THE POLARIZATION OF LIGHT.
This branch of the phenomena of light includes some of the most
remarkable and gorgeous chromatic effects; at the same time, regarded
philosophically, it is certainly a most difficult subject to place in a
purely elementary manner before the youthful minds of juvenile
philosophers, and unless the previous chapter on the diffraction of
light is carefully examined, the rationale of the illustrations of
polarized light will hardly be appreciated. We have first to ask, "What
is polarized light?" The answer requires us again to carry our thoughts
back to the consideration of the undulatory theory of light, already
illustrated and partly explained at pages 262, 330.
After perusing this portion of the subject, it might be considered that
waves of light were constituted of one motion only, and that an
undulation might be either perpendicular or horizontal, according to
circumstances. (Fig. 320.)
[Illustration: Fig. 320.--No. 1. A wire bent to represent a
perpendicular vibration, which if kept in the latter position, will only
pass through a perpendicular aperture.--No. 2. A wire bent to represent
a horizontal wave which will only pass through a horizontal aperture.]
This simple condition of the waves of light could not, however, be
reconciled theoretically with the actual facts, and it is necessary in
regarding a ray of light, to consider it as a combination of two
vibrating motions, one of which, for the sake of simplicity, may be
considered as perpendicular, and the other horizontal; and this idea of
the nature of [Page 336] an undulation of light originated with the
late Dr. Young, who while considering the results of Sir D. Brewster's
researches on the laws of double refraction, first proposed the theory
of transversal (cross-wise) vibration. Dr. Young illustrated his theory
with a stretched cord, which if agitated or violently shaken
perpendicularly, produces a wave that runs along the cord to the other
end, and may be often seen illustrated on the banks of a river overhung
with high bushes; the bargemen who drive the horses pulling the vessel
by a rope, would be continually stopped by the stunted thick bushes, but
directly they approach them, they give the horse a lash, and then
violently agitate the rope vertically, which is thrown into waves that
pass along the rope, and clear the bushes in the most perfect manner.
(Fig. 321.)
[Illustration: Fig. 321. Bargeman throwing his tow-rope into waves to
get it over the thick bushes.]
[Illustration: Fig. 322. A section of a wave of common light made up of
the transversal vibration, A B and C D.]
Now if a similar movement is made with the stretched rope from right to
left, another wave will be produced, which will run along the cord in an
horizontal position, and if the latter is compared with the
perpendicular undulation, it will be evident that each set of waves will
be in planes at right angles to and independent of each other. This is
supposed to be the mechanism of a wave of common light, so that if a
section is taken of such an undulation, it will be represented by a
circle A B C D (Fig. 322), with two diameters A B, and C D; or a better
mechanical notion of a wave of common [Page 337] light is acquired from
the inspection of another of Mr. Woodward's cardboard models. (Fig.
323.)
[Illustration: Fig. 323. Model of a wave of common light.]
The existence of an _alternating motion of some kind_ at minute
intervals along a ray is, says Professor Baden 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 must have reference to certain
directions _transverse_ to that of the ray is equally established as a
consequence of the phenomena; and these _two_ principles must form the
basis of any explanation which can be attempted." A beam of common light
is therefore to be regarded as a rapid succession of systems of waves in
which the vibrations take place in different planes.
If the two systems of waves are separated the one from the other, viz.,
the horizontal from the perpendicular, they each form separately a ray
of polarized light, and as Fresnel has remarked, _common light_ is
merely _polarized light_, having _two planes_ of polarization at _right
angles_ to each other. To follow up the mechanical notion of the nature
of polarized light, it is necessary to refer again to Woodward's card
wave model (Fig. 323), and by separating the two cards one from the
other it may be demonstrated how a wave of common light reduced to its
skeleton or primary form is reducible into two waves of polarized light,
or how the two cards placed together again in a transversal position
form a ray of common light. (Fig. 324.)
[Illustration: Fig. 324.--No. 1. Common light, made up of the two waves
of polarized light, Nos. 2 and 3. ]
The query with respect to the nature of polarized light being answered,
it is necessary, in the next place, to consider how the separation of
these transversal vibrations may be effected, and in fact to ask what
optical arrangements are necessary to procure a beam of polarized light?
Light may be polarized in four different ways--viz., by reflection,
single refraction, double refraction, and by the tourmaline--viz., by
absorption.
