Finally, our order comprises the Hepatica, with its blue or pink blossoms and three-lobed leaves, which, from their fancied resemblance to the form of the liver, procured the plant its English name of liverwort; the Flos Adonis, or pheasant's eye,—the goutte-de-sang of the French,—so called because the ancients fabled that it sprang from the blood of Adonis, when wounded by the bear; the marsh marigold; the gay and vivacious larkspur; the deadly wolfsbane, or aconite, which secretes so potent a poison; and the aromatic love-in-a-mist, or French flower.[55]

B.—Animals.

Under the soft moss, under the stones, in all localities where mouldiness is easily developed, under the closed doors of cellars, you must certainly have more than once observed a tiny creature of the form of a horse-bean, of a gray leaden colour, and supplied with a considerable number of feet. This last characteristic will induce you immediately to abandon the idea that you have before you an insect.

Catch hold of it, and count its feet.

Well said; but it runs much more quickly than I would have suspected from its previously dilatory movements.

Because it knows that danger threatens it, instinct impels it to escape at its utmost speed. Do not be afraid to handle it; the poor creature can do you no harm.

Unable to escape, it counterfeits death, and remains perfectly immovable.

Now examine it. It has fourteen feet, symmetrically arranged in couples; their size perceptibly increases from the first to the last. When the animal is at rest, they are coiled inside, so as to form an angle whose opening faces the medial line. But here is something much more curious; its body, which does not possess the vestige of a wing, is also without those segments which would divide it visibly, as in the case of insects, into head, thorax, and abdomen; but is composed of rings, hard and scaly, like those of a shrimp.

Can it be a crustacean?

Yes; the animal you hold in your hand, and which everybody knew by the name of wood-louse long before our naturalists knew how to classify it, belongs to the great animal division of the Articulata, which, instead of having their skeleton inside the body, like the Vertebrates, have it externally. The Crustacea form a class of this division, to which also belong the Insects, the Arachnida, and the Myriapoda.

Let us continue to anatomise our crustacean.

In front of its first ring (a transversal segment) you see a little black head, with two lateral bead-like eyes, and a couple of antennæ. The latter are each composed of three joints, which are extremely mobile; their base is covered by the edges of the sloping head. The most conspicuous rings of the body are seven in number; their lateral borders are pointed in front, and rounded behind. But, if you look closely, you will see some other rings, a little less projecting than the former; they circumscribe the abdominal region, the belly, properly so called, in which the intestines are lodged. These rings, or abdominal segments, are six in number; but they have not all the same form. The one which occupies the tail is triangular, pointed, and surrounded by four (caudal) appendages. The three next segments, counting from the front to the rear, are prolonged laterally in a very marked manner; the two anterior, on the contrary, have no such distinction. As for the caudal appendages, the two outer ones are very strong, conical, and composed of two articulations, while the inner, situated above the former, are frail, cylindrical, and terminated by a tuft of hairs, whence issues a viscous liquid. (See Fig. 35, a.)

An enumeration of these characteristics is tedious, but necessary for the determination of the genus and the species. They belong to the Oniscus asellus, or common wood-louse. But why, you ask, why such a strange conjunction of names,—one Greek, ὀνίσκος, the other Latin, asellus? Both carry the same meaning: why not, then, have called our tiny crustacean an ass-ass (if such a compound be possible)? Why, neither close at hand nor at a distance, has it the slightest resemblance to an ass; and to say that we have only borrowed these names from the ancients, is neither an explanation nor a justification.

But we have not yet done with the wood-lice. Are these interesting little creatures (they are interesting, are they not?) oviparous or viviparous? I defy you to show me anywhere a single wood-louse's egg. Have the patience to observe our crustaceans more nearly. Among the crowd, you will remark some—they are the females—with a kind of membraneous pouch underneath the body, stretching from the head to the fifth pair of legs. The pellicle which forms this pouch is so thin, and so transparent, that you can distinguish the eggs within it. These eggs, instead of being expelled for incubation, remain in the mother's pouch until they are hatched. At that felicitous moment, the membranous bag splits cross-wise, longitudinally and transversally, to permit the emergence of the young wood-lice. The latter are extremely small, and in form resemble nothing in the world so much as a little white line (Fig. 35, b). They differ from their parents only in having one pair of feet, and one ring less than they have. They undergo no metamorphoses. After their birth, the little ones, which have proportionally very large antennæ, do not immediately separate from their mother. By a wonderful act of forethought on the part of Nature,[56] they keep themselves concealed in the middle of the respiratory laminæ, which garnish the under part of the tail.

