Chapter VII: Light (2)

It was a beautiful idea that real impressions of external objects are made upon the seat of vision, and that they are viewed, as in a picture, by something behind the screen,--that these pictures become dormant, but are capable of being revived by the operations of the mind in peculiar conditions; but we can only regard it as a philosophical speculation of a poetic character, the truth or falsehood of which we are never likely to be enabled to establish.[108]

That which sees will never itself be visible. The secret principle of sensation,--the mystery of the life that is in us,--will never be unfolded to finite minds.

Numerous experiments have been made from time to time on the influence of light upon animal life. It has been proved that the excitement of the solar rays is too great for the healthful growth of young animals; but, at the same time, it appears probable that the development of the functional organs of animals requires, in some way, the influence of the solar rays. This might, indeed, have been inferred from the discovery that animal life ceases in situations from which light is absolutely excluded. The instance of the Proteus of the Illyrian lakes may appear against this conclusion. This remarkable creature is found in the deep and dark recesses of the calcareous rocks of Adelsburg, at Sittich; and it is stated, also in Sicily, and in the Mammoth caves of Kentucky. Sir Humphry Davy describes the Proteus anguinus as “an animal to whom the presence of light is not essential, and who can live indifferently in air and in water, on the surface of the rock, or in the depths of the mud.” The geological character of rocks, however, renders it extremely probable that these animals may have descended with the water, percolating through fissures from very near the surface of the ground. All the facts with which science has made us acquainted--and both natural and physical science has been labouring with most untiring industry in the pursuit of truth--go to prove that light is absolutely necessary to organization. It is possible the influence of the solar radiations may extend beyond the powers of the human senses to detect luminous or thermic action, and that consequently a development of animal and vegetable forms may occur where the human eye can detect no light; and under such conditions the Proteus may be produced in its cavernous abodes, and also those creatures which live buried deep in mud. Some further consideration of the probable agency of light will occupy us, when we come to examine the phenomena of vital forces.

Light is essentially necessary to vegetable life; and to it science refers the powers which the plant possesses of separating carbon from the air breathed by the leaves, and secreting it within its tissues for the purpose of adding to its woody structure. As, however, we have, in the growing plant, the action of several physical powers exerted to different ends at the same time, the remarkable facts which connect themselves with vegetable chemistry and physiology are deferred for a separate examination.

The power of the solar rays to produce in bodies that peculiar gleaming light which we call phosphorescence, and the curious conditions under which this phenomenon is sometimes apparent, independent of the sun’s direct influence, present a very remarkable chapter in the science of luminous powers.

The phosphorescence of animals is amongst the most surprising of nature’s phenomena, and it is not the less so from our almost entire ignorance of the cause of it. Many very poetical fancies have been applied in description of these luminous creations; and imagination has found reason why they should be gifted with these extraordinary powers. The glow-worm lights her lamp to lure her lover to her bower, and the luminous animalcules of the ocean are employed in lighting up the fathomless depths where the sun’s rays cannot penetrate, to aid its monsters in their search for prey. “The lamp of love--the pharos--the telegraph of the night,--which scintillates and marks, in the silence of darkness, the spot appointed for the lover’s rendezvous,”[109] is but a pretty fiction; for the glow-worm shines in its infant state, in that of the larva, and when in its aurelian condition. Of the dark depths of the ocean it may be safely affirmed that no organized creation lives or moves in its grave-like silence to require this fairy aid. Fiction has frequently borrowed her creations from science. In these cases science appears to have made free with the rights of fiction.

The glow-worms (_lampyris noctiluca_), it is well known, have the power of emitting from their bodies a beautiful pale bluish-white light, shining during the hours of night in the hedge-row, like crystal spheres. It appears, from the observations of naturalists, that these insects never exhibit their light without some motion of the body or legs;--from this it would seem that the phosphorescence was dependent upon nervous action, regulated at pleasure by the insect; for they certainly have the power of obscuring it entirely. If the glow-worm is crushed, and the hands or face are rubbed with it, luminous streaks, similar to those produced by phosphorus, appear. They shine with greatly increased brilliancy in oxygen gas and in nitrous oxide. From these facts may we not infer that the process by which this luminosity is produced, whatever it may be, has a strong resemblance to that of respiration?