[Page 338]
_Polarization by Reflection, and by Single Refraction._
[Illustration: Fig. 325.--No. 1. A is the lime light. B. The condenser
lenses. C. The beam of _common_ light. Here the glass plates are
removed.--No. 2. A. Lime light. B. The condenser lenses. C C. The bundle
of plates of glass at an angle of 56° 45´. D is the ray of light
polarized by reflection from the glass plates, C C, and E is the beam of
polarized light by single refraction, having passed through the bundle
of plates of glass, C C.]
In the year 1810, the celebrated French philosopher, Mons. Malus, while
looking through a prism of Iceland spar, at the light of the setting
sun, reflected from the windows of the Luxemburg palace in Paris,
discovered that a beam of light reflected from a plate of glass at an
angle of 56 degrees, presented precisely the same properties as one of
the rays formed by a rhomb of Iceland spar, and that it was in fact
polarized. _One_ of the transversal waves of polarized light of the
common light, being reflected or thrown off from the surface of the
glass, whilst the other and second transversal vibration passed
_through_ the plate of glass, and was likewise polarized in another
plane, but by _single refraction_, so that the experiment illustrates
two of the modes of polarizing light-viz., by reflection, and by single
refraction. This important elementary truth is beautifully illustrated
by Mr. J. T. Goddard's new form of the oxy-hydrogen polariscope, by
which a beam of common light traverses a long square tin box without
change; but directly a bundle of plates of glass composed of ten plates
of thin flattened crown glass, or sixteen plates of thin parallel glass
plates used for microscopes, are slid into the box at an angle of 56°
45´, then the beam of common [Page 339] light is split into two beams
of polarized light, which pursue their respective paths, one passing by
single refraction through the glass, and the other being reflected, and
rendered apparent by opening an aperture over the glass plates, and then
again by using a little smoke from brown paper, the course of the rays
becomes more apparent.
[Illustration: Fig. 326. A A. Model in wood of a bundle of plates of
glass at an angle of 56° 45´. B. Beam of common light, with transversal
vibration. C. Light polarized by reflection. D. Light polarized by
refraction.]
The same truth is well illustrated by the cardboard model wave and a
wooden plane with horizontal and perpendicular slits, placed at an angle
of 56° 45´, as at Fig. 326.
POLARIZATION BY DOUBLE REFRACTION.
The name of _Double_-refracting or Iceland Spar is given to a very
clear, limpid, and perfectly transparent mineral, composed of carbonate
of lime, and found on the eastern coast of Iceland. Its crystallographic
features are well described by the Rev. Walter Mitchell in his learned
work on mineralogy and crystallography, and it is sufficient for the
object of this article to state that it crystallizes in rhombs, and
modifications of the rhomboidal system. It must not be confounded with
rock or mountain crystal, which, under the name of quartz, crystallizes
in six-sided prisms with six-sided pyramidal tops; quartz being composed
of silica, or silicic acid and calcareous spar of carbonate of lime.
Very large specimens of the latter mineral are rare and valuable, and
the _lion_ of specimens of calcareous, or double-refracting spar, is now
in the possession of Professor Tennant, the eminent mineralogist of the
Strand. It is nine inches high, seven and three-quarters inches broad,
and five and a half inches thick; its estimated value being 100_l._ This
beautiful specimen has been photographed, and its stereograph
illustrates in a very striking manner the double refracting properties
of the spar.
If a printed slip of paper is placed behind a rhomb of Iceland spar, two
images of the former are apparent, and the stereograph already alluded
to shows this fact very perfectly, at the same time illustrates the
value of the stereoscope. Out of the stereoscope the words "Stereoscopic
Magazine" appear doubled, but seem to lie in the same plane; but
directly the picture is placed in the instrument, then it is clearly
seen that one image is evidently in a very different plane from the
other. The double-refracting power of this mineral is illustrated by
holding a small rhomb of Iceland spar, placed in a proper brass tube
before the orifice as at Fig. 327, from which the rays of common light
are [Page 340] passing; if an opaque screen of brass perforated with a
small hole is introduced behind the rhomb, then, instead of one circle
of light being apparent on the screen, two are produced, and both the
rays issuing in this manner are polarized, one being termed the ordinary
and the other the extraordinary ray. (Fig. 327.)
[Illustration: Fig. 327. A. The condensers. B. The hole in the brass
screen or stop. C. The rhomb of Iceland spar. O. The ordinary, and E the
extraordinary, ray, both of which are polarized light.]
The polarizing property of the rhomb is perhaps better shown by the next
diagram, where A B represents the obtuse angles of the Iceland spar, and
a line drawn from A to B, would be the axis of the crystal. The
incidental ray of common light is shown at C, and the oppositely
polarized transmitted rays called the ordinary ray O, and extraordinary
ray E, emerge from the opposite face of the rhomboid. If a black line is
ruled on a sheet of paper as at K K, and examined by the eye at C, it
appears double as at K K and J J. (Fig. 328.)
[Illustration: Fig. 328. Rhomb of Iceland spar.]
The cardboard model is again useful in demonstrating the polarization of
light by double refraction, and if a model of a rhomb of Iceland [Page
341] spar is made of glass plates, one face of which has an aperture
like a cross, and the other a horizontal and perpendicular slit, as at
Nos. 1 and 2 (Fig. 329), the production of the ordinary and
extraordinary rays is demonstrated in a familiar manner, and is easily
comprehended.
[Illustration: Fig. 329.--No. 1. One face of the model rhomb to admit
the transversal vibration, represented by the cardboard model.--No. 2.
The opposite face of the rhomb, from which issue the polarized,
ordinary, and extraordinary rays.--No. 3. Side view of the model.]
In Newton's "Optics" we find the following description of Iceland
spar:--"This crystal is a pellucid fissile stone, clear as water or
crystal of the rock (quartz), and without colour.... Being rubbed on
cloth it attracts pieces of straw and other light things like amber or
glass, and with aquafortis it makes an ebullition.... If a piece of this
crystalline stone be laid upon a book, every letter of the book seen
through it will appear double by means of a double refraction."
POLARIZATION BY THE TOURMALINE.
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 quantity of lime, or potash, or
soda, boracic acid, and sometimes oxide of iron or manganese. When light
is passed 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.)
[Illustration: 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 mass 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.)
[Illustration: 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 passes through both the
slices, whilst in other positions the light is completely cut off.
[Page 343]
When the axes of both plates coincide, the light polarized by one
tourmaline will pass 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
pass. 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 pass. (Fig. 332.)
[Illustration: Fig. 332. A. Model of the first slice of tourmaline into
which the transversal vibrations, B, are passing; 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 pass 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 glass, 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 glass, or a bent cold glass
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 glass 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-glasses 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 glass 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.)
[Illustration: Fig. 333. Duboscq's polarizing apparatus, A. The light
and the condenser lens. B. The plates of glass at the proper angle, C.
The selenite object, D. The focusing lens. E. The second bundle of
plates of glass called the analyser, F. A stop for extraneous rays of
light, G. The image of the film of selenite most beautifully coloured.
Goddard'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
glass or mica, called the analyser. 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
glass 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 glass called the analyser, at an angle of 56° 45´,
but at right angles to the first set of glass plates, they interfere,
because in the passage 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 retarded, 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 glass plates called the
analyser; and when this is brought into a position at right angles to
the first set of reflecting glass plates, half the ordinary wave
interferes with half the extraordinary wave; and being transmitted
through the analyser, 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 versâ_. By rotating the
analyser, the reflected and refracted rays change colours, and if the
former is red and the latter green, by moving the analyser 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.
[Illustration: 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
pass through a very narrow aperture at B.]
[Illustration: Fig. 335. A A. Card model of a beam of polarized light
coming from the first bundle of plates of glass, 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 glass called the analyser 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 glass for cathedral windows), the most beautiful
designs were made by the late Mr. J. T. Cooper, jun., which have since
been manufactured in great quantity 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 glass, 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 analyser.
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 glass_
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 glass, 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.
[Page 347]
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.)
[Illustration: 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 analyser (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.]
[Illustration: Fig. 337. No. 1. Unannealed glass for the polariscope.
Nos. 2 and 3. Appearance of the black cross and coloured circles in a
square and circular piece of unannealed glass in the polariscope.]
The chromatic effects described are not confined to selenite objects
only, but are obtained from glass, provided the particles are in a state
of unequal tension, as in masses of unannealed glass 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-glass, 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.
[Illustration: Fig. 338. A B. Bar of glass 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 analyser.
Another series of beautiful appearances present themselves when a ray of
white polarized light is made to pass perpendicularly through a slice of
any crystallized substance with a single axis; if the analyser 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.)
[Illustration: Fig. 339. Crystal of nitre with two axes, as seen in
polarized light.]
Mr. Goddard 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 passing 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 space
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.
[Illustration: 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 analysing.]
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 assistance 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.)
[Illustration: 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 glass. 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 Institution,
Professor Grove demonstrated the production of all the other kinds of
force from light, using the following arrangement for the purpose:
A prepared daguerréotype plate is enclosed in a box full of water having
a glass front with a shutter over it; between this glass 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 passes 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 passage 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 Institutions 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 God, which they can read by day as well as by
night, for light in their case is not an essential."
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