The specific characters of the Oniscus asellus are tolerably well defined. By its rings of dark gray, a little lighter at the edges, which form for it an articulated, glossy carapace, marked with white spots, longitudinally arranged; by the uniform pale gray colour of its belly and its legs, covered with scattered hairs; and, particularly, by its habits, our wood-louse, which the Germans call cellar-louse (Kellerlaus), is distinguished from its kindred species, of which naturalists have made distinct genera. Thus, the asellus found generally under stones, which counterfeits death by rolling itself up in a ball like a hedgehog, and will rather suffer itself to be crushed than unfold, is the Oniscus armadillo, which some naturalists transform into the Armadillo vulgaris. (See Fig. 36.) This species prefers the solitude of the field to inhabited places. Its body is considerably expanded, and its rings do not terminate in a point on their lateral and posterior edges.

Another species, equally common underneath stones, has its head and tail covered over with granulations; its antennæ are composed of seven joints, of which the fourth and fifth are perceptibly situated lengthwise. This is the Oniscus granulatus of some entomologists; others have designated it the Porcellio scaber. Why not simplify the study of species?

The wood-lice seem to live upon decomposed vegetable matter. But in default of other food, they devour their own kind; in this respect resembling beings who are supposed to rank much higher in the animal hierarchy.

In the pharmacopœia of the ancients our wood-lice found a place. Reduced into powder, and mixed with various substances, they were prescribed as diuretic and aperient; but they were long ago abandoned in medicine.

The Dragon-Flies (Libellulæ).

In walking along the banks of a river, you must frequently have seen hovering around you a cloud of insects, whom you would readily take to be butterflies, were not you arrested in your conjecture by the largeness of their head, the length of their body, the form of their vivid, diaphanous, gauze-like wings, and, generally,—which will most astonish you,—by their carnivorous instincts. You have about you and before, then, not butterflies, but Dragon-flies—the Libellulæ of naturalists. They are the demoiselles, or "ladies," of the French; so called, perhaps, in allusion to their airy and graceful flight.

Among these Libellulæ, one is called Eleanora. If she does not shine so brightly as the others—if her colours are less brilliant—she has, at least, the advantage of being so common that you can easily obtain a specimen.

But, first, let us pause to think of the strange dissimilarity in the names bestowed on the Libellulæ by the English and French respectively. They are the Dragon-flies of the former,—fierce, rapacious, formidable; the Ladies of the latter,—elegant, light, and radiant. Here we have a glimpse of national character. With the Frenchman, "appearance" counts for so much; with the Englishman, everything depends upon the "reality." Yet our English poets can appreciate their gay exterior. Moore speaks of them as—

Poor Clare, the Northamptonshire poet, correctly studied—

And Mary Howitt has seen them—

Beautiful as they are, they must be ranked among Nature's fiercest and most insatiable destroyers. They are the terror of the insect world. On this point we shall hereafter enlarge, but before I forget it, I would fain relate an anecdote, in illustration of their voracity, which I have read somewhere or other. A naturalist recounts with what interest he has often watched the proceedings of the dragon-fly. He has seen it, in a locality where white butterflies were numerous, dart down as a hawk upon a quarry, seize with its legs a firm hold of a butterfly, and carry it to a branch of an adjoining tree. In a moment one of the white wings would drop from the boughs, and then another would come wavering downwards, and so on, until all four had fallen; and the dragon-fly, after a short pause, would again dart forth in pursuit of a fresh victim. He never launched himself on his prey when on a perfect horizontal line with it; but took care to be either somewhat higher, or somewhat lower, so that he could seize it with his feet.

But now let us consider by what characteristics the reader is to recognise his Libellulæ.

The eyes are very large, the size of the insect considered; of a brown colour, and nearly joined together at the top of the head; in front of this point of junction a tiny vesicle is visible, carrying three ocelli (i.e., little, simple, glossy eyes), two on each side, and the third on the anterior margin: the thorax is large, hairy, and composed of two yellow plates; the abdomen laterally depressed, in such a manner as to give great prominence to the medial line. This last and well-marked feature it is which has procured for the Eleanora the scientific name of Libellula depressa,—a name proposed by Linnæus, and unanimously adopted; such unanimity being a rare occurrence among naturalists, though, to parody a phrase of Sheridan's, when naturalists do agree, it is something wonderful!