There are several varieties of flies, and three species of beetles of the genus _Elater_, which have the power of emitting luminous rays. The great lantern-fly of South America is one of the most brilliant, a single insect giving sufficient light to enable a person to read. In Surinam a very numerous class of these insects are found, which often illuminate the air in a remarkable manner. In some of the bogs of Ireland a worm exists which gives out a bright green light; and there are many other kinds of creatures which, under certain circumstances, become luminous in the dark. This is always dependent upon vitality; for all these animals, when deprived of life, cease to shine.

At the same time we have many very curious instances of phosphorescence in dead animal and vegetable matter; the lobster among the Crustacea, and the whiting among fishes, are striking examples; decayed wood also emits much light under certain conditions of the atmosphere. This development of light does not appear to be at all dependent upon putrefaction; indeed, as this process progresses, the luminosity diminishes. We cannot but imagine that this light is owing, in the first place, to direct absorption by, and fixation within, the corpuscular structure of those bodies, and that it is developed by the decomposition of the particles under the influence of our oxygenous atmosphere.

The pale light emitted by phosphorus in the dark is well known; and this is evidently only a species of slow combustion, a combination of the phosphorus with the oxygen of the air. Where there is no oxygen, phosphorus will not shine; its combustion in chlorine or iodine vapour is a phenomenon of a totally different character from that which we are now considering. This phosphorescence of animal and vegetable matter has been regarded as something different from the slow combustion of phosphorus; but, upon examination, all the chemical conditions are found to be the same, and it is certainly due to a similar chemical change.

The luminous matter of the dead whiting or the mackerel may be separated by a solution of common salt or of sulphate of magnesia; by concentrating these solutions the light disappears; but it is again emitted when the fluid is diluted. The entire subject is, however, involved in the mystery of ignorance, although it is a matter quite within the scope of any industrious observer. The self-emitted light of the carbuncle of the romancer is realized in these remarkable phenomena.

The phosphorescence of some plants and flowers is not, perhaps, of the same order as that which belongs to either of the conditions we have been considering. It appears to be due rather to an absorption of light and its subsequent liberation. If a nasturtium is plucked during sunshine, and carried into a dark room, the eye, after it has reposed for a short time, will discover the flower by a light emitted from its leaves.

The following remarkable example, and an explanation of it by the poet Goethe, is instructive:--

“On the 19th of June, 1799, late in the evening, when the twilight was deepening into a clear night, as I was walking up and down the garden with a friend, we very distinctly observed a flame-like appearance near the oriental poppy, the flowers of which are remarkable for their powerful red colour. We approached the place, and looked attentively at the flowers, but could perceive nothing further, till at last, by passing and repassing repeatedly, while we looked side-ways on them, we succeeded in renewing the appearance as often as we pleased. It proved to be a physiological phenomenon, and the apparent corruscation was nothing but the spectrum of the flower in the complementary blue-green colour. The twilight accounts for the eye being in a perfect state of repose, and thus very susceptible, and the colour of the poppy is sufficiently powerful in the summer twilight of the longest days to act with full effect, and produce a complementary image.”[110]

The leaves of the _œnothera macrocarpa_ are said to exhibit phosphoric light when the air is highly charged with electricity. The agarics of the olive-grounds of Montpelier have been observed to be luminous at night; but they are said to exhibit no light, even in darkness, _during the day_. The subterranean passages of the coal mines near Dresden are illuminated by the phosphorescent light of the _rhizomorpha phosphoreus_, a peculiar fungus. On the leaves of the Pindoba palm, a species of agaric grows which is exceedingly luminous at night; and many varieties of the lichens, creeping along the roofs of caverns, lend to them an air of enchantment by the soft and clear light which they diffuse. In a small cave near Penryn, a luminous moss is abundant; and it is also found in the mines of Hesse. According to Heinzmann, the _rhizomorpha subterranea_ and _aidulæ_ are also phosphorescent.

It is but lately that a plant which abounds in the jungles in the Madura district of the East Indies was sent to this country, which, although dead, was remarkably phosphorescent; and, when in the living state, the light which it emitted was extraordinarily vivid, illuminating the ground for some distance. Those remarkable effects may be due, in some cases, to the separation of phosphuretted hydrogen from decomposing matter, and, in others, to some peculiar electric manifestation.

