refracting prisms. H.
Image, or screen where the deviation of the plane of polarization by the magnetic force is shown.]
By another and equally beautiful experiment at the London Inst.i.tution, Professor Grove demonstrated the production of all the other kinds of force from light, using the following arrangement for the purpose:
A prepared daguerreotype plate is enclosed in a box full of water having a gla.s.s front with a shutter over it; between this gla.s.s and the plate is a gridiron of silver wire; the plate is connected with one extremity of a galvanometer coil, and the gridiron of wire with one extremity of a Breguet"s helix; the other extremities of the galvanometer and helix are connected by a wire, and the needles brought to zero. As soon as a beam of either daylight or the oxy-hydrogen light is, by raising the shutter, permitted to impinge upon the plate, the needles are deflected. Thus, light being the initiatory force, we get
_Chemical action_ on the plate, _Electricity_ circulating through the wires, _Magnetism_ in the coil, _Heat_ in the helix, _Motion_ in the needle.
Such, then, are some of the glorious phenomena that we have endeavoured to explain in this and the preceding chapters on light. Here we have noticed specially how completely we owe their appreciation to the sense of sight operating through the eye, the organ of vision. Well may those who have lost this divine gift speak of their darkness as of a lost world of beauty to be irradiated only by better [Page 351] and more enduring light; and most feelingly does Sir J. Coleridge speak on this point when he says:--
"Conceive to yourselves, for a moment, what is the ordinary entertainment and conversation that pa.s.ses around any one of your family tables; how many things we talk of as matters of course, as to the understanding and as to the bare conception of which sight is absolutely necessary. Consider, again, what an affliction the loss of sight must be, and that when we talk of the golden sun, the bright stars, the beautiful flowers, the blush of spring, the glow of summer, and the ripening fruit of autumn, we are talking of things of which we do not convey to the minds of these poor creatures who are born blind, anything like an adequate conception. There was once a great man, as we all know, in this country, a poet--and nearly the greatest poet that England has ever had to boast of--who was blind; and there is a pa.s.sage in his works which is so true and touching that it exactly describes that which I have endeavoured, in feeble language, to paint. Milton says:--
"Thus with the year Seasons return; but not to me returns Day, or the sweet approach of even, or morn, Or sight of vernal bloom, or summer"s rose, Or flocks, or herds, or human face divine; But cloud instead, and ever-during dark Surrounds me; from the cheerful ways of men Cut off, and for the book of knowledge fair Presented with a universal blank Of Nature"s works, to me expunged and rased, And wisdom at one entrance quite shut out.
So much the rather, thou, celestial light, Shine inward, and the mind through all her powers Irradiate; there plant eyes; all mist from thence Purge and disperse, that I may see and tell Of things invisible to mortal sight."
The great poet, when intent upon his work, sought for celestial light to accomplish it. And this brings me to that part of the labours of our Blind Inst.i.tutions upon which I dwell the most and which, after all, is the greatest compensation we can afford to the inmates for the affliction they suffer; and that is, the means we provide for them to read the blessed Word of G.o.d, which they can read by day as well as by night, for light in their case is not an essential."
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