In the male Libellulæ, the upper surface of the abdomen is bluish in hue, and covered as it were with an ashy dust, while in the female it is olive; in both sexes the first and last abdominal segment are of a deeper shade than the other segments. The feet are black, and bristle all over with stiff hairs; the thighs are of a brownish red. The two pairs of wings, each strongly reticulated, present, towards the extremity of the upper border, a black rectangular spot; their base, moreover, is edged with brown spots; the spots of the lower pair are triangular and larger than those of the upper pair, which are nearly linear. They form, to a certain extent, the reservoir of the liquid which nourishes and maintains the circulation of the network of the wings.

The Libellula, which Geoffroy calls (in his "Histoire des Insectes") the Philinta, is simply the male of the species we have just been describing. You may see them

pursuing one another with an abrupt, jerking—I had almost said staccato—flight.

A species less common than the preceding, but closely resembling it, is the Françoise of Geoffroy, the Libellula quadrimaculata of Linnæus. It owes its descriptive or specific designation to the colours which diversify its wings. On the outer edge of each, two brown marginal spots are conspicuous: the first at the place where in the Eleanora (and other species) the black spot is found, and the second nearly in the centre of the external border, which, at this point, is considerably compressed; moreover, the lower wings are marked, beneath their yellow base, with a kind of triangular spot of blackish brown, finely reticulated with yellow. Externally, there is no difference of appearance between male and female, except that the abdomen of the latter is somewhat the larger.

Now we come to the Sylvia, the Libellula cancellata of Linnæus. Its eyes and thorax wear a greenish hue; the diaphanous wings are spotted with brown near the outer margin; the abdomen is of a bluish-gray; the extremity of the sixth segment, and the following segments, are wholly black.

The Julia, or Libellula grandis (Linn.), has been separated from the Libellulæ by the great entomologists, such as Fabricius and Latreille, and included in the genus Aeshna (Phœbus, what a name!), preserving the descriptive adjective grandis. What are the reasons put forward to justify this separation? Principally, the form of the abdomen, and the position of those little smooth and simple eyes, like tiny pearls or pearlets, which we call ocelli. In the Libellulæ, properly so called, the ocelli, three in number, are situated on either side and on the exterior margin of a kind of semi-triangular vesicle, and the abdomen is slightly depressed, and not unlike a club; while in the Aeshnæ elongated, and almost cylindrical.

The Aeshna grandis (or the "Julia") is one of the largest of the British species. Its head is large, and its eyes are of a brown colour shading into blue; the yellow thorax has two bright yellow bands or stripes obliquely painted on each side. The abdomen is of a reddish or even rusty brown; generally spotted with white and yellow at the top and bottom of each wing. This species haunts the vicinity of streams and "silent pools."

We come, in due order, to the Aeshna forcipata, or "Carolina," with its dirty yellow head, and its greenish-yellow thorax, the latter marked on each side with three oblique lines of black: the abdomen is black, and composed of segments laterally spotted with yellow.

The Libellula, which the illustrious Geoffroy designates "Louisa," is, according to modern entomologists, neither a Libellula nor a Aeshna, but a species of Agarion,—the Agarion virgo (Libellula virgo, Linn.) The Agarions are distinguishable from the Libellula and the Aeshna by their remarkably thin, filiform, and exceedingly elongated abdomen, and by the three ocelli arranged triangle-wise on the top of the head.

The "Louisa" (AgarionAgarion virgo) is by no means uncommon in England, and may be found along the upper course of the Thames, the Avon, and other rivers. It is easily recognised by its frail slender body, shining with metallic blue reflexes. There are numerous varieties, distinguished by their varieties of "light and shade." The spotless green-winged species is the "Ulrica" of Geoffroy. The two sexes are not alike. In the centre of their delicately-reticulated wings the males have a large bluish-brown spot, which is wanting in the females.

