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Two concentric shells of atmosphere, perfectly distinct in character, clasped the earth this morning. That which hugged the surface was of a deep neutral tint, too shallow to reach more than midway up the loftier mountains. Upon this, as upon an ocean, rested the luminous higher atmospheric layer, both being separated along the horizon by a perfectly definite line. This higher region was without a cloud; the arrowy streamer that had shot across the firmament during our ascent, first reduced to feathery streaks, had long since melted utterly away. Blue was supreme above; while all round the horizon the intrinsic brilliance of the upper air was enhanced by contrast with the dusky ground on which it rested. But this gloomier portion of the atmosphere was also transparent. It was not a cloud-stratum cutting off the view of things below it, but an attenuated mist, through which were seen as through a glass darkly the lower mountains, and out of which the higher peaks and ridges sprung into sudden glory.

But the pomp of peak and crag has already palled upon the public mind; why, then, dwell upon it? I do so because my own enjoyment of it was fresh, notwithstanding the number of times that I had seen it. We will now, however, quit this region of the sublime and beautiful. The emotions excited by natural grandeur are all very well in their way, but they are evanescent, and something is needed to fill the vacuity created by their departure. Here the action of the intellect comes to our aid, and fills the shores of life after the feelings have retreated.

The vision of an object always implies a differential action on the retina of the observer. The object is distinguished from surrounding space by its excess or defect of light in relation to that space. By altering the illumination, either of the object itself or of its environment, we alter the appearance of the object. Take the case of clouds floating in the atmosphere with patches of blue between them. Anything that changes the illumination of either alters the appearance of both, that appearance depending, as stated, upon differential action. Now the light of the sky, being polarised, may, as the reader of this Review already knows, be in great part quenched by a Nicol's prism, while the light of a cloud, being unpolarised, cannot be thus extinguished. Hence the possibility of very remarkable variations, not only in the aspect of the firmament, which is really changed, but also in the aspect of the clouds which have that firmament as a background. It is possible, for example, to choose clouds of such a depth of shade that when the Nicol quenches the light behind them, they shall vanish, being undistinguishable from the residual dull tint which outlives the extinction of the brilliance of the sky. A cloud less deeply shaded, but still deep enough, when viewed with the naked eye, to appear dark on a bright ground, is suddenly changed to a white cloud on a dark ground by the quenching of the sky behind it. This was the case to-day with the lower atmospheric stratum above referred to. When the light of the upper firmament was removed it no longer appeared dark, but whitish ; being changed into a milky haze by contrast with the superjacent darkness. When a reddish cloud at sunset chances to float in the region of maximum polarisation, the quenching of the sky behind it causes it to flash with a brighter crimson. Last Easter eve the Dartmoor sky, which had just been cleansed by a snow storm, wore a very wild appearance. Round the horizon it was of steely brilliancy, while reddish cumuli and cirri floated southwards. When the sky was quenched behind them these floating masses seemed like dull embers suddenly blown upon, brightening into fire. In the Alps we have the most magnificent examples of crimson clouds and snows, so that the effects just referred to may be here studied under the best possible conditions. On the 23rd of August the evening Alpen-glow was very fine, though it did not reach its maximum depth and splendour. Towards sunset I walked up the slopes to obtain a better view of the Weisshorn. The side of the peak seen from the Bel Alp, being turned from the sun, was tinted mauve ; but I wished to see one of the rose-coloured buttresses of the mountain. Such was visible from a point a few hundred feet above the hotel. The Matterhorn also, though for the most part in shade, had a crimson projection, while a deep ruddy red lingered along its western shoulder. Four distinct peaks and buttresses of the Dom, in addition to its dominant head—all covered with pure snow—were reddened by the light of sunset. The shoulder of the Alphubel was similarly coloured, while the great mass of the Fletschorn was all aglow, and so was the snowy spine of the Monte Leone.

Looking at the Weisshorn through the Nicol, the glow of its protuberance was strong or weak according to the position of the prism. The summit also underwent a change. In one position of the prism it exhibited a pale white against a dark background ; in the rectangular position, it was a dark mauve against a light background. The red of the Matterhorn changed in a similar manner; but the whole mountain also passed through striking changes of definition. The air at the time was highly opalescent-filled in fact with a silvery haze, in which the Matterhorn almost disappeared. This could be wholly quenched by the Nicol, and then the mountain sprang forth with astonishing solidity and detachment from the surrounding air. The changes of the Dom were still more wonderful. A vast amount of light could be removed from the sky behind it, for it occupied the position of maximum polarisation. By a little practice with the Nicol it was easy to render the extinction of the light or its restoration almost instantaneous. When the sky was quenched, the four minor peaks and buttresses and the summit of the Dom, together with the shoulder of the Alphubel, glowed as if set suddenly on fire. This was immediately dimmed by turning the Nicol through an angle of 90°. It was not the stoppage of the light of the sky alone which produced this startling effect; the air between the Bel Alp and the Dom was, as I have said, highly opalescent, and the quenching of this intermediate glare augmented remarkably the distinctness of the mountain.

On the morning of the 24th of August similar effects were finely shown. At 10 A.M. all three mountains, the Dom, the Matterhorn, and the Weisshorn, were powerfully affected by the Nicol. But in this instance also the line drawn to the Dom being accurately perpendicular to the direction of the solar shadows, and consequently very nearly perpendicular to the solar beams, the effects on this mountain were most striking. The grey summit of the Matterhorn at the same time could scarcely be distinguished from the opalescent haze around it; but when the Nicol quenched the haze, the summit became instantly isolated, and stood out in bold definition. It is to be remembered that in the production of these effects the only things changed are the sky behind and the luminous haze in front of the mountains; that these are changed because the light emitted from the sky and from the haze is plane polarised light, and that the light from the snows and from the mountains being sensibly unpolarised, is not directly affected by the Nicol. It will also be understood that it is not the interposition of the haze as an opaque body that renders the mountains indistinct, but that it is the light of the haze which dims and bewilders the eye, and thus weakens the definition of objects seen through it.

