Imágenes de páginas
PDF
EPUB

the distance of a quarter of a million of miles would tend to soften and equalize its colouring. The reason of this should be clearly understood. It would not be the direct consequence of distance alone. The transparency of the intervening space being supposed, distance has no effect upon either the brightness or the colour of objects. In comparing degrees of brightness, we must consider those equal in which the eye receives equal amounts of light from equal surfaces. But the amount of light received by the eye, and the apparent magnitude of the surface, both follow the same lawthat of "inverse squares;” that is, each of them diminishes as the square of the distance increases ; and consequently the result is an invariable one. At greater distances the light received by the eye is less, but the magnitude of the object being lessened in precisely the same degree, the brightness continues unchanged. This being equally true, whatever may be the colour of the light, it might at first be supposed that a distant prospect of our globe would exhibit a wonderfully chequered and mottled aspect as to local tints. So undoubtedly it would, if the effect, or rather want of effect, of distance upon colour were alone concerned. But another consideration comes in to interfere with the result. Though colours are not enfeebled, the spaces they occupy become less conspicuous with increasing distance: till beyond a certain limit, the eye ceases to take cognizance of them : the space which their image occupies upon the retina is no longer an extended object of vision, but is contracted into a point; and the colour of its light must of course merge into and become confused with the hue of its immediate neighbourhood, which it will modify in. proportion to its intensity. But white is the product of a combination of all the colours in the solar spectrum, the effect, therefore, upon the retina of the juxtaposition of a number of particoloured areas, too small to be separately distinguished, will be that of whiteness; and this will be more or less pure, in proportion as the several tints approach more or less to the purity and relative intensity of those in the solar spectrum.

This is, in fact, the celebrated experiment of Newton, in which, after haviny decomposed white light by the prism into what are called “the colours of the rainbow,” he sought to reproduce whiteness by an artificial mixture of hues corresponding as nearly as might be with the prismatic tints. For this purpose, having first fixed upon seven as the original colours (though in reality the gradations are literally innumerable), he mixed carefully certain coloured powders, of such tints and in such proportions as seemed

to him best suited to match the natural phenomenon. The result of course was an imperfect one. The colour of no powder can equal the purity of refracted light; and no skill of man could correctly apportion their quantity or intensity: yet the success was remarkable, and fully attested the sagacity of that marvellous man.

The powder so compounded was by no means absolutely white; but it was so fair an approximation that when it was laid in the sunshine by the side of a white paper in the shade, the eye could scarcely perceive any difference between them. Yet a powerful microscope, or a minute insect creeping amongst the powder, if endued with the sense of colour, would still have distinguished the separate hue of each single grain.

Now it is evident that the two causes conspiring in the result of this experiment, namely, that a due mixture of all colours produces white, and that the eye has no separate perception of coloured areas beneath a certain magnitude, would both come into play in a distant view of our globe. The larger surfaces of oceans and deserts would doubtless retain their tints, as presenting an extent fully appreciable by the eye; but the patchwork hues of our cultivated lands, the contrast of our rocks and woods, would disappear in the general intermixture; and though no such accurate balance of tints could be expected as to compose a pure white, the result no doubt would be some kind of neutral grey, warmer or cooler according to the predominant colour entering into its composition.

If we now apply to the Moon the reasoning thus drawn from the Earth, we shall readily see that the apparent whiteness of her light does not of necessity infer a colourless surface. It merely shows that the coloured areas, if such there are, are not sufficiently continuous to make a separate impression upon the eye; and that they are so far balanced in point of tint, that their combinations do not all verge towards either end of the spectrum—that is, there is no general tendency to a ruddy or bluish cast.

We must not, however, entertain an exaggerated impression as to the perfect whiteness of the Moon. The telescope, while reducing our apparent distance from it in proportion to the magnifying power employed, begins to bring out traces of those colours which would doubtless be much more decided could we actually place ourselves among them.

The grey in several parts is found to be stained with a touch of yellow or brown; and Beer and Mädler have pointed out that the Mare Serenitatis, a large and very regular specimen of those wide plains which characterize the lunar surface, exbibits about the time

of Full Moon a paie, clear, green tint in its interior, set within a more shadowy border of iron grey. They admit, however, that it is not readily seen, and might not be perceived by other observers, as in fact it has never been very distinctly made out by the writer, who, in the use of very superior optical means, has always thought it too yellow to correspond with their description. This is probably an instance of the difference between eyes or judgments which astronomers call “chromatic personal equation.” The contrast, however, between the centre and its fringe in point of hue is too obvious to be overlooked by any observer. In other places, slight vestiges of colouring have been pointed out; and especially by that accoinplished artist, Professor Piazzi Smyth, Astronomer Royal for Scotland, who found it impossible to represent, by any admixture of black and white alone, the portion of the Moon which he had undertaken to delineate at the request of the then-existing Moon Committee.

It must notwithstanding be admitted that no positive colour of any intensity has ever been discovered in the Moon. Nothing in fact is to be seen more marked than a warmer, or cooler, or greener tone of the universal greyish white. And this, as we now understand, may either result from the natural colour of the soil, or from a combination of a variety of tints, confined respectively to areas insufficient to produce a separate impression upon the eye.

The preceding remarks, however, will be found more expressly suitable to those great dusky levels which used to be called seas, and certainly have much the aspect of dried ocean-beds, but whose analogues at present seem to be the deserts or prairies of our globe. For in the more elevated and rugged parts of the lunar surface, the light is usually, though not without some exceptions, of a considerably whiter character; and some spots exhibit an almost dazzling amount of reflection, in which the eye detects no trace of colour. Here, it is obvious, the full extent of our explanation is less applicable, as we could not imagine the juxtaposition of minute patches of colour, sufficiently pure, and vivid, and accurate in their balance; and we must therefore have greater recourse to the idea of a real whiteness in the soil. In this there will be nothing to surprise us, as we are sufficiently familiar with it nearer home. If the analogy of our chalky cliffs should be considered inapplicable, inasmuch as the class of aqueous deposits to which they belong is considered to be unrepresented upon the Moon, we may still refer to the loftiest crests of Kinchinjunga, one of the highest of the Himalayas, where the granite rock, according to Hooker, is scarcely

to be distinguished from the everlasting snow. It is, nevertheless, evident that there is a far greater proportional amount of such white materials on the Moon than on the Earth, and that the products of eruptive action on the former body are far more generally uncoloured than on the latter, where basalts, lava, and scoriæ are frequently of a dark hue.

Thus far, no great difficulty has started up in the way of our inquiry. We can understand that there is no occasion for the supposition of ice or snow-a supposition quite unwarranted by observation-either actual whiteness of soil, or the juxtaposition of inappreciably small patches of colour, or both causes in combination, will sufficiently explain, in a general view, the character of the lunar light: nor need we suppose that it differs from that of the Earth (our seas, and snows, and clouds excluded) in kind, but only in degree. We need only imagine a greater extent of uncoloured material, or a greater subdivision and closer intermixture of patches of colour, to account for all that we see; and we can easily conceive that in this respect the Moon may not differ from the Earth more than one region of the Earth differs from another.

But we should be greatly mistaken if we were to think that we had thus mastered the whole subject. We may stop here, but if we do, we stop short of the most interesting portion; we may proceed, but it will be into a region of perplexity and difficulty. Terrestrial analogy will be of less avail, and we shall be abandoned to the guidance of conjecture. This will soon be apparent, even in replying to the natural inquiry, Is there anything on the Moon corresponding with the changes of colour which a distant view of the Earth may be supposed to give, during the progress of the seasons? Here we have first to observe that the axis of the Moon is so slightly inclined to its orbit that there is no change of seasons which could reasonably be expected to show itself by any indication of this kind. The difference between the winter and the summer of the Moon would be far less than often exists in Europe between two successive seasons of the same name. But on the other hand, the very slow rotation of the Moon on its axis makes its day, in a certain sense, the equivalent of a year; and we have every reason to suppose that if vegetable life is present there, it would—at least as to deciduous species-run its whole course in the space of a terrestrial month, or lunar day. Are there, then, changes of colour during this period such as would bear out an idea of this nature? The reply is dubious. Changes of colour, when the whole amount is so slight, we could not expect to see; but

changes in depth of tone, which may be regarded as equivalent, we do see in some places—in others they are wholly absent. The majority of the spots preserve the same reflective power, whatever may be the height of the Sun above their horizon: others of similar character-Cleomedes may be specified, and the Paludes Amarecome out in deeper shades with the advance of the lunar day, in a manner which is certainly suggestive of the development of a vegetable covering ; but the suggestion remains incapable of proof, and is less probable than it would have appeared, had those changeable localities been of more general distribution.

The idea of vegetation naturally introduces the subtle and difficult question of an atmosphere. It is so frequently taken for granted that no such gaseous envelope exists, that some of our readers may be surprised to find the question treated as still an open one: but we venture to think that the negative has been too confi. dently asserted. The disproof of refractive action in the occultations of stars, on which Bessel relied, has been recently set aside by the deeper researches of Airy. His most careful as well as impartial scrutiny of the Greenwich observations has shown that the very disparity of measures exists, the supposed absence of which was thought to disprove the idea of an atmosphere. De la Rue's remark, that it would be difficult to suppose the existence of chemical change, such as must have occurred on the Moon, without an atmosphere, is a very important one; and while oxygen enters so very largely into the composition of the Earth's crust, of which, according to Humboldt, it actually makes up half the weight, its non-existence on the Moon would infer a greater dissimilarity of constitution than we are warranted in supposing; and if present in a combined, it would hardly be wholly absent in a gaseous form. Of course any atmosphere in that situation, however constituted, would from the inferior attractive power of the lunar globe be of great tenuity; and this corresponds exactly with the want of more visible indications of its existence in other ways. Its denser portion may, agreeably to Schröter's suggestion, be confined to the low lying plains or valleys, and circular cavities; and it is chiefly, if not exclusively, in such localities that we find such periodical changes of tone as have been described. These, we must remember, would be absent, as far as vegetation is concerned, from the loftier moun. tains of the Earth, and would naturally be still less likely to be found on those of the Moon. And it may perhaps be worthy of consideration that there appears no antecedent necessity that the hue of vegetable matters should be green. That it has pleased the Almighty

[merged small][ocr errors]
« AnteriorContinuar »