The genera Libellula, Aeshna, and Agarion compose the small family of the Libellulites—a family plainly and conspicuously characterised by the size of their head, and by the two pairs of diaphanous wings of almost equal dimensions (the posterior pair is a little shorter than the anterior), which, while the animal is at rest, are kept horizontally extended.

These are the characteristics which present themselves to the observant eye at the first glance. Do you doubt me? Betake yourself, when

betake yourself, I say, to any sunny pool, half-fringed with trees, or pleasant river-margin; go, armed with net and microscope, and, having secured a specimen of these terrors of the insect world, where

devote yourself to its patient examination.

It is only such an examination that can reveal to us some equally important, but less obvious features of the insect. But to catch a Dragon-fly is not always an easy task; the Libellulæ are very timorous, or else they are suspicious of the prowling naturalist who seeks what he can entrap. Their flight is livelier and swifter than even that of the butterfly: when disturbed in their repose, they fly away abruptly, their wings rustling or crackling like a sheet of parchment; if obstinately pursued, they grow irritated, and in their quick jerking movements exhibit all the rage of the Carnivora.

But can our Libellulæ be carnivorous? Most undoubtedly. To convince yourself of it, you have but to glance at their mouth, which is wholly unlike a butterfly's. What an arsenal of weapons adapted for seizing and crushing a victim! How strong are those saw-toothed scaly mandibles! How strong their auxiliaries, the jaws, which terminate in that dentated spring projection, furnished internally with ciliæ! Surely, such instruments testify to their ferocious instincts, and should induce our French neighbours to deny them the graceful name of "les demoiselles." What a libel on tender woman,—on man's "ministering angel!" Do but observe them. They do not rest upon the blossom to extract its nectared sweets; in truth, they could not do so, for they are not furnished with a proboscis. Warlike as the Amazons,—the only portion of the female sex they can justly be said to resemble,—they hover in the air to pounce, like vultures, upon whatever insects may come within their reach; they quickly transfix, and as quickly devour them. If they love to fly about the pools, the marshes, and the streams, it is because they are sure of prey in these localities. And, in fact, they there encounter and devour an innumerable quantity of flies, moths, gnats, and the like.

But there is another reason why the Dragon-flies, obeying the secret impulse of nature, resort to the haunts we have been describing. They were their cradles or nurseries, and they become, in due time, the scenes of their espousals.

Before speaking of the singular metamorphosis they afterwards accomplish, we must touch lightly on the subject of the mode of reproduction of our brilliant demoiselles.

It is laid down as a law that, in the insect world, the males are invariably smaller or weaker than the females. Yet this law does not hold good with reference to the Libellulæ, whose males are, on the contrary, larger and stronger than their females. Man may lay down laws, and extort obedience to them, within his own domain; but nature laughs at human rules, and gives up her secrets only to the free thought, unshackled by the fetters of authority.

But why is the male Dragon-fly stronger than the female? Because the former must make the first advances, and carry off his aërial companion to celebrate their bridal. For this purpose, he holds her tightly by the neck, and continues to fly in this way for some few minutes. At length, he perches himself on the branch of a willow, or the leaf of an aquatic plant, along with his companion.

The eggs laid by the female are oblong in shape, and sometimes united together in clusters: the female deposits them in the water, or on some water-floated leaf plant, shortly after their fecundation.

Metamorphosis of the Dragon-fly.

If in your walks abroad you should meet with a pool of turbid water, do not object to linger for a moment on its brink. Nature, as a reward for your trouble, will gratify you with an extraordinary surprise. Stir up the mud with a stick, or, better still, with your hand; and then, as the children say, you shall see—what you shall see!

Oh, what an ugly creature is struggling in this handful of slime! It looks like a large spider!

Nay, examine it more attentively; it can do you no harm.

Well, it is a singular animal; greenish in colour, bristling with hairs, and covered with mud. It is not a spider, for it has only six legs and these are exactly like the legs of insects. I can distinctly make out three joints (articulations) to each tarsus, which terminates in a simple hook. The belly is formed of regular segments; is rounded above, and flat underneath. What do I see? On the top, and nearly in the middle, each ring is armed with a spine, so that the row of spiny projections remind one of the back of a crocodile. Pray tell me, what is this curious creature?

Before I tell you, I would have you more thoroughly acquainted with it. Continue your examination.

