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 At certain times and places it is almost as blue as the sky itself; but to see its colour, the attention must be withdrawn from the mountains and from the trees which cover them.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 and foliage. Trees and meadows thus regarded exhibit a (:) See FORTNIGHTLY REVIEW, February, 1869, p. 244.

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 here 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

without lateral scattering-the earth would lose the light of the sky. To form an idea of the magnitude of this loss we must have a clear idea of the quality of the light under consideration. It is now known to everybody that the vegetable world is nourished by the rays of the sun; and as animal life is sustained by vegetables, that life also is supported in the long run by the solar rays. Now, these rays are as composite as the coins of the realm. As regards their power to produce the chemical actions necessary to vegetable life, they differ from each other in value as widely as gold does from copper. It is the gold of the solar beams that is showered down upon us from the sky. In the article above referred to, the chemical potency of the shorter waves of light was dwelt upon; and Professor Roscoe has shown that the light of the sky, which is mainly produced by these shorter waves, has a chemical value at Kew Observatory greater than that of the unclouded sun at a height of 42° above the horizon. This would be the measure of the loss to the vegetable world at Kew if the sky were abolished. Roscoe's experiments were made with chemical substances sensitive to solar light, and they appear open to the objection that the rays effective in the plant-world may not be those which were effective upon his salts. But taking everything into account, and assuming the correctness of the observations, I think the probability great that the value of sky-light as a feeder of the vegetable world, and through it of the animal, cannot be much less than Roscoe makes it to be.

Our descent from the Aletschhorn was conducted with the same care and success that attended our ascent. I have already stated it to be a new thing for one man to lead a traveller up the mountain, and my guide in ascending had informed me that his wife was in a state of great anxiety about him. But until he had cleared all dangers he did not let me know the extent of her devotion, nor the means she had resorted to to insure his safety. When we were once more upon the lower glacier, having left all difficulties behind us, he remarked with a chuckle that she had been in a terrible state of fear, and had informed him of her intention to have a mass celebrated for his safety by the village priest. But if he profited by this mediation, I must have done so equally; for in all dangerous places we were tied together by a rope which was far too strong to break, had I slipped. My safety was, in fact, bound up in his, and I therefore thought it right to pay my share of the expense. "How much did the mass cost?" I asked. "Oh, not much, sir," he replied; "only ninety centimes." Not deeming it worth dividing, I let him pay for my fourpennyworth of celestial intervention.

JOHN TYNDALL.

(1) Proceedings of the Royal Institution, vol. iv. p. 657. The whole article here referred to is exceedingly interesting.

CONDORCET.

Or the illustrious thinkers and writers who for two generations had been actively scattering the seed of revolution in France, only Condorcet survived to behold the first bitter ingathering of the harvest. Those who had sown the wind were no more; he only was left to see the reaping of the whirlwind, and to be swiftly and cruelly swept away by it. Voltaire and Diderot, Rousseau and Helvétius, had vanished, but Condorcet both wrote in the Encyclopædia and sat in the Convention; the one eminent man of those who had tended the tree, who also came in due season to partake of its fruit; at once a precursor and a sharer in the fulfilment. In neither character has he attracted the good-will of any of those considerable sections and schools into which criticism of the Revolution has been mainly divided. As a thinker he is roughly classed as an Economist, and as a practical politician he figured first in the Legislative Assembly, and next in the Convention. Now, as a rule, the political parties that have most admired the Convention have had least sympathy with the Economists, and the historians who are most favourable to Turgot and his followers, usually are most hostile to the action and associations of the great revolutionary chamber alternately swayed by a Vergniaud, a Danton, a Robespierre. Between the two, Condorcet's name has been allowed to lie hidden for the most part in a certain obscurity, or else has been covered with those taunts and inuendoes which partisans are wont to lavish on men of whom they do not know exactly whether they are with or against them.

Generally, the men of the Revolution are criticised in blocks and sections, and Condorcet cannot be accurately placed under any of these received schools. He was an Economist, but he was something more; for the most characteristic article in his creed was a passionate belief in the infinite perfectibility of human nature. He was more of a Girondin than a Jacobin, yet he did not always act, any more than he always thought, with the Girondins, and he did not fall when they fell, but was proscribed by a decree specially levelled at himself. Isolation of this kind is assuredly no merit in political action, but it explains the coldness with which Condorcet's memory has been treated; and it flowed from some marked singularities both of character and opinion which are of the highest interest, if we consider the position of the man, and the lustre of that ever-memorable time. Condorcet, said D'Alembert, is a volcano covered with snow. Said another less picturesquely, He is a sheep in a passion. "You may say of the intelligence of Condorcet, in

relation to his person," wrote Madame Roland, "that it is a subtle essence soaked in cotton." The curious mixture disclosed by sayings like these, of warm impulse and fine purpose with immovable reserve, only shows that he of whom they were spoken belonged to the class of natures which may be called non-conducting. They are not effective, because without this effluence of power and feeling from within, the hearer or onlooker is stirred by no sympathetic thrill. They cannot be the happiest, because consciousness of the inequality between expression and meaning, between the influence intended and the impression conveyed, must be as tormenting as to one who dreams is the vain effort to strike a blow. If to be of this non-conducting temperament is impossible in the really greatest sorts of men-like St. Paul, St. Bernard, or Luther-at least it is no proper object of blame, for it is constantly the companion of lofty and generous aspiration. It was, perhaps, unfortunate that Condorcet should have permitted himself to be drawn into a position where his want of that magical quality by which even the loathed and loathsome Marat could gain the sympathies of men, should be so conspicuously made visible. Frankly, the character of Condorcet, unlike so many of his contemporaries, offers nothing to the theatrical instinct. None the less on this account should we weigh the contributions which he made to the stock of science and social speculation, and recognise the fine elevation of his sentiments, his noble solicitude for human well-being, his eager and resolute belief in its indefinite expansion, and the devotion which sealed his faith by a destiny that was as tragical as any in those bloody and most tragical days.

I.

Until the outbreak of the Revolution, the circumstances of Condorcet's life were as little externally disturbed or specially remarkable as those of any other geometer and thinker of the time. He was born at a small town in Picardy, in the year 1743. His father was a cavalry officer, but as he died when his son was only three years old, he could have exerted no influence upon the future philosopher, save such as comes of transmission through blood and tissue. Condillac was his uncle, but there is no record of any intercourse between them. His mother was a devout and trembling soul, who dedicated her child to the Holy Virgin, and for eight years or more made him wear the dress of a little girl, by way of sheltering him against the temptations and unbelief of a vile world. So long as women are held by opinion and usage in a state of educational and political subjection, which prevents the growth of a large intelligence, made healthy and energetic by knowledge and by activity, we may expect to read of pious extravagances of this kind. Condorcet was