The phosphorescence of the sea, or that condition called by fishermen _brimy_, when the surface, being struck by an oar, or the paddle-wheels of a steamer, gives out large quantities of light, has been attributed to the presence of myriads of minute insects which have the power of emitting light when irritated. The night-shining nereis (_Nereis noctiluca_) emits a light of great brilliancy, as do several kinds of the mollusca. The nereides attach themselves to the scales of fishes, and thus frequently render them exceedingly luminous. Some of the crustaceæ possess the same remarkable property;--twelve different species of _cancer_ were taken up by the naturalists of the Zaire in the Gulf of Guinea.[111] The _cancer fulgens_, discovered by Sir Joseph Banks, is enabled to illuminate its whole body, and emits vivid flashes of light. Many of the medusæ also exhibit powerful phosphorescence.[112] These noctilucous creatures are, many of them, exceedingly minute, several thousands being found in a tea-cup of sea water. They float near the surface in countless myriads, and when disturbed they give out brilliant scintillations, often leaving a train of light behind them.[113] By microscopic examination no other fact has been elicited than that these minute beings contain a fluid which, when squeezed out, leaves a line of light upon the surface of water. The appearance of these creatures is almost invariably on the eve of some change of weather, which would lead us to suppose that their luminous phenomena must be connected with electrical excitation; and of this, the investigations of Mr. C. Peach, of Fowey, communicated to the British Association at Birmingham, furnish the most satisfactory proofs we have as yet obtained.

Benvenuto Cellini gave a curious account of a carbuncle which shone with great brilliancy in the dark.[114] The same thing has been stated of the diamond; but it appears to be necessary to procure these emissions of light, that the minerals should be first warmed near a fire. From this it may be inferred that the luminous appearance is of a similar character to that of fluor spar, and of numerous other earthy minerals, which, when exposed to heat, phosphoresce with great brilliancy. Phosphorescent glow can also be excited in similar bodies by electricity, as was first pointed out by Father Beccaria, and confirmed by Mr. Pearsall.[115] These effects, it must be remembered, are distinct from the electric spark manifested upon breaking white sugar in the dark, or scratching sulphuret of zinc.

In the instances adduced there is not necessarily any exposure to the sunshine required. It is probable that two, if not three, distinct phenomena are concerned in the cases above quoted, and that all of them are distinct from animal phosphorescence, or the luminous appearance of vegetables. They, however, certainly prove, either that light is capable of becoming latent, or that it is only a condition of matter, in which it may be made manifest by any disturbance of the molecular forces. We have, in answer to this, very distinct evidence that some bodies do derive this property from the solar rays. Canton’s phosphorus, which is a sulphuret of calcium, will, having been exposed to the sun, continue luminous for some time after it is carried into the dark; as will also the Bolognian stone,--a sulphuret of barium. This result appears to be due to a particular class of the solar rays; for it has been found, if these sulphurets, spread smoothly on paper, are exposed to the influence of the solar spectrum for some little time, and then examined in the dark, that luminous spaces appear, exactly corresponding with the most refrangible rays, or those which excite chemical change; and one very remarkable fact must not be forgotten--the dark rays of the spectrum beyond the violet produce a lively phosphorescence, which is _extinguished_ by the action of the rays of least refrangibility, or the heat rays--whilst artificial heat, such as a warm iron, produces a very considerable elevation of the phosphorescent effect.[116] It is not improbable, that the fluorescent rays of Mr. Stokes may be materially concerned in producing the phenomena of phosphorescence: experiments are, however, required to prove this.

In these allied phenomena we have effects which are evidently dependent upon several dissimilar causes. The phosphorescence of the living animal is due, without doubt, to nervous excitation: that of the living vegetable to solar luminous influence; and in the case of the mosses of caverns, &c. to the chemical agency of the sun’s rays, which appears to be capable of conduction. In the dead organic matter we have a purely chemical action developing the light, and in the inorganic bodies we have peculiar molecular constitution, by which an absorption of light appears to take place.

The subject is one of the greatest difficulty; the torch of science is too dim to enable us to see the causes at work in producing these marvellous effects. The investigation leads, to a certain extent, to the elucidation of many of the secrets of luminous action; and the determination of the question, whether light is an emanation from the sun, or only a subtile principle diffused through all matter, which is excited by solar influence, is intimately connected with the inquiry.

It has been stated that matter is necessary to the development of light; that no luminous effect would be produced if it were not for the presence of matter. Of this we not only have no proof, but such evidence as we have is against the position. There is no loss of light in the most perfect vacuum we can produce by any artificial means, which should be the case if matter was concerned in the phenomena of light, as a cause.

Colour is certainly a property regulated by material bodies; or rather, the presence of matter is necessary to the production of colour. Chlorine gas is a pale yellow, and nitrous vapour a yellowish red. These and one or two other vapours, which are near the point of condensation into fluids, are the only coloured gaseous or vaporiform bodies. The sky is blue, because the material particles of the atmosphere reflect back the blue rays. But we have more practical illustrations than this. The flame of hydrogen burning with oxygen gives scarcely any light; allow it to impinge on lime, a portion of which is carried off by the heat of the flame, and the most intense artificial light with which we are acquainted is produced. Hydrogen gas alone gives a flame in which nearly all but the blue rays are wanting: place a brush of steel or asbestos in it, and many of the other rays are at once produced. An argand lamp, and more particularly the lamp in which camphine--a purified turpentine,--is burnt, gives a flame which emits most of the rays found in sunlight. Spirit of wine mixed with water, warmed and ignited, gives only yellow rays; add nitrate of strontian and they become red; but nitrate of barytes being mixed with the fluid, they are changed to green and yellow; salts of copper afford fine blue rays, and common salt intense yellow ones. Many of these coloured rays and others can be produced in great power by the use of various solid bodies introduced into flame. This has not been sufficiently pointed out by authors; but it is clear from experiments that light requires the presence of matter to enable it to diffuse its coloured glories. How is it that the oxygen and hydrogen flame gives so little light, and with a solid body present, pours forth such a flood of brilliancy?

The production of artificial light by electrical and chemical agencies will necessarily find some consideration under their respective heads. There are numerous phenomena which connect themselves with luminous power, or appear to do so, which, in the present state of our knowledge, cannot come immediately under our attention. We are compelled to reserve our limited space for those branches of science which we are enabled to connect with the great natural operations constantly going on around us. Many of these more abstruse results will, however, receive some incidental notice when we come to examine the operation of the combined physical forces on matter.

We see in light a principle which, if it has not its source in the sun, is certainly dependent upon that luminary for its manifestations and powers. From that “fountain of light” we find this principle travelling to us at a speed which almost approaches the quickness of thought itself; yet by the refinements of science we have been enabled to measure its velocity with the utmost accuracy. The immortal poet of our own land and language, in his creations of Ariel, that “tricksy spirit,” who could creep like music upon the waters, and of the fantastic Puck, who could girdle the earth in thirty minutes, appears to have approached to the highest point to which mere imagination could carry the human mind as to the powers of things ethereal. Science has, since then, shown to man that this “spirit, fine spirit,” was a laggard in his tasks, and a gross piece of matter, when compared with the subtile essences which man, like a nobler Prospero, has now subdued to do him service.

Light is necessary to life; the world was a dead chaos before its creation, and mute disorder would again be the consequence of its annihilation. Every charm which spreads itself over this rolling globe is directly dependent upon luminous power. Colours, and probably, forms, are the result of light; certainly the consequence of solar radiations. We know much of the mysterious influences of this great agent, but we know nothing of the principle itself. The solar beam has been tortured through prismatic glasses and natural crystals; every chemical agent has been tried upon it, every electrical force in the most excited state brought to bear upon its operations, with a view to the discovery of the most refined of earthly agencies; but it has passed through every trial without revealing its secrets, and even the effects which it produces in its path are unexplained problems, still to tax the intellect of man.

Every animal and every plant alike proclaim that life and health are due to light; and even the crystallizing forms of inorganic matter, by bending towards it, confess its all-prevailing sway. From the sun to each planet revolving around that orb, and to the remotest stars which gleam through the vast immensity of heaven, we discover this power still in its brightness, giving beauty and order to these unnumbered creations; no less completely than to this small island of the universe which we call our Earth. Through every form of matter we can mark its power, and from all, we can, under certain conditions, evoke it in lustre and activity. Over all and through all light spreads its ethereal force, and manifests, in all its operations, powers which might well exalt the mind of Plato to the idea of an omniscient and omnipresent God. Science, with her Ithuriel wand, has, however, shown that light is itself the effect of a yet more exalted cause, which we cannot reach.

Indeed, the attentive study of the fine abstractions of science lifts the mind from the grossness of matter, step by step, to the refinements of immateriality, and there appear, shadowed out beyond the physical forces which man can test and try, other powers still ascending, until they reach the Source of every good and every perfect gift.

FOOTNOTES:

[84] “These--oxygen, hydrogen, nitrogen, and carbon--are the four bodies, in fact, which, becoming animated at the fire of the sun, the true torch of Prometheus, approve themselves upon the earth the eternal agents of organisation, of sensation, of motion and of thought.”--Dumas, _Leçons de Philosophie Chimique_, p. 100. Paris, 1837.