The face has a strange expression; it looks—the insect, I mean—like a masked knight. Its mask, formed of a scaly substance and thoroughly compact, is composed of two pieces, which are separated from one another by a transversal suture: the upper, which is broader than it is high, we may call the vizor; it consists of two lobes, soldered together longitudinally; the lower, which is higher than it is broad, will be the chin-piece; it is triangular in shape, and its base rests against the vizor, while the summit is jointed or articulated with a support, which acts as a hinge when the mask is raised or lowered.

These movements, let me tell you, are voluntary; the animal raises its mask to arrest on their way the Infusoria and other animalcules on which it feeds. For crushing them, it is provided with strong mandibles, which you can detect by lowering the mask with a pin, or the point of a penknife. The eyes, which resemble little mammillary protuberances, are situated above and outside of the vizor. On the animal's back, where the belly joins the thorax, do you observe those four little sheath-like or scabbard-like tongues? The extremity of the body, the tail, is marked by three conspicuous triangular points, lying close to the opening, through which the water enters and issues, as if it were alternately sucked in and poured out by a piston. This, you must understand, is the respiratory apparatus. (See Fig. 40.)

Well, then, what is the name of this most singular creature, this masked knight?

It is the larva of the Libellula depressa; the dismal envelope whence will issue the gaudy Dragon-fly. Listen to a graphic description of the mask you are looking at:—

"Conceive your under-lip to be horny instead of fleshy, and to be elongated perpendicularly downwards, so as to wrap over your chin and extend to its bottom; that this elongation is there expanded into a triangular convex plate attached to it by a joint, so as to bend upwards again and fold over the face as high as the nose, concealing not only the chin and the first-mentioned elongation, but the mouth and part of the cheeks; conceive, moreover, that to the end of this last-mentioned plate are fixed two other convex ones, so broad as to cover the nose and temples; that these can open at pleasure transversely, like a pair of jaws, so as to expose the nose and mouth, and that their inner edges, where they meet, are cut into numerous short teeth, or spines, or armed with one or more sharp claws;—you will then have as accurate an idea as any powers of description can give you of the strange conformation of the under-lip of the larva of those insects, which conceals the mouth and face precisely as I have supposed a similar construction of your lip would do yours. When at rest, this mask applies closely to and covers the face; when they would make use of it, they unfold it like an arm, catch the prey at which they aim by means of the mandibuliform plates, and then partly refold it, so as to hold the prey to the mouth in the most convenient position for the operation of the two pairs of jaws with which they are provided."

And so the creature I have been examining is only a larva! How strange to compare it,—thick, ugly, unwieldy,—with the insect that issues from it, so aërial, so graceful, so light, so beautiful! The more I think of the contrast, the more it interests me.

This larva, however, has all the characteristics of a perfect insect, and I will wager that more than one observer has described it as such, and classified it among aquatic insects. Yet it is but a larva! And each species has its own special larva (see Figs. 40 and 41).

This seems to me as difficult to believe as if you told me that John, James, Peter,—in fact, all men,—were only temporary bodies from which more perfect beings would one day emerge. For the medium in which these masked creatures live,—the troubled and muddy medium for which they are adapted by their organisation, and in which they seem destined to live out their life,—differs wholly and absolutely from the aërial medium wherein, you tell me, they will one day rejoice.

Do you see, on the brink of the pond, those dried-up bodies? They are those of your aquatic insects, the mortal remains of Libellulites; they have rent open their vest, as you would do a garment which had become too narrow; the rent is very conspicuous along the back; and through the fissure issue the winged and aërial resuscitated Dragon-flies, summoned to live in a world differing so widely from their former one! Under the tongues were folded up the wings, and the mask that puzzled you so greatly is split open on the level of the sutures, so as to represent a ⟙, wanting only a handle to reproduce one of those hieroglyphics (☥) which are found so frequently on the Egyptian monuments, and which, according to some Egyptologists, signify eternal life.

And here, my friend and companion, we may take leave of

CHAPTER I.
WHAT MAY BE SEEN IN THE HEAVENS.

T The unequal duration of Day and Night, the succession and regular return of the seasons, all the phenomena observable upon the earth, are but the effects of a cause which we must seek in the heavens. It is impossible to explain them unless we contemplate it on high, relegating our planet into the great chorus of the worlds, where it holds but a modest rank. Only, to perform this miracle, we must for a moment repress in ourselves the senses which deceive us by their exaggeration or appearances, and give free course to enlightened thought. It is by this means alone that we can succeed in fully demonstrating the close and perpetual relationship which exists between our planet and the other spheres composing what we call the Solar System.