These results have a direct bearing upon what artists call “ aërial perspective.” As we look from the summit of the Aletschhorn, or from a lower elevation, at the serried crowd of peaks, especially if the mountains be darkly coloured—covered with pines, for example, every peak and ridge is separated from the mountains behind it by a thin blue haze which renders the relations of the mountains as to distance unmistakable. When this haze is regarded through the Nicol perpendicular to the sun's rays, it is in many cases wholly quenched, because the light which it emits in this direction is wholly polarised. When this happens, aërial perspective is abolished, and mountains very differently distant appear to rise in the same vertical plane. Close to the Bel Alp, for instance, is the gorge of the Massa, a river produced by the ablation of the Aletsch glacier, and beyond the gorge is a high ridge darkened by pines. This ridge may be projected upon the dark slopes at the opposite side of the Rhone valley, and between both we have the blue haze referred to, throwing

(1) See FortniGHTLY Review, February, 1869, p. 239.

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the distant mountains far away. But at certain hours of the day this haze may be quenched, and then the Massa ridge and the mountains beyond the Rhone seem almost equally distant from the eye. The one appears, as it were, a vertical continuation of the other. The haze varies with the temperature and humidity of the atmosphere. At certain times and places it is almost as blue as the sky itself; but to see its colour, the attention must be withdrawn from the mountains and from the trees which cover them. In point of fact, the haze is a piece of more or less perfect sky ; it is produced in the same manner, and is subject to the same laws, as the firmament itself. We live in the sky, not under it.

These points were further elucidated by the deportment of the selenite plate, with which the readers of this Review are already acquainted.' On some of the sunny days of August the haze in the valley of the Rhone, as looked at from the Bel Alp, was very remarkable. Towards evening the sky above the mountains opposite to my place of observation yielded a series of the most splendidlycoloured iris-rings; but on lowering the selenite until it had the darkness of the pines at the opposite side of the Rhone valley, instead of the darkness of space as a background, the colours were not much diminished in brilliancy. I should estimate the distance across the valley, as the crow flies, to the opposite mountains, at nine miles; so that a body of air nine miles thick can, under favourable circumstances, produce chromatic effects of polarisation almost as vivid as those produced by the sky itself.

Again : the light of a landscape, as of most other things, consists of two parts; the one part comes purely from superficial reflection, and this light is always of the same colour as that which falls upon the landscape; the other part comes to us from a certain depth within the objects which compose the landscape, and it is this portion of the total light which gives these objects their distinctive colours. The white light of the sun enters all substances to a certain depth, and is partially ejected by internal reflection ; each distinct substance absorbing and reflecting the light in accordance with the laws of its own molecular constitution. Thus the solar light is sifted by the landscape, which appears in such colours and variations of colours as, after the sifting process, reach the observer's eye. Thus the bright green of grass, or the darker colour proper to the pine, never comes to us alone, but is always mingled with an amount of really foreign light derived from superficial reflection. A certain hard brilliancy is conferred upon the woods and meadows by this superficially-reflected light. Under certain circumstances, it may be quenched by a Nicol’s prism, and we then obtain the true colour of the grass aud foliage. Trees and meadows thus regarded exhibit a

(:) See FORTNIGHTLY Review, February, 1869, p. 244.

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richness and softness of tint which they never show as long as the superficial light is permitted to mingle with the true interior emission. The needles of the pines show this effect very well, largeleaved trees still better ; while a glimmering field of maize exhibits the most extraordinary variations when looked at through the rotating Nicol.

Thoughts and questions like those herè referred to took me to the top of the Aletschhorn. The effects described in the foregoing paragraphs were for the most part reproduced in the summit of the mountain. I scanned the whole of the sky with my Nicol. Both alone and in conjunction with the selenite it pronounced the perpendicular to the solar beams to be the direction of maximum polarisation. But at no portion of the firmament was the polarisation complete. The artificial sky produced in the experiments already recorded in the FORTNIGHTLY REVIEW could, in this respect, be rendered more perfect than the natural one; while the gorgeous “ residual blue” which makes its appearance when the polarisation of the artificial sky ceases to be perfect, was strongly contrasted with the lack-lustre hue which, in the case of the firmament, outlived the extinction of the brilliance. With certain substances, however, artificially treated, this dull residue may also be obtained.

All along the arc from the Matterhorn to Mont Blanc the light of the sky immediately above the mountains was powerfully acted upon by the Nicol. In some cases the variations of intensity were astonishing. I have already said that a little practice enables the observer to shift the Nicol from one position to another so rapidly as to render the alternate extinction and restoration of the light immediate. When this was done along the arc to which I have referred, the alternations of light and darkness resembled the play of sheet lightning behind the mountains. My notes state that there was an element of awe connected with the suddenness with which the mighty masses, ranged along the line referred to, changed their aspect and definition under the operation of the prism.

In a former essay printed in this Review I endeavoured to show that the colour and polarisation of the sky could be reproduced artificially, and that the only condition necessary to their production was the smallness of the particles by which the light was scattered. The effects were proved to be totally independent of the optical character of the substances from which the particles were derived. The parallelism of the artificial and the natural phenomena is so perfect as to leave no doubt upon the mind that they are due to a common cause. And here a practical issue of immense import reveals itself. Supposing those particles which now throw down upon us the blue light of the firmament to be abolished, what would be the result ? The sun's rays would pass through the atmosphere

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