[85] It will be found in examining any of the works of the alchemists,--particularly those of Geber, _De inveniendi arte Auri et Argenti_, and his _De Alchemiâ_; Roger Bacon’s _Opus Majus, or Alchymia Major_; Helvetius’ _Brief of the Golden Calf_; or Basil Valentine’s _Currus Triumphalis_,--that in the processes of transmutation the solar light was supposed to be marvellously effective. In Boyle’s _Sceptical Chemist_ the same idea will be found pervading it.

Amid all their errors, the alchemists were assiduous workmen, and to them we are indebted for numerous facts. Of them, and of their age, as contrasted with our own, Gibbon remarks:--“Congenial to the avarice of the human heart, it was studied in China, as in Europe, with equal eagerness and equal success. The darkness of the middle ages ensured a favourable reception to every tale of wonder; and the revival of learning gave new vigour to hope, and suggested more specious arts of deception. Philosophy, with the aid of experience, has at length banished the study of alchemy; and the present age, however desirous of riches, is content to seek them by the humbler means of commerce and industry.”--_Decline and Fall_, vol. ii. p. 137.

[86] On the two theories the following maybe consulted:--Young, _Supplement to Encyclopædia Britannica_, article _Chromatics_; Fresnel, _Supplément à la Traduction Française de la 5ième édition du Traité de Chimie de Thomson_, par Riffault, Paris, 1822; Herschel’s Article, _Light_, in the Encyclopædia Metropolitana, and the French Translation of it by Quetelet and Verhulst; Airy’s _Tract on the Undulatory Theory_, in his Tracts, 2nd edition, Cambridge, 1831; Powel, _The Undulatory Theory applied to Dispersion_, &c. p. 184; Lloyd’s _Lectures_, Dublin, 1836-41; Cauchy, _Sur le Mouvement des Corps élastiques_, Mémoires de l’Institut, 1827, vol. ix. p. 114; _Théorie de la Lumière_, Ibid. vol. x. p. 293; M’Cullagh, _On Double Refraction_, Ibid., vol. xvi.; _Geometrical Propositions applied to the Wave Theory of Light_, Ibid., vol. xvii.; Sir David Brewster’s papers in the Transactions of the Royal Society of Edinburgh, and the Philosophical Magazine.

[87] _Results of Astronomical Observations made during the years 1834-38, at the Cape of Good Hope, &c._ By Sir John Herschel, Bart., K.H., D.C.L., F.R.S.--“In the contemplation of the infinite, in number and in magnitude, the mind ever fails us. We stand appalled before this mighty spectre of boundless space, and faltering reason sinks under the load of its bursting conceptions. But, placed as we are on the great locomotive of our system, destined surely to complete at least one round of its ethereal course, and learning that we can make no apparent advance on our sidereal journey, we pant with new ardour for that distant bourne which we constantly approach without the possibility of reaching it. In feeling this disappointment, and patiently bearing it, let us endeavour to realise the great truth from which it flows. It cannot occupy our mind without exalting and improving it.”--_Sir D. Brewster_: North British Review.

[88] For examples of this, consult Graham’s _Elements of Chemistry_; Brande’s _Manual of Chemistry_; or, indeed, any work treating of the science. The formation of ink, by mixing two colourless solutions, one of gallic acid and another of sulphate of iron, may be taken as a familiar instance.

[89] Sir John Herschel, in his paper _On the Chemical Action of the Rays of the Solar Spectrum on Preparations of Silver_, remarks that, “it may seem too hazardous to look for the cause of this very singular phenomenon in a real difference between the chemical agencies of those rays which issue from the central portion of the sun’s disc, and those which, emanating from its borders, have undergone the absorptive action of a much greater depth of its atmosphere; and yet I confess myself somewhat at a loss what other cause to assign for it. It must suffice, however, to have thrown out the hint; remarking only, that I have other, and, I am disposed to think, decisive evidence (which will find its place elsewhere) of the existence of an absorptive solar atmosphere, extending beyond the luminous one. The breadth of the border, I should observe, is small, not exceeding 0·5 or 1/7 part of the sun’s radius, and this, from the circumstances of the experiment, must necessarily err in excess.”--Philosophical Transactions, 1840.

[90] _Experiments and Observations on some Cases of Lines in the Prismatic Spectrum, produced by the passage of Light through Coloured Vapours and Gases, and from certain Coloured Flames._ By W. A. Miller, M.D., F.R.S., Professor of Chemistry in King’s College, London.--Philosophical Magazine, vol. xxvii.

[91] _Report on the Mollusca and Radiata of the Ægean Sea, and on their distribution, considered as bearing on Geology._ By Edward Forbes, F.R.S., &c.--Reports of the British Association, vol. xii. Professor Forbes remarks:--“A comparison of the testacea, and other animals of the lowest zones, with those of the higher, exhibits a very great distinction in the hues of the species, those of the depths being, for the most part, white or colourless, while those of the higher regions, in a great number of instances, exhibit brilliant combinations of colour. The results of an enquiry into this subject are as follows:--

“The majority of shells of the lowest zone are white or transparent; if tinted rose is the hue, a very few exhibit markings of another colour. In the seventh region, white species are also very abundant, though by no means forming a proportion so great as the eighth. Brownish red, the prevalent hue of the brachiopoda, also gives a character of colour to the fauna of this zone; the crustacea found in it are red. In the sixth zone the colours become brighter, reds and yellows prevailing,--generally, however, uniformly colouring the shell. In the fifth region many species are banded or clouded with various combinations of colours, and the number of white species has greatly diminished. In the fourth, purple hues are frequent, and contrasts of colour common. In the second and third, green and blue tints are met with, sometimes very vivid; but the gayest combinations of colour are seen in the littoral zone, as well as the most brilliant whites.

“The animals of Testacea, and the Radiata of the higher zones, are much more brilliantly coloured than those of the lower, where they are usually white, whatever the hue of the shell may be. Thus the genus _Trochus_ is an example of a group of forms mostly presenting the most brilliant hues both of shell and animal; but whilst the animals of such species as inhabit the littoral zone are gaily chequered with many vivid hues, those of the greater depth, though their shells are almost as brightly covered as the coverings of their allies nearer the surface, have their animals, for the most part, of a uniform yellow or reddish hue, or else entirely white. The chief cause of this increase of intensity of colour as we ascend is, doubtless, the increased amount of light above a certain depth.”--p. 172.

[92] Ἁμὁρφωτα. _On the Epipolic Dispersion of Light_, being a paper entitled, _On a case of Superficial Colour presented, by a homogeneous liquid internally colourless_. By Sir J. F. W. Herschel, Bart, K.H., F.R.S., &c.--_An epipolized beam of light_ (meaning thereby a beam which has once been transmitted through a quiniferous solution, and undergone its dispersing action) _is incapable of further undergoing epipolic dispersion_. In proof of this the following experiment may be adduced,--

A glass jar being filled with a quiniferous solution, a piece of plate glass was immersed in it vertically, so as to be entirely covered, and to present one face directly to the incident light. In this situation, when viewed by an eye almost perpendicularly over it, so as to graze either surface very obliquely, neither the anterior nor posterior face showed the slightest trace of epipolic colour. Now, the light, at its egress from the immersed glass, entered the liquid under precisely the same circumstances as that which, when traversing the anterior surface of the glass jar, underwent epipolic dispersion on first entering the liquid. It had, therefore, lost a property which it originally possessed, and could not, therefore, be considered _qualitatively_ the same light.--Philosophical Transactions, vol. cxxxvi.

[93] In connection with this view, the Newtonian theory should be consulted, for which see--_A Letter of_ Mr. Isaac Newton, _Professor of the Mathematicks in the University of Cambridge; containing his new Theory about Light and Colors: sent by the Author to the Publisher, from Cambridge, Feb. 6, 1671-72, in order to be communicated to the Royal Society._

[94] In that admirable work, _The Physical Atlas_ of Dr. Berghaus, of which a very complete edition by Alexander Keith Johnstone is published in this country, the following order of the distribution of plants is given:--

1. The region of palms and bananas Equatorial zone.
2. Tree ferns and figs Tropical zone.
3. Myrtles and laurels Sub-tropical zone.
4. Evergreen trees Warm temperate zone.
5. European trees Cold temperate zone.
6. Pines Sub-arctic zone.
7. Rhododendrons Arctic zone.
8. Alpine plants Polar zone.

Consult Humboldt, _Essai sur la Géographie des Plantes_, Paris, 1807; _De Distributione Geographicâ Plantarum_, Paris, 1817. Schouw, _Grundzüge der Pflanzengeographie_. Also his _Earth, Plants, and Man_; translated by Henfrey, in _Bohn’s Scientific Library_. Lamouroux, _Géographie Physique_. _The Plant, a Biography_: by Schleiden; translated by Henfrey. _Physical Geography_: by Mrs. Somerville.

[95] Fraunhofer’s measure of illuminating power is as follows:--

At the 22nd degree of the red 0·032
" 34th degree of the red 0·094
" 22nd degree of the orange 0·640
" 10th degree of the yellow 1·000
" 42nd degree of the yellow 0·480
" 2nd degree of the blue 0·170
" 16th degree of the indigo 0·031
" 43rd degree of the violet 0·0056

[96] Herschel, _On the Action of Crystallized Bodies on Homogeneous Light, and on the causes of the deviation from Newtons scale in the tints which many of them develope on exposure to a polarized ray_.--Phil. Trans., vol. cx., p. 88.

[97] _On the Nature of Light and Colours_: Lecture 39, in Young’s _Lectures on Natural Philosophy_, Kelland’s Edition, p. 373, and the authorities there quoted.

[98] Brewster’s _Optics_: Lardner’s Cabinet Cyclopædia. Herschel, _On Light_: Encyclopædia Metropolitana.

[99] Malus, _Sur une Propriété de la Lumière Réfléchie_: Mémoires d’Arcueil. Numerous memoirs by Sir David Brewster, in the Philosophical Transactions.

[100] Bartholin, _On Iceland Crystals_: Copenhagen, 1669. _An Accompt of sundry Experiments made and communicated by that Learn’d Mathematician_ Dr. Erasmus Bartholin, _upon a Chrystal like Body sent to him out of Island_: in connection with which Dr. Matthias Paissenius writes:--The observations of the excellent Bartholin upon the Island Chrystal are, indeed, considerable, as well as painful. We have here, also, made some tryals of it upon a piece he presented me with, which confirm his observations. Mean time he found it somewhat scissile and reducible by a knife into thin laminas or plates, which, when single, shew’d the object single, but laid upon one another shew’d it double; the two images appearing the more distant from one another, the greater the number was of those thin plates laid on one another. With submission to better judgements I think it to be a kind of Selenites. Some of our curious men here were of opinion that the Rhomboid figure proper to this stone was the cause of the appearances doubled thereby. But having tryed whether in other transparent bodies of the like figure the like would happen, we found no such thing in them, which made us suspect some peculiarity in the very Body of the stone.--Phil. Trans. for 1670, vol. v.

[101] _On the Application of the Laws of Circular Polarization to the Researches of Chemistry_: by M. Biot.--Nouvelles Annales du Muséum d’Histoire Naturelle, vol. iii., and Scientific Memoirs, vol. i. p. 600. _On Circular Polarization_: by Dr. Leeson.--Memoirs of the Chemical Society.

[102] In Sir David Brewster’s Treatise _On Optics_, chap, xviii., _On Polarization_, the best arrangements for a polarizing apparatus will be found described.

[103] This beautiful application was recently made by Professor Wheatstone, the particulars of which will be found in his interesting communication.--_On a means of determining the apparent Solar Time by the diurnal changes of the Plane of Polarization at the Northern Pole of the Sky_: Report of the Eighteenth Meeting of the British Association.

[104] _On the Polarization of the Chemical Rays of Light_: by John Sutherland, M.D., in which the author refers to the following experiment of M. J. E. Bérard--“I received the chemical rays directed into the plane of the meridian on an unsilvered glass, under an incidence of 35° 61'. The rays reflected by the first glass were received upon a second, under the same incidence. I found that when this was turned towards the south, the muriate of silver exposed to the invisible rays which it reflected was darkened in less than half an hour; whereas, when it was turned towards the west, the muriate of silver exposed in the place where the rays ought to have been reflected, was not darkened, although it was left exposed for two hours. It is consequently to be presumed that the chemical rays can undergo double refraction in traversing certain diaphanous bodies; and lastly, we may say that they enjoy the same physical properties as light in general.”--Philosophical Magazine, vol. xx.

Dr. Leeson has stated that Daguerreotype pictures can be taken more readily under the influence of polarized light, than by ordinary radiation.

[105] _On the Magnetization of Light, and the Illumination of Magnetic Lines of Force_: by Michael Faraday, D.C.L., F.R.S.--Philosophical Transactions, vol. cxxxvii.--The following remarks are to the point of doubt referred to in the text.--“The magnetic forces do not act on the ray of light directly and without the intervention of matter, but through the mediation of the substance in which they and the ray have a simultaneous existence; the substances and the forces giving to and receiving from each other the power of acting on the light. This is shown by the non-action of a vacuum, of air or gases, and it is also further shown by the special degree in which different matters possess the property. That magnetic force acts upon the ray of light always with the same character of manner, and in the same direction, independent of the different varieties of substance, or their states of solid or liquid, or their specific rotative force, shows that the magnetic force and the light have a direct relation; but that substances are necessary, and that these act in different degrees, shows that the magnetism and the light act on each other through the intervention of the matter. Recognising or perceiving _matter_ only by its powers, and knowing nothing of any imaginary nucleus abstract from the idea of these powers, the phenomena described must strengthen my inclination to trust in the views I have advanced in reference to its nature.”--Phil. Mag. vol. xxiv.

[106] The invention of the camera obscura certainly belongs to Giambattista Porta, and is described in his _Magiæ Naturalis, sive de Miraculis Rerum Naturalium, Libri Viginti_; Antwerp, 1561. An English translation made in 1658 exists, but I have not seen it.

Hooke, in one of the earliest volumes of the Philosophical Transactions, describes as new many of the phenomena mentioned by Porta, and particularly the images of the dark chamber.

[107] Herschel, _On Light_,--Encyclopædia Metropolitana.

[108] “I would here observe that a consideration of many such phenomena (the obliteration and revival of photographic drawings) has led me to regard it as not impossible that the retina itself may be _photographically_ impressed by strong light, and that some at least of the phenomena of visual spectra and secondary colours may arise from the sensorial perception of actual changes in progress in the physical state of that organ itself subsequent to the cessation of the direct stimulant.”--_On the action of the Rays of the Solar Spectrum on Vegetable Colours, &c._: by Sir J. F. W. Herschel, Bart.

[109] Dumeril.

[110] _Theory of Colours_: by Goethe; translated by Eastlake.

[111] See Tuckey’s Narrative of the Expedition of the Zaire.

[112] The most complete examination of this subject will be found in two Memoirs:--

1. _Experiments and observations on the light which is spontaneously emitted with some degree, of permanency from various bodies._--Phil. Trans., vol. xc.

2. _A continuation of the above, with some experiments and observations on solar light, when imbibed by Canton’s phosphorus_: by Nathaniel Hulm, M.D.--Phil. Trans., vol. xci.; and in the _Monograph of the British Naked-eyed Medusæ_, by Professor Edward Forbes (published for the Ray Society). See Wilson’s note to the account of _Pennalata phosphorea_ in Johnston’s Zoophytes, 2nd edition.

[113] _A General Outline of the Animal Kingdom_: by Thomas Rymer Jones, F.L.S.--Acalephæ, p. 64. _Lettre à M. Dumas sur la Phosphorescence des Vers luisants_: par M. Ch. Matteucci.--Annales de Chimie, vol. ix. p. 71, 1843.

[114] _Memoirs of Benvenuto Cellini--Bohn’s Standard Library._ See also his Treatise on his Art as a Sculptor and Engraver. Florence, 1568. 4to.

[115] _Phosphorescence of the Diamond_: by M. Reiss (Revue Scientifique et Industrielle, vol. xxiii. p. 185).--“The diamond, phosphorescent by insulation, lost rapidly its phosphorescence when submitted to the action of the red rays of the solar spectrum. On the contrary, the blue rays are those which render the diamond the most luminous in the dark. It is probable that the phosphorescence produced by heat is equally diminished by the action of the red rays of the solar spectrum.” Giovanni Battista Beccaria published his experiments in 1769. See Priestley’s _History of Electricity_; and _On the Effects of Electricity upon Minerals which are Phosphorescent by Heat_; and _Further Experiments on the communication of Phosphorescence and Colour to bodies of Electricity_; by Thomas J. Pearsall.--Journal of the Royal Institution of Great Britain, Oct. 1830, Feb. 1831.--These two memoirs contain the most complete set of experiments on this subject which have yet been made; see _Placidus Heinrich_, _Phosphorescenz der Körper_, vol. iv.; Gmelin’s _Handbuch der Chemie_, part 1.;--_On the Phosphorescence of Minerals_, Brewster: Edinburgh Philosophical Journal, vol. i. p. 137.;--_The Aërial Noctiluca, or some New Phenomena, and a process of a factitious self shining substance_: Boyle’s Works, vol. iv.

[116] _Des Effets produits sur les corps par les Rayons Solaires_: par M. Edmond Becquerel.--Annales de Chimie, vol. ix. p. 257. 1843.

M. Becquerel has applied the term _phosphorogénique_ to those rays producing phosphorescence.