Permit us here a parenthesis, or, shall we say, a digression?

The whole secret of science, the whole secret of human knowledge—in truth, the whole future of humanity—lies in these two words—Enlightened Thought.

And here, gentle reader, I solicit your assistance in endeavouring to elucidate a question which has a for a long time puzzled me. Why do we apply the word "light" to that which sets in motion the eye of the body, and to that which induces the operation of the eye of the mind,—to the singularly mysterious physical agent without whose intervention all the external world would be to man a dream or a void, as well as to the still more mysterious moral agent which illuminates the world within?

I anticipate your answer. "The light," you say, "which opens up to me the material world is a reality; the other is only an image."

And it is true that this is the solution which first presents itself to the mind. But the longer you reflect upon it, the more you will be inclined to agree with me that it is unsatisfactory. What, then, means this agreement among all peoples, past and present, to designate by the same words—as

—those actions, processes, or operations which take place or are produced in the outer world, physical or material, and in the inner world, intellectual or moral.

Were your pretended "image" the result of purely personal impressions,—all persons are not equally apt in apprehending those fictitious relations which are the food of poetry,—were it, in a word, no more than an individual conception, and, therefore, eminently variable, I should not hesitate to accept your opinion: we should simply be discussing what are called, in scholastic language, the individual sense (sens propre) and the imaginative sense (sens figuré). But there exists upon this point an unanimous and universal agreement: all languages, living or dead, attest it; it embraces the aggregate of the members of the human family.

This is the first point which induces me to doubt the legitimacy of the proposed explanation. But I am confirmed in my hesitation by numerous other facts.

Let us emerge from the domain of rhetoric to enter that of experimental physiology and psychology. Too powerful a light blinds the organs of seeing. No mortal eye, unless with the aid of artificial appliances, can gaze upon the sun, the great source of light. If the vision is to act clearly, we must proceed step by step. If from a very light apartment we pass rapidly, and without transition, into a darkened room, or vice versâ, we feel temporarily blinded: the eye demands a short time to recover, as it were, from its astonishment. If we fix our gaze for one or two minutes on a star of the first magnitude, as, for example, Sirius, and afterwards turn away abruptly, the eye will for a while remain insensible to stars of a less intense splendour.

These are incontestable physiological facts, which anybody may easily verify for himself.

Well, in the psychological order facts exist which are in all respects analogous. Take one of those truths which the human thought, labouring generation after generation, has taken centuries to discover or elucidate; place it suddenly before an unprepared mind; however luminous may be this truth, it will be simply shadow and darkness to the mind we speak of; it will not comprehend an iota of it. Why? For the very obvious reason that it lies outside its sphere of ideas, in every respect comparable to the sphere of vision, beyond which and within which there is no more room for the sensorial impressions. There, as here, we must proceed gradually, and arrange the transitions so as to produce the desired results.

It would be easy for me to develop this parallel by other and still more remarkable facts; but what I have just said will suffice to show that the line of demarcation which philosophers have, upon principle, desired to trace between the physical and moral order, has turned the mind aside from many fertile fields of research and speculation. Let us cite an example. The sun is the visible centre of light, heat, organic life; in fine, of all the movements of our material world. Yet it is but a relative focus, since the sun, with its planetary train, revolves, probably, in company with other suns or systems, around a centre as yet unknown; and as there is no reason why we should pause in this cycloidal progression, this second centre or focus of systems may revolve around a third, the third around a fourth, and so on. Thus we shall have an indeterminate series of relative centres; for the term does not exist of which we can say,—there is the beginning, or here is the end of the series.

We do not meet with the absolute in the material, any more than in the intellectual world. Truth, by its power of attraction, sets in motion all the wheels of our understanding; we seek after it eagerly, in the doubt which torments us, in the obscurity which surrounds us; we all feel the need of being enlightened by it, and warmed, and revivified; we all are in need of belief, and, at the same time, of possessing—let there be no illusion in this respect—a certainty or demonstration of what we believe.

But the truth which we think our own does not leave the mind at rest; a slight effort suffices, in fact, to teach us that the truth we accept depends upon another and remoter truth, and that the world of thought is thus carried onward in an interminable series of relative truths; unless we find it more convenient to pause here at a primary cause, as elsewhere at a primordial centre, which we may ever identify with the primary cause. But is this truth, supposed to be final, capable of satisfying equally every mind? With this question, dear reader, I close my parenthesis.

To understand clearly the variable duration of the day at all points of the terrestrial surface, let us so place ourselves as to see our planet distinctly in front. I need hardly say that this invitation is not addressed to the visual organs of the body, but to the "mind's eye, Horatio." With the former, fixed as we are on the surface of the globe, we can only see the shadows of objects standing out in relief upon it, and still, with the sun at our back, can perceive but a comparatively insignificant portion. With the second, on the contrary, we may detach ourselves from the earth, may mount afar into space, may plant ourselves on some aërial height, whence we may embrace, at a single glance, all the illuminated hemisphere of our globe: we shall have but a single shadow on the prospect,—that which our brightened planet casts behind it, and which, in four-and-twenty hours, traverses the entire surface of the globe, producing light wherever it passes.

Having completed our preparations, let us note what we perceive with our eye thus disjoined from our body,—an eye equally well adapted to fix every moving object, and to see at any distance bodies extremely small, or bodies extremely large. What a marvellous eye! And it is for us to develop its power, and freely to increase its range. What an arduous task!

The luminous envelope, the photosphere of the sun, simultaneously darts its rays in all directions, with an intensity which diminishes with the square of the distance.

Let us follow those which direct their course towards our planet.

So long as the rays do not come into collision, or strike against resistant matter, they neither warm nor enlighten. Confirming the Newtonian hypothesis of emission, they travel straight as arrows through the icy space, the shadowy ocean in which the waves of the terrestrial atmosphere are lost. But the moment they encounter a material obstacle, the rays partly penetrate it, and are partly thrown back, in such a manner as to form a series of undulations like those which the falling of a stone into a pond produces. As they recede from these repeated shocks, the rays come under that theory of undulation which Huygens promoted.[57]

Let us trace the rays of light back to their origin. These encounter first the globes nearest to the sun: the almost imperceptible asteroids, which, on account of the solar splendour, can scarcely be detected, and which, as yet, have received no baptismal names. Yonder beams illuminate other revolving globes,—Mercury, Venus, the Earth. If we here arrest our gaze, we are influenced by a vague and mournful recollection; the Earth was our place of sojourn at an epoch when thought, formerly shackled, had become free. It is thus that a seed, removed from the stem which nourished it, wanders afar to diffuse and perfect its species. Our planet, of an ochreous yellow, relieved with green and white, possesses no special privileges; it shares in the ponderated movements of the spheres; it is neither the largest, nor the smallest, nor the heaviest, nor the lightest, nor the nearest to, nor the farthest from, the sun. And shall Earth alone, of all the planets, nourish that kind of "thinking seed" which we name the human race? It seems improbable.

But let us return to the great orb of light. It illuminates exactly one half of our earth; the other half lies in shade. And as the earth rotates upon its own axis, every point of its surface is necessarily exposed to the action of the solar rays. This action varies in duration and intensity.

All this I was taught, when I was still at school on our revolving planet. I remember, too, as a lesson learned by heart, that, under the Equator, or, more exactly, in 0° 0´ latitude, as well as at the Poles, or under 90° latitude; the duration of the day is equal to that of the night, with this difference, that, while under the Equator, a day of twelve hours alternates invariably with a night of the same duration, at the Poles a day of six months succeeds continually to a night of six months. I also recollect that, at a given moment, namely, at the spring and autumn equinoxes, the duration of the day, over all the terrestrial surface, is equal to that of the night, just as under the Equator; and that, after these two epochs, under the intermediary latitudes, between the Equator and the Poles, the length of the days and their corresponding nights varies according to the seasons; that, in our northern hemisphere, after the spring equinox, the days increase while the nights diminish, to such an extent that, at the summer solstice,—21-22 June,—they attain their maximum of length (and the nights their minimum), the reverse taking place during the period that elapses between the autumn equinox and the winter solstice.

I remember that I learned these data when I was living rooted to or anchored upon the earth; but the explanation which my masters gave me was not so clear as I could have wished. Their considerations on the declensions of the sun, on the obliquity of the ecliptic, on the necessity of exactly reducing the earth to a simple point in relation to the distance of the stars, to the end that the phenomena occurring (rapportés) on parallel planes would be nearly identical with those observed from the centre of the globe, or from a point situated on its surface; all these fine things, which demanded a certain faculty of geometrical intuition, left a curious vagueness of idea upon my mind. I accepted them, under the influence of authority, as beyond discussion, but I was by no means satisfied whenever I wished to ascertain their foundation.

But now—in the regions of space—everything grows simple before the mind, in which, apparently, all my power of thought is concentrated.

Behold the illuminated atmosphere; it reminds me vividly of the disc of the full moon. Ah, what exquisite iris colours! They mark the meeting-points of the bright with the obscured hemisphere: a circular line carried through all these points would exactly separate day and night. On the one side, motion and life and glow; on the other, silence and shade and calm. This line moves, carrying with it in its movement day and night, the illuminated and the darkened hemisphere; it moves from east to west, so that the bright hemisphere and the shadowy one, whose union forms what may be called the photo-adumbrated sphere, revolves, in four-and-twenty hours, round an axis which coincides at this moment—the 21st day of March, according to the "terrestrial worms,"—with the axis of the earth rotating on itself in an inverse direction, that is to say, from west to east. Let us note this coincidence: it is remarkable; inasmuch as, at the equinoxes, the terrestrial equator divides exactly the illuminated and the obscured hemisphere into two equal parts, one of which is situated to the north, the other to the south, and their line of separation coincides with a meridian circle.

But I see another, and much slower movement, very clearly defined. The axis of rotation of the photo-adumbrated sphere does not remain parallel with the terrestrial axis of rotation; it retires from it little by little, so as to form with it an angle which attains its maximum at the summer solstice (21-22 June); afterwards, returning upon itself, it coincides anew with the terrestrial axis of rotation (the autumn equinox), to make an angle in the contrary direction, whose maximum, of the same value as the former, corresponds to the winter solstice (21-22 December). This movement, which is annual, complicates itself with the diurnal. It is rendered visible by the displacement of the polar shroud of snow: the portion which, at the moment of the spring equinox, belonged, in the northern hemisphere, to the obscured hemisphere, moves onward, as a result of the inclination of its axis, to become an integral part of the illuminated hemisphere; while, at the same time, the portion which, in the southern hemisphere, belonged to the illuminated hemisphere, moves onward to become an integral part of the darkened hemisphere. Owing to this displacement, the sun shows itself for six consecutive months above the horizon, for the North Pole; at the spring equinox it begins to rise, at the summer solstice it attains its maximum elevation, and from the summer solstice it begins to decline. Thus, then, we have in reality a day six months long, of which the morning and the evening are the two equinoxes, its noon the summer solstice.

In the same periods an exactly opposite order of things prevails in the southern hemisphere.

Everything, even to the minutest detail, is in this way very clearly explained. Two facts—like the touches of a painter's brush—suffice to impress the whole upon the mind, namely:—

That, first, one half of the terrestrial surface is constantly illuminated by the sun, while the other half remains in darkness;

That, secondly, the rotation of the earth upon its axis produces day and night, by carrying from east to west the illuminated hemisphere, always diametrically opposite to the darkened hemisphere.

This being thoroughly understood, let us place ourselves in the equatorial plane, so as to embrace at a single glance a quarter of the illuminated and a quarter of the darkened hemisphere. If the equator of the photo-adumbrated sphere perpetually coincided with the terrestrial equator,—if, in other words, the earth, in revolving round the sun, invariably occupied the plane of the Equator, which, when prolonged, would pass through the centre of the illuminating orb,—the diurnal rotation would not cease to divide equally the light and the darkness over the earth's surface, as is shown in Fig. 42 a, where S indicates the sun, EE the terrestrial equator, and N S the extremities (or North and South Poles) of the axis which divides the globe into an illuminated and a darkened half. This phenomenon of coincidence exists; but only for a very brief period, and is only repeated twice a year,—that is, at the equinoxes. At all other times, the equator of the photo-adumbrated sphere, in whose plane the orb of light is situated, passes sometimes above and sometimes below the terrestrial equator.