the weather was very fine; I distinguished with great clearness the chain of the High Alps, but every thing was confused on the plain. The country, extending beyond the Lake of Brienz seemed covered with a veil; only the summits of Pilate, Forêt-Noire, and the Vosges, at a great distance, were clearly defined, whilst nothing was distinct in the plain between the Alps and the Jura. It was with much difficulty that I could distinguish, during serene weather, the town of Berne through a glass; and yet the Faulhorn is very visible from that town. This is easily explained: indeed, while mountains make a contrast by their opacity and their deep colour with the transparency of the sky, and are very distinctly defined, all the objects on the plain are clad in a sombre and uniform greenish tint: so that at a certain distance an isolated object is not relieved from among those surrounding it.

But it is not only rays coming from terrestrial objects that are partially absorbed by the atmosphere; it is the same with those coming from the sun. The curved surface that bounds the atmosphere being parallel to that of the earth, and its thickness being nothing when compared with the mass of the terrestrial spheroid, we may suppose, without sensible error, that the plain of the portion of the atmosphere, which the eye can embrace, is sensibly parallel to the horizon. If the sun were in the zenith, its rays would traverse the shortest road to reach us; the more the sun approaches the horizon, the greater is the thickness of atmosphere to be traversed by its rays, and, consequently, the more is the brilliancy of the rays enfeebled: : experience proves this every day. The light of the sun or of the moon in their passage to the meridian is dazzling, whilst we can gaze at these bodies when they are near the horizon; for the same reason, the regions situated near the horizon appear devoid of stars. The latter are actually invisible, because their rays cannot reach us through the thick stratum of atmosphere that they have to traverse; but they become perfectly visible as this part of the vault of heaven rises above the horizon. If it were possible to measure the intensity of solar light at different elevations, we might also indicate the quantity of this absorption; but the methods employed to measure this intensity, and the results obtained, are still subject to very grave difficulties." The actinometer, or the heliothermometer, described in p. 149, may be employed for this purpose. The absorption of solar rays is such, that in the plains of Germany, if the sun were in 21 Vide Note u, Appendix, No. II.

the zenith, and the sky perfectly clear, the earth would only receive two-thirds of the rays that arrive at the upper surface of the atmosphere; all the rest is partly absorbed, and partly reflected by the air and the particles of vapour : but the numerical value of these elements is still unknown.* BLUE COLOUR OF THE AIR. One part of the luminous rays is absorbed, the other reflected by the air; the latter, however, does not act equally on all the coloured rays, of which white light is composed; it acts like a milky glass, it rather allows the rays of the red extremity of the spectrum to pass, and, on the contrary, reflects the blue rays; but this difference is not sensible, until the light has traversed large masses of air. De Saussure has shewn that the blue colour of the sky is due to the reflection of light, and not to a peculiar colour belonging to the aërial particles. If the air were blue, he said, mountains that are very distant, and that are covered with snow, ought to appear blue, which is not the case. An experiment made by Hassenfratz also proves that the blue ray undergoes the greatest reflection. Indeed, the thicker the stratum of atmosphere in which the ray traverses, the more do the blue rays disappear, which make the red ray visible: now, when the sun is near the horizon, the ray traverses a greater thickness of the atmosphere; thus this body appears to us red, purple, or yellow. The predominance of red, and the absence of blue, when the sun is near the horizon, have been confirmed by an experiment of Hassenfratz: he passed the solar light through an opening, and received it on a prism; he then measured the width of the prism at a certain distance; the observation was repeated when the sun was at different heights above the horizon. In the long days of summer, at mid-day, the length of the prism was 185 parts; and in winter, during the shortest days, at sunset, only 70 parts. All the rays of the extreme violet were wanting; for the spectrum was only composed of red, orange, and green: an evident proof that all the blue rays had been absorbed. The blue rays also are often wanting in rainbows that appear a short time before sunset.

In order to measure the intensity of the blue, de Saussure invented the cyanometer. Imagine a band of paper divided into rectangles, of which the first is of the deepest

*It follows, from BOUGUER'S experiments, that while the barometer is at 760 millimetres, if we take as unity the intensity of a star, when it enters into the atmosphere, its intensity, when it reaches the observer, and when the star is at the zenith, is reduced to 0,8123. (LAPLACE, Ezposition du Système du Monde, t. i. p. 191.)

cobalt blue, whilst the last is almost white, the intermediate rectangles presenting all imaginable shades, between deep blue and white. If we find that the blue of one of these rectangles is identical with that of the sky, we then express this identity by a number corresponding to one of the rectangles, and every thing is obtained for drawing out the scale of the instrument. To accomplish this, de Saussure rested on this principle, that the difference between two very similar colours disappears the farther we recede, so that finally they are confounded. De Saussure, therefore, takes two shades of blue that are very similar, and lays beside each other two sheets of paper coloured with these shades; then he recedes until a black circle, four millimetres in diameter, painted on a white ground, and placed beside these sheets of paper, becomes invisible; if the differences of the shades disappear at a distance greater or less than that at which the circle disappears, one of these must be exchanged for another, until the required shade is obtained. In this manner, de Saussure obtained between white and black fifty-one shades, and consequently 53 degrees in all. The white was marked 0; and he satisfied himself, by other experiments, that these degrees corresponded to combinations of white and deep blue mixed in definite proportions.

Other apparatus have been devised, but all are intended for measuring the intensity of the blue. Now, as the atmosphere presents other colours, such as yellow, red, greyish blue, &c., instruments should be constructed for each of these colours. The following apparatus might serve to indicate the shade of colour; but I leave others the care of verifying by actual experiments the utility of this idea. The colour of objects is due to the want of certain of the colours of white light; thus, then, if we knew the principal elementary colours in white, and in the light coming from any body, we might know the colour of that body. In order to determine the number of elementary colours, we should select a perfect prism of flint-glass, and fix it at the extremity of a tube three or four decimetres long. The light of a body whose colour we desire to know is received through a narrow opening, and the prism decomposes it; but, in order to distinguish the colours well, they are received, as they pass out from the prism, on the achromatic object-glass of an astronomical telescope. By means of a micrometer-screw, the length of the spectrum and the width of each colour is measured: in this way, we may not only indicate, with great accuracy, the different shades of the sky, but, on repeating the experiment, when

the sun is at different heights above the horizon, we arrive at a positive knowledge of the number and the nature of the different elementary colours of solar light.

The mere contemplation of the sky at once proves to us that its colour is not the same at all points of the same vertical; it is generally deeper in the zenith; then it becomes brighter toward the horizon, when it is frequently completely white. This contrast becomes still more striking by the use of the cyanometer. Thus, de Saussure found one day that the colour, corresponding to No. 23 of his cyanometer, was in the neighbourhood of the zenith, and that corresponding to No. 4, near the horizon; M. de Humboldt arrived at analogous results. But the colour of the same part of the sky changes very regularly during the day; in that it becomes deeper from morning to mid-day, and becomes clearer from this time until evening.

When we ascend from the plain to mountains, the sky appears deeper and deeper; the chamois-hunters and shepherds have long known this. Deluc was the first to direct attention to this fact, which de Saussure verified in the Alps, and M. de Humboldt on the Cordilleras. In our climates, the sky has the deepest blue colour when, after several days' rain, the east wind drives away the clouds. According to M. de Humboldt the sky is bluer between the tropics than in the higher latitudes; but paler at sea than in the interior of countries.

The colour of the sky is modified by the combination of three tints: blue, which is reflected by the particles of air; the black of the vault of heaven, that forms the ground of the atmosphere; and finally, the white of the vesicles of fog and flakes of snow that swim in the atmosphere. Indeed, the tint of the blue rays is darkened by the black colour of space; and, on the other hand, