Chapter VII: Light (1)

Theories of the Nature of Light--Hypotheses of Newton
and Huygens--Sources of Light--The Sun--Velocity of
Light--Transparency--Dark Lines of the Spectrum--Absorption
of Light--Colour--Prismatic Analysis--Rays of the
Spectrum--Rainbow--Diffraction--Interference--Goethe’s
Theory--Polarisation--Magnetisation of Light--Vision--The
Eye--Analogy--Sound and Light--Influence of Light on Animals and
Vegetables--Phosphorescence arising from several Causes--Artificial
Light--Its Colour dependent on Matter.

Light, the first creation, presents to the enquiring mind a series of phenomena of the most exalted character. The glowing sunshine, painting the earth with all the brilliancy of colour, and giving to the landscape the inimitable charm of every degree of illumination, from the grey shadow to the golden glow;--the calm of evening, when, weary of the “excess of splendour,” the eye can repose in tranquillity upon the “cloud-land” of the west, and watch the golden and the ruddy hues fade slowly into the blue tincture of night;--and the pale refulgence of the moon, with the quiet sparkle of the sun-lit stars,--all tend to impress upon the soul, the great truth that, where there is light, organisation and life are found, and beyond its influence death and silence hold supreme dominion.[84] Through all time we have evidences that this has been the prevailing feeling of the human race, derived, of course, from their observation of the natural phenomena dependent upon luminous agency. In the myths of every country, impersonations of light prevail, and to these are referred the mysteries of the perpetual renewal of life on the surface of the earth.

This presentiment of a philosophic truth, in the instance of the poet sages of intellectual Greece, was advanced to the highest degree of refinement; and the sublime exclamation of Plato: “Light is truth, and God is light,” approaches nearly to a divine revelation.

As the medium of vision--as the cause of colour--as a power influencing in a most striking manner all the forms of organisation around us, light presented to the inquiring minds of all ages a subject of the highest interest.

The ancient philosophers, although they lost themselves in the metaphysical subtleties of their schools, could not but discover in light an element of the utmost importance in natural operations. The alchemists regarded the luminous principle as a most subtile fluid, capable of interpenetrating and mingling with gross matter: gold being supposed to differ from the baser metals only in containing a larger quantity of this ethereal essence.[85] Modern science, after investigating most attentively a greater number of the phenomena of light, has endeavoured to assist the inquiry by the aid of hypotheses. Newton, in a theory, which exhibits the refined character of that great philosopher’s mind, supposes luminous particles to dart from the surfaces of bodies in all directions--that these infinitely minute particles are influenced by the attracting and repelling forces of matter, and thus turned back, or reflected, from their superficies in some cases, and absorbed into their interstitial spaces in others.

Huyghens, on the contrary, supposes light to be caused by the waves or vibrations of an infinitely elastic medium--ETHER--diffused through all space, which waves are propagated in every direction from the luminous body. In the first theory, a luminous particle is supposed actually to come from the sun to the earth; in the other, the sun only occasions a disturbance of the _ether_, which extends with great rapidity, in the same manner as a wave spreads itself over the surface of a lake.

Nearly all the facts known in the time of Newton, and those discovered by him, were explained most satisfactorily by his hypothesis; but it was found they could be interpreted equally as the effects of undulation, with the exception of the production of colour by prismatic refraction. Although the labours of many gifted minds have been given, with the utmost devotion, to the support of the vibratory theory, this simple fact has never yet received any satisfactory explanation; and there are numerous discoveries connected with the molecular and chemical disturbances produced by the sun’s rays, which do not appear to be explained by the hypothesis of emission or of undulation.

In both theories a wave motion is admitted, and every fact renders it probable that this mode of progression applies not only to light, but to the so-called _imponderable forces_ in general. Admitting, therefore, the undulatory movement of luminous rays, we shall not stop to consider those points of the discussion which have been so ably dealt with by Young, Laplace, Fresnel, Biot, Fraunhofer, Herschel, Brewster, and others, but proceed at once to consider the sources of light, and its more remarkable phenomena.[86]

The sun is the greatest permanently luminous body we are acquainted with, and that orb is continually pouring off light from its surface in all directions at the rate, through the resisting medium of space and of our own atmosphere, of 192,000 miles in a second of time. It has been calculated, however, that light would move through a vacuum with the speed of 192,500 miles in the same period. We, therefore, learn that a ray of light requires eight minutes and thirteen seconds to come from the sun to us. In travelling from the distant planet Uranus, nearly three hours are exhausted; and from the nearest of the fixed stars each ray of light requires more than six years to traverse the intervening space between it and the earth. Allow the mind to advance to the regions of nebulæ, and it will be found that hundreds of years must glide away during the passage of their radiations. Consequently, if one of those masses of matter, or even one of the remote fixed stars, was “blotted out of heaven” to-day, several generations of the finite inhabitants of this world would fade out of time before the obliteration could be known to man. Here the immensity of space assists us in our conception, limited though it be, of the for-ever of eternity.[87]

All the planets of our system shine with reflected light, and the moon, our satellite, also owes her silvery lustre to the sun’s radiations. The fixed stars are, in all probability, suns shining from the far distance of space, with their own self-emitted lights. By the photometric researches of Dr. Wollaston, we learn, however, that it would take 20,000 millions of such orbs as Sirius, the brightest of the fixed stars, to afford as much light as we derive from the sun. The same observer has proved that the brightest effulgence of the full moon is yet 801,072 times less than the luminous power of our solar centre.

The cultivators of modern science are a bold race; not contented with endeavouring to understand the physical earth, they are endeavouring to comprehend the condition of the solar surface. The mind of man can penetrate far into nature, and, as it were, feel out the mysteries of untraversed space. The astronomer learns of a peculiar condition of light, which is termed polarisation, and he learns by this, too, that he can determine if from a bright luminous disc the light is derived from a solid mass in a state of intense ignition, or from vapour in an incandescent condition. He adds a polarising apparatus to his telescopes, and he determines that the light we derive from the sun is due to an envelope of vapour--burning, in all probability--only with greater intensity, as the gas which we now employ. This _Photosphere_--as it has been called by the late French philosopher Arago, is found to be subjected to violent disturbances, and the dark spots seen on the sun’s disc are now known to be openings through this mysterious envelope of light, which enable us to look in upon the dark body of the sun itself.

Luminous phenomena may be produced by various means--chemical action is a source of light; and, under several circumstances in which the laws of affinity are strongly exerted, a very intense luminous effect is produced. Under this head all the phenomena of combustion are included. In the electric spark we have the development of light; and the arc which is formed between charcoal points at the poles of a powerful voltaic battery affords us the most intense artificial illumination with which we are acquainted. In addition to these, we have the peculiar phenomena of phosphorescence arising from chemical, calorific, electrical, actinic, and vital excitation, all of which must be particularly examined.

From whatever source we procure light, it is the same in character, differing only in intensity. In its action upon matter, we have the phenomena of transmission, of reflection, of refraction, of colour, of polarisation, and of vision, to engage our attention.

A beam of white light falls upon a plate of colourless glass, and it passes freely through it, losing but little of its intensity; the largest portion being lost by reflection from the first surface upon which the light impinges. If the glass is roughened by grinding, we lose more light by absorption and by reflection from the asperities of the roughened surface; but if we cover that face with any oleaginous fluid, as, for instance, turpentine, its transparency is restored. We have thus direct proof that transparency to light is due to molecular condition. This may be most strikingly shown by an interesting experiment of Sir David Brewster’s:--

If a glass tube is filled with nitrous acid vapour, which is of a dull red colour, it admits freely the passage of the red and orange rays with some of the others, and, if held upright in the sunshine, casts a red shadow on the ground; by gently warming it with a spirit-lamp, whilst in this position, it acquires a much deeper and blacker colour, and becomes almost impervious to any of the rays of light; but upon cooling it again recovers its transparency.

It has also been stated by the same exact experimentalist, that having brought a purple glass to a red heat, its transparency was improved, so that it transmitted green, yellow, and red rays, which it previously absorbed; but the glass recovered its absorptive powers as it cooled. A piece of yellowish-green glass lost its transparency almost entirely by being heated. Native yellow orpiment becomes blood-red upon being warmed, when nearly all but the red rays are absorbed; and pure phosphorus, which is of a pale yellow colour, and transmits freely all the coloured rays upon being melted, becomes very dark, and transmits no light.

Chemistry affords numerous examples of a very slight change of condition, producing absolute opacity in fluids which were previously diaphanous.[88]

Charcoal absorbs all the light which falls upon it, but in some of its states of combination, and in the diamond, which is pure carbon, it is highly transparent. Gold and silver beaten into thin leaves are permeated by the green and blue rays, and the metals in combination with acids are all of them more or less transparent. What becomes of the light which falls upon and is absorbed by bodies, is a question which we cannot yet, notwithstanding the extensive observations that have been made by some of the most gifted of men, answer satisfactorily. In all probability, as already stated, it is permanently retained within their substances; and many of the experiments of exciting light in bodies when in perfect darkness, by the electric spark and other means, appear to support the idea of light becoming latent or hidden.

No body is absolutely transparent; some light is lost in passing even through ethereal space, and still more in traversing our atmosphere.

Amongst the most curious instances of absorption is that which is uniformly discovered in the solar spectrum, particularly when we examine it with a telescope. We then find that the coloured rays are crossed by a great number of dark bands or lines, giving no light; these are generally called Fraunhofer’s dark lines, as it was to the indefatigable exertions of that experimentalist, and by the aid of his beautiful instruments, that most of them were discovered and measured, and enumerated, although they were previously noticed by Dr. Wollaston. It is quite clear that those lines represent rays which have been absorbed in their passage from the sun to the earth: although some of them have no doubt undergone absorption within the limits of the earth’s atmosphere, we have every reason to believe, with Sir John Herschel, that the principal absorption takes place in the atmosphere of the sun.[89]

It has been proved by Dr. Miller, that the number of those dark lines is continually varying with the alteration of atmospheric conditions;[90] and the evidences which have been afforded, of peculiar states of absorption by the gaseous envelope of the earth,--during the prosecution of investigations on the chemical agencies of the sun’s rays,--are of a sufficiently convincing character.

It has been calculated by Bouguer, that if our atmosphere, in its purest state, could be extended rather more than 700 miles from the earth’s surface instead of nearly 40, as it is at present, the sun’s rays could not penetrate it, and this globe would roll on in darkness and silence, without a vestige of vegetable form or of animal life. In the Hebrew version of the Mosaic History, the reading is, “Let light appear:” may not this really mean that the earth’s atmosphere was so cleared of obstructing vapours, that the solar rays were enabled to reach the earth? The same calculation supposes that sea-water loses all its transparency at the depth of 730 feet; but a dim twilight must prevail much deeper in the ocean.

The researches of Professor Edward Forbes have proved, that at the depth of 230 fathoms in the Ægean sea, the few shelled animals that exist are colourless: no plants are found within that zone; and that industrious naturalist fixes the zero of animal life of those waters at about 300 fathoms.[91] Since these zones mark the rapidly diminishing light, it is evident that where life ceases to be must be beyond the limits to which life can penetrate.

Our atmosphere, charged with aqueous vapour, serves to shield us from the intense action of the solar powers. By it we are protected from the destructive influences of the sun’s light and heat; enjoy those modified conditions which are most conducive to the healthful being of organic forms; to it we owe “the blue sky bending over all,” and those beauties of morning and evening twilight of which

---- Sound and motion own the potent sway.
Responding to the charm with its own mystery.

To defective transparency, or rather to the different degrees of it, we must attribute, in part, the colours of permeable media. Thus, a glass or fluid appears yellow to the eye, because it has the property of admitting the permeation of a larger quantity of the yellow rays than of any others;--red, because the red rays pass it with the greatest freedom; and so on for every other colour. In most cases the powers of transmission and of reflection are similar; but it is not so in all; a variety of fluor spar, which, while it transmits green light, reflects blue, and the precious opal, are striking instances to the contrary. Some glasses, which transmit yellow light have the singular power of dispersing blue rays from one surface; and a solution of quinine in water acidulated with sulphuric acid, although perfectly transparent and colourless when held between the eye and the light, exhibits, if viewed in a particular direction, a lively cerulean tint. These effects being supposed to be due to the conditions of the surface, have been called _epipolic_ phenomena.[92]

The careful investigation of these phenomena has made us acquainted with some very interesting facts, and indeed discovered to us a set of luminous rays which were previously unknown. The dispersion of blue light from the surface of some yellow glasses--such as have been coloured by the oxide of silver--is of a different order from that which takes place with the solution of sulphate of quinine, or with the fluor spar. The first depends upon a peculiar condition of the surface, while the latter phenomena are due to a dispersion which takes place _within_ the solid or fluid. In addition to the sulphate of quinine, and the fluor spar, we obtain the same results in a very marked manner by a canary yellow glass, coloured with the oxide of uranium, and by a decoction of the inner bark of the horse-chesnut tree. Mr. Stokes, who has investigated this class of phenomena, and proposes to call it _Fluorescence_, from its being naturally seen in fluor-spar, has shown that the peculiar internal dispersion, and the consequent alteration of the colour of the ray, is due to an alteration in its refrangibility. Whether this hypothesis prove to be the correct one or not, it is certain that there exists a set of rays of far higher refrangibility than those seen in the ordinary Newtonian spectrum. This may be shown in the following manner: taking either of the solutions named, or a block of uranium glass, throw upon one face, by means of a prism, a very pure spectrum. On looking _into_ the glass or fluid there will be seen, commencing amidst the most refrangible rays, a new set of spectral rays, struggling to make their way through the absorbent medium. These are of a blue colour in the quinine or chesnut solution, and green in the uranium glass, and are seen extending themselves far beyond the most refrangible rays of the ordinary Newtonian spectrum. This is the space over which those rays which have the power of producing chemical changes, such as are rendered familiar by the practice of Photography, are detected in their greatest activity. It has, therefore, been supposed that these fluorescent rays are the chemical rays rendered luminous by the alteration of their refrangibility. This view has received much support from the fact that the extra spectral rays are crossed with numerous dark lines, and that in the chemical impressions these lines are marked by unchanged spaces which exactly coincide with them. There is, however, much doubt of the correctness of this, since, in the uranium glass of such a thickness that these visible rays are quite absorbed, the chemical rays still pass.

However, the whole question requires, and is receiving, the most searching investigation. The discovery of these phenomena, which are included under the term of Fluorescence, is of that interesting and important character, that it must be ranked as the most decided advance which has been made in physical optics since the days of Newton.

It is not improbable that those rays of such high refrangibility may, although they are under ordinary circumstances invisible to the human eye, be adapted to produce the necessary degree of excitement upon which vision depends in the optic nerves of the night-roaming animals. The bat, the owl, and the cat, may see in the gloom of night by the aid of rays which are invisible to, or inactive on the eyes of man, or of those animals which require the light of day for perfect vision.

It is a general law of the radiant forces, that whenever they fall upon any surface, a portion is thrown back or reflected at the same time as other portions are absorbed or transmitted. Upon this peculiarity appear to depend the phenomena of natural colour in bodies.

The white light of the sun is well known to be composed of several coloured rays. Or rather, according to the theory of undulations, when the rate at which a ray vibrates is altered, a different sensation is produced upon the optic nerve. The analytical examination of this question shows, that to produce a red colour the ray of light must give 37,640 undulations in an inch, and 458,000000,000000 in a second. Yellow light requires 44,000 undulations in an inch, and 535,000000,000000 in a second; whilst the effect of blue results from 51,110 undulations within an inch, and 622,000000,000000 of waves in a second of time.[93] The determination of such points as these is among the highest refinements of science, and, when contrasted with the most sublime efforts of the imagination, they must appear immeasurably superior.

If a body sends back white light unchanged, it appears white; if the surface has the property of altering the vibration to that degree which is calculated to produce redness, the result is a red colour: the annihilation of the undulations produces blackness. By the other view, or the corpuscular hypothesis, the beam of white light is supposed to consist of certain coloured rays, each of which has physical properties peculiar to itself, and thus is capable of producing different physiological effects. These rays falling upon a transparent or an opaque body suffer more or less absorption, and being thus dissevered, we have the effect of colour. A red body absorbs all the rays but the red; a blue surface, all but the blue; a yellow, all but the yellow; and a black surface absorbs the whole of the light which falls upon it.

That natural colours are the result of white light, and not innate properties of the bodies themselves, is most conclusively shown by placing coloured bodies in monochromatic light of another kind, when they will appear either of the colour of that light, or, by absorbing it, become black; whereas, when placed in light of their own character, the intensity of colour is greatly increasing.

Every surface has, therefore, a peculiar constitution, by which it gives rise to the diversified hues of nature. The rich and lively green, which so abundantly overspreads the surface of the earth, the varied colours of the flowers, and the numberless tints of animals, together with all those of the productions of the mineral kingdom, and of the artificial combinations of chemical manufacture, result from powers by which the relations of matter to light are rendered permanent, until its physical conditions undergo some change.

There is a remarkable correspondence between the geographical position of a region and the colours of its plants and animals. Within the tropics, where

“The sun shines for ever unchangeably bright,”

the darkest green prevails over the leaves of plants; the flowers and fruits are tinctured with colours of the deepest dye, whilst the plumage of the birds is of the most variegated description and of the richest hues. In the people also of these climes there is manifested a desire for the most striking colours, and their dresses have all a distinguishing character, not of shape merely, but of chromatic arrangement. In the temperate climates everything is of a more subdued variety: the flowers are less bright of hue; the prevailing tint of the winged tribes is a russet brown; and the dresses of the inhabitants of these regions are of a sombre character. In the colder portions of the earth there is but little colour; the flowers are generally white or yellow, and the animals exhibit no other contrast than that which white and black afford. A chromatic scale might be formed, its maximum point being at the equator, and its minimum at the poles.[94]

The influence of light on the colours of organized creation is well shown in the sea. Near the shores we find sea-weeds of the most beautiful hues, particularly on the rocks which are left dry by the tides; and the rich tints of the actiniæ, which inhabit shallow water, must have been often observed. The fishes which swim near the surface are also distinguished by the variety of their colours, whereas those which live at greater depths are grey, brown, or black. It has been found that after a certain depth, where the quantity of light is so reduced that a mere twilight prevails, the inhabitants of the ocean become nearly colourless. That the sun’s ray alone gives to plants the property of reflecting colour is proved by the process of blanching, or _etiolation_, produced by artificially excluding the light.

By a triangular piece of glass--a prism,--we are enabled to resolve light into its ultimate rays. The white pencil of light which falls on the first surface of the prism is bent from its path, and coloured bands of different colours are obtained. These bands or rays observe a curious constancy in their positions: the red ray is always the least bent out of the straight path: the yellow class comes next in the order of refrangibility; and the blue are the most diverted from the vertex of the prism. The largest amount of illuminating power exists in the yellow ray, and it diminishes towards either end.[95] It is not uninteresting to observe something like the same variety of colour occurring at each end of the prismatic spectrum. The strict order in which the pure and mixed coloured rays present themselves is as follows:--

1. The _extreme red_: a ray which can only be discovered when the eye is protected from the glare of the other rays by a cobalt blue glass, is of a crimson character--a mixture of the _red_ and the _blue_, red predominating.[96]

2. The _red_: the first ray visible under ordinary circumstances.

3. The _orange_: red passing into and combining with yellow.

4. The _yellow_: the most intensely luminous of the rays.

5. The _green_: the yellow passing into and blending with the blue.

6. The _blue_: in which the light very rapidly diminishes.

7. The _indigo_: the dark intensity of blue.

8. The _violet_: the _blue_ mingled again with the _red_--blue being in excess.

9. The _lavender grey_: a neutral tint, produced by the combination of the red, blue, and yellow rays, which is discovered most easily when the spectrum is thrown upon a sheet of turmeric paper.

10. The fluorescent rays: which are either a _pure silvery blue_ or a delicate _green_.

Newton regarded the spectrum as consisting of seven colours of definite and unvarying refrangibility. Brewster and others appear to have detected a great diffusion of the colours over the spectrum, and regard white light as consisting only of three rays, which in the prismatic images overlap each other; and from these--red, yellow, and blue--all the others can be formed by combination in varying proportions. The truth will probably be found to be, that the ordinary prismatic spectrum is a compound of two spectra:--that is, as we have the ordinary rainbow, and a supplementary bow, the colours of which are inverted, so the extraordinary may be somewhat masked by the intense light of the ordinary spectrum; and yet by overlapping produce the variations of colour in the rays. We have already examined the heating power found in these coloured bands, which, although shown to be in a remarkable manner in constant agreement with the colour of a particular ray, is not directly connected with it; that is, not as the effect of a cause, or the contrary. The chemical action of the solar rays, to which from its important bearings we shall devote a separate chapter, has, in like manner with heat, been confounded with the sun’s luminous power; but although associated with light and heat, and modified by their presence, it must be distinguished from them.

We find the maximum of heat at one end of the spectrum, and that of chemical excitation at the other--luminous power observing a mean point between them. Without doubt we have these powers acting reciprocally, modifying all the phenomena of each other, and thus giving rise to the difficulties which beset the inquirer on every side.

We have beautiful natural illustrations of luminous refraction in the rainbow and in the halo: in both cases the rays of light being separated by the refractive power of the falling rain drop, or the vesicles which form the moisture constituting a fog. In the simple toy of the child--the soap-bubble floating upon the air--the philosopher finds subjects for his contemplation; and from the unrivalled play of colours which he discovers in that attenuated film, he learns that the varying thicknesses of surfaces influence, in a most remarkable manner, the colours of the sunbeam. Films of oil floating upon water present similar appearances; and the colours developed in tempering steel are due entirely to the thickness of the oxidized surface produced by heat. There have lately been introduced some beautiful specimens of paper rendered richly iridescent by the following process:--A solution of a gum resin in chloroform is floated upon water, where it forms a film giving all the colours of Newton’s rings. A sheet of paper which has been previously sunk in the water is carefully lifted, and the film thus removed adheres with great firmness to the paper, and produces this rich and curious play of colour. The rich tints upon mother-of-pearl, in the feathers of many birds, the rings seen in the cracks of rock-crystal, or between the unequal faces of two pieces of glass, and produced by many chemical and indeed mechanical operations--are all owing to the same cause;--the refraction of the luminous pencil by the condition of the film or surface. If we take one of those steel ornaments which are formed by being covered with an immense number of fine lines, it will be evident that these striæ present many different angles of reflection, and that, consequently, the rays thrown back will, at some point or another, have a tendency to cross each other. The result of this is, that the quantity of light is augmented at some points of intersection, and annihilated at others.[97] Out of the investigation of the phenomena of diffraction, of the effects of thin and thick plates upon light, and the results of interference, has arisen the discovery of one of the most remarkable conditions within the range of physical science.

_Two bright lights may be made to produce darkness._--If two pencils of light radiate from two spots very close to each other in such a manner that they cross each other at a given point, any object placed at that line of interference will be illuminated with the sum of the two luminous pencils. If we suppose those rays to move in waves, and the elevation of the wave to represent the maximum of luminous effect, then the two waves meeting, when they are both at the height of their undulation, will necessarily produce a spot of greater intensity. If now we so arrange the points of radiation, that the systems of luminous waves proceed irregularly, and that one arrives at the screen half an undulation before the other, the one in elevation falling into the depression of the other, a mutual annihilation is the consequence. This fact, paradoxical as it may appear, was broadly stated by Grimaldi, in the description of his experiments on the inflection of light, and has been observed by many others. The vibratory hypothesis, seizing upon the analogy presented by two systems of waves in water, explains this plausibly, and many similar phenomena of what is called the _interference of light_; but still upon examination it does not appear that the explanation is quite free from objection.[98]

Another theory, not altogether new to us, it being indicated in Mayer’s hypothesis of three primary colours (1775), and to be found as a problem in some of the Encyclopædias of the last century, has been put forth, in a very original manner, by that master-mind of intellectual Germany, Goethe; and from the very comprehensive views which this poet-philosopher has taken of both animal and vegetable physiology (views which have been adopted by some of the first naturalists of Europe), we are bound to receive his theory of colours with every respect and attention.

Goethe regards colour as the “thinning” of light; for example, by obstructing a portion of white light, yellow is produced; by reducing it still farther, red is supposed to result; and by yet farther retarding the free passage of the beam, we procure a blue colour, which is the next remove from blackness, or the absence of light. There is truth in this; it bears about it a simplicity which will satisfy many minds; by it many of the phenomena of colour may be explained: but it is insufficient for any interpretation of several of those laws to which the other theories do give us some insight.

Newton may have allowed himself to be misled by the analogy presented between the seven rays of the spectrum and the notes in an octave. The mystic number, seven, may have clung like a fibre of the web of superstition to the cloak of the great philosopher; but the attack made by Goethe upon the Newtonian philosophy betrays the melancholy fact of his being diseased with the lamentable weakness of too many exalted minds--an overweening self-esteem.

The polarization of light, as it has been unfortunately called--unfortunately, as conveying an idea of determinate and different points or poles, which only exists in hypothetical analogy--presents to us a class of phenomena which promise to unclose the mysterious doors of the molecular constitution of bodies.

This remarkable condition, as produced by the reflection of light from glass at a particular angle, was first observed by Malus, in 1808,[99] when amusing himself by looking at the beams of the setting sun, reflected from the windows of the Luxembourg Palace through a double refracting prism. He observed that when the prism was in one position, the windows with their golden rays were visible; but that turned round a quarter of a circle from that position, the reflected rays disappeared although the windows were still seen.

The phenomenon of double refraction was noticed, in the first instance, by Erasmus Bartholin, in Iceland-spar, a crystal the primary form of which is a rhombohedron; who perceived that the two images produced by this body were not in the same physical conditions.[100] It was also studied by Huyghens and Sir Isaac Newton, and to our countryman we owe the singular idea that a ray of light emerging from such a crystal has _sides_. This breaking up of the beam of light into two,--which is shown by looking through a pin-hole on a card through a crystal of Iceland spar, when two holes become visible, is due to the different states of tension in which the different layers constituting the crystal exist.

In thus separating the ray of light into two rays, the condition called polarisation has been produced, and by experiment we discover that the single ray has properties different from those of the compound or ordinary ray.

It is somewhat difficult to explain what is meant by, and what are the conditions of, _polarised light_. In the first instance let us see by what methods this peculiar state may be brought about.

If we reflect a ray of light from the surface of any body, fluid or solid, but not metallic, at an angle between 53° and 68° it undergoes what has been called _plane polarisation_. It may also be produced by the refraction of light from several refracting surfaces acting upon the pencil of light in succession; as by a bundle of plates of glass. Each surface polarises a portion of the pencil, and the number of plates necessary to polarise a whole beam depends upon the intensity of the beam and the angle of incidence. Thus, the light of a wax candle is wholly polarised by forty-seven plates of glass at an angle of 40° 41'; while at an angle of 79° 11' it is polarised by eight plates. Again, plane polarisation may be produced by the double refraction of crystals. Each of the two pencils is polarised, like light reflected from glass at an angle of 56° 45', but in opposite planes.

Non-scientific readers will still ask,--What is this mysterious condition of light which is produced by reflection and refraction at peculiar angles to the incident ray. It is one of the most difficult of problems to express in popular language. The conditions are, however, these:--

An ordinary ray of light will be reflected from a reflecting surface at whatever angle that surface may be placed in relation to the incident beam.

A polarised ray of light is not reflected in all positions of the reflecting surface.

An ordinary ray of light is freely transmitted through a transparent medium, as glass, in whatever position it may be placed relative to the source of light.

A polarised ray of light is not transmitted in all the positions of the permeable medium.

Supposing a plate of glass is presented at the angle 56° to a polarised ray, and the plane of incidence or reflexion is at right angles to the plane of polarisation of the ray, _no light is reflected_. If we turn the plate of glass round through 90°, when the plane of reflexion is parallel to that of polarisation _the light is reflected_. If we turn the plate round another 90°, so that the plane of reflexion and of polarisation are parallel to each other, again _no light is reflected_; and if we turn it through another 90° the reflection of the ray again takes place.

Precisely the same result takes place when, instead of being reflected, the polarised ray is transmitted.

Some substances have peculiar polarizing powers: _the tourmaline_ is a familiar example. If a slice of tourmaline is taken, and we look at a common pencil of light through it, we see it in whatever position we may place the transparent medium. If, however, we look at a pencil of polarised light, and turn the crystal round, it will be found that in two positions the light is stopped, and that in two other positions it passes freely through it to the eye.

By way of endeavouring to conceive something of what may be the conditions which determine this very mysterious state, let us suppose each ray of light to vibrate in two planes at right angles to each other: one wave being vertical and the other horizontal. We have many examples of this compound motion. The mast of a ship, by the force with which she is urged through the water, describes a vertical wave, while by the roll of the billows across which she sails, a lateral undulation is produced at the same time. We may sometimes observe the same thing when a field of corn is agitated by a shifting wind on a gusty day.

The hypothesis therefore is, that every ray of ordinary light consists of two rays vibrating in different planes; and that these rays, separated one from the other, have the physical conditions which we call _polarized_.

The most transparent bodies may be regarded as being made up of atoms arranged in certain planes. Suppose the plane of lamination of any substance to be vertical in position, it would appear that the ray which has a vertical motion passes it freely, whereas if we turn the body round so that the planes of lamination are at right angles to the plane of vibration of the ray, it cannot pass.

That some action similar to that which it is here endeavoured to express in popular language does take place, is proved by the correctness of the results deduced by rigid mathematical analyses founded on this hypothesis.

There are two other conditions of the polarization of light--called _circular_ and _elliptical_ polarization. The first is produced by light when it is twice reflected from the second surface of bodies at their angle of maximum polarization, and the second by reflexions from the surfaces of metals at angles varying from 70° 45' to 78° 30'. The motion of the wave in the first is supposed to be circular, or to be that which is represented by looking along the centre of a corkscrew as it is turned round. At every turn of the medium effecting _circular polarization_ the colour of the ray of light is changed after a uniform order. If turned in one direction, they change through red, orange, yellow, green, and violet; and if in the other direction, the colours appear in the contrary order.

The variety of striking effects produced by the polarization of light; the unexpected results which have sprung from the investigation of the laws by which it is regulated; and the singular beauty of many of its phenomena, have made it one of the most attractive subjects of modern science.

Ordinary light passes through transparent bodies without producing any very striking effects in its passage; but this _extraordinary_ beam of light has the power of insinuating itself between the molecules of bodies, and by illuminating them, and giving them every variety of prismatic hue, of enabling the eye to detect something of the structure of the mass. The chromatic phenomena of polarized light are so striking, that no description can convey an adequate idea of their character.

Spectra more beautiful and intense than the prismatic image,--systems of rings far excelling those of thin plates,--and forms of the most symmetric order, are constantly presenting themselves, as the polarized ray is passed through various transparent substances; the path of the ray indicating whether the crystal has been formed round a single nucleus or axis, or whether it has been produced by aggregation around two axes. The coloured rings, and the dark or luminous crosses which distinguish the path of the polarized ray, are respectively due to different states of tension amongst the particles, although those differences are so slight, that no other means is of sufficient delicacy to detect the variation.

The poetry which surrounds these, in every way, mysterious conditions of the solar beam, is such, that it is with difficulty that imagination is restrained by the stern features of truth. The uses of this peculiar property in great natural phenomena are not yet made known to us; but, since we find on every side of us the natural conditions for thus separating the beam of light, and effecting its polarization, there must certainly be some most important end for which it is designed by Him who said, “Let there be Light.”

It must not be forgotten that we have at command the means of showing that the chromatic phenomena of polarized light are due to atomic arrangement. By altering the molecular arrangement of transparent bodies, either by heat or by mere mechanical pressure, the unequal tension or strain of the particles is at once indicated by means of the polarized ray of light and its rings of colour. Differences in the chemical constitution of bodies, too slight to be discovered by any other mode of analysis, can be most readily and certainly detected by this luminous investigator of the molecular forces.[101]

Although we cannot enter into an examination of all the conditions involved in the polarization of, and the action of matter on, ordinary light, it will be readily conceived, from what has been already stated, that some most important properties are indicated, beyond those which science has made known.

Almost every substance in nature, in some definite position, appears to have the power of producing this change upon the solar ray, as may be satisfactorily shown by examining them with a polarizing apparatus.[102] The sky at all times furnishes polarized light, which is most intense where it is blue and unclouded, and the point of maximum polarization is varied according to the relative position of the sun and the observer. A knowledge of this fact has led to the construction of a “Solar Clock,”[103] with which the hour can be readily determined by examining the polarized condition of the sky. It has been stated, that chemical change on the Daguerreotype plates and on photographic papers is more readily produced by the polarized than by the ordinary sunbeam.[104] If this fact be established by future investigations, we advance a step towards the discovery so much desiderated of the part it plays in natural operations.

The refined and accurate investigations of Dr. Faraday stand prominently forward amid those which will redeem the present age from the charge of being superficial, and they will, through all time, be referred to as illustrious examples of the influence of a love of truth for truth’s sake, in entire independence of the marketable value, which it has been unfortunately too much the fashion to regard. The searching examination made by this “interpreter of nature” into the phenomena of electricity in all its forms, has led him onward to trace what connexion, if any, existed between this great natural agent and the luminous principle.

By employing that subtile analyzer, a polarized ray, Dr. Faraday has been enabled to detect and exhibit effects of a most startling character. He has proved magnetism to have the power of influencing a ray of light in its passage through transparent bodies. A polarized ray is passed through a piece of glass or a crystal, or along the length of a tube filled with some transparent fluid, and the line of its path carefully observed; if, when this is done, the solid or fluid body is brought under powerful magnetic influence, such as we have at command by making a very energetic voltaic current circulate around a bar of soft iron, it will be found that the polarized light is disturbed; that, indeed, it does not permeate the medium along the same line.[105] This effect is most strikingly shown in bodies of the greatest density, and diminished in fluids, the particles of which are easily moveable over each other, and has not hitherto been observed in any gaseous medium. The question, therefore, arises,--does magnetism act directly upon the ray of light, or only indirectly, by producing a molecular change in the body through which the ray is passing? This question, so important in its bearings upon the connexion between the great physical powers, will, no doubt, before long receive a satisfactory reply. A medium is necessary to the production of the result, and, as the density of the medium increases, the effect is enlarged: it would therefore appear to be due to a disturbance by magnetic force of the particles which constitute the medium employed.

Without any desire to generalize too hastily, we cannot but express a feeling,--amounting to a certainty in our own mind,--that those manifestations of luminous power, connected with the phenomena of terrestrial magnetism, which are so evident in all the circumstances attendant upon the exhibition of Aurora Borealis, and those luminous clouds which are often seen, independent of the Northern Lights, that a very intimate, relation exists between the solar radiations and that power which so strangely gives polarity to this globe of ours.

In connexion with the mysterious subject of solar light, it is important that we should occupy a brief space in these pages with the phenomena of vision, which is so directly dependent upon luminous radiation.

The human eye has been rightly called the “masterpiece of divine mechanism;” its structure is complicated, yet all the adjustments of its parts are as simple as they are perfect. The eye-ball consists of four coats. The cornea is the transparent coat in front of the globe; it is the first optical surface, and this is attached to the sclerotic membrane, filling up the circular aperture in the white of the eye; the choroid coat is a very delicate membrane, lining the sclerotic, and covered with a perfectly black pigment on the inside; and close to this lies the most delicately reticulated membrane, the retina, which is, indeed, an extension of the optic nerve. These coats enclose three humours,--the aqueous, the vitreous, and the crystalline humours.

The eye, in its more superficial mechanical arrangements, presents exactly the same character as a camera obscura, the cornea and crystalline lens receiving the images of objects refracting and inverting them; but how infinitely more beautiful are all the arrangements of the organ of vision than the dark chamber of Baptista Porta![106] The humours of the eye are for the purpose of correcting the aberrations of light, which are so evident in ordinary lenses, and for giving to the whole an achromatic character. Both spherical and chromatic aberration are corrected, the latter not entirely, and by the agency of the cornea and the crystalline lens perfect images are depicted on the retina, in a similar way to those very charming pictures which present themselves in the table of the camera obscura.

The seat of vision has been generally supposed to be the retina; but Mariotte has shown that the base of the optic nerve, which is immediately connected with the retina, is incapable of conveying an impression to the brain. The choroid coat, which lies immediately behind the retina, is regarded by Mariotte and Bernoulli as the more probable seat of vision. The retina, being transparent, offers no obstruction to the passage of the light onward to the black surface of the choroid coat, from which the vibrations are, in all probability, communicated to the retina and conveyed to the brain. Howbeit, upon one or the other of these delicate coats a distinct image is impressed by light, and the communication made with the brain possibly by a vibratory action. We may trace up the phenomena of vision to this point; we may conceive undulations of light, differing in velocity and length of wave, occasioning corresponding tremors in the neuralgic system of the eye; but how these vibrations are to communicate correct impressions of length, breadth, and thickness, no one has yet undertaken to explain.

It has, however, been justly said by Herschel:--

“It is the boast of science to have been able to trace so far the refined contrivances of this most admirable organ, not its shame to find something still concealed from scrutiny; for, however anatomists may differ on points of structure, or physiologists dispute on modes of action, there is that in what we _do_ understand of the formation of the eye, so similar, and yet so infinitely superior to a product of human ingenuity; such thought, such care, such refinement, such advantage taken of the properties of natural agents used as mere instruments for accomplishing a given end, as force upon us a conviction of deliberate choice and premeditated design, more strongly, perhaps, than any single contrivance to be found whether in art or nature, and renders its study an object of the greatest interest.”[107]

Has the reader ever asked himself why it is, having two eyes, and consequently two pictures produced upon the tablets of vision, that we see only one object? According to the law of visible direction, all the rays passing through the crystalline lenses converge to one point upon the retina,--and as the two images are coincident and nearly identical, they can only produce the sensation of one upon the brain.

When we look at any round object, as the ornamented moderator lamp before us, first with one eye, and then with the other, we discover that, with the right eye, we see most of the right-hand side of the lamp, and with the left eye more of the left-hand side. These two images are combined, and we see an object which we know to be round.

This is illustrated in a most interesting manner by the little optical instrument, _the Stereoscope_. It consists either of two mirrors placed each at an angle of 45°, or of two semi-lenses turned with their curved sides towards each other. To view its phenomena, two pictures are obtained by the camera obscura on photographic paper of any object in two positions, corresponding with the conditions of viewing it with the two eyes. By the mirrors or the lenses these dissimilar pictures are combined within the eye, and the vision of an actually solid object is produced from the pictures represented on a plane surface. Hence the name of the instrument; which signifies, _Solid I see_.

Analogy is often of great value in indicating the direction in which to seek for a truth; but analogical evidence, unless where the resemblance is very striking, should be received with caution. Mankind are so ready to leap to conclusions without the labour necessary for a faithful elucidation of the truth, that too often a few points of resemblance are seized upon, and an inference is drawn which is calculated to mislead.

There is an idea that the phenomena of sound bear a relation to those of light,--that there exists a resemblance between the chromatic and the diatonic scales. Sound, we know, is conveyed by the beating of material particles--the air--upon the auditory membrane of the ear, which have been set in motion by some distant disturbance of the medium through which it passes. Light has been supposed to act on the optic nerve in the same manner. If we imagine colour to be the result of vibrations of different velocities and lengths, we can understand that under some of these tremors, first established on the nerves, and through them conveyed to the brain, sensations of pain or pleasure may result, in the same way as sharp or subdued sounds are disagreeable or otherwise. Intensely coloured bodies do make an impression upon perfectly blind men; and those who, being born blind, know no condition of light or colour, will point out a difference between strongly illuminated red and yellow media. When the eyes are closed we are sensible to luminous influence, and even to differences of colour. We must consequently infer that light produces some peculiar action upon the system of nerves in general; this may or may not be independent of the chemical agency of the solar radiations; but certainly the excitement is not owing to any calorific influence. The system of nerves in the eye is more delicately organized, and of course peculiarly adapted to all the necessities of vision.

Thus far some analogy does appear to exist between light and sound; but the phenomena of the one are so much more refined than those of the other--the impressions being all of them of a far more complicated character, that we must not be led too far by the analogical evidence in referring light, like sound, to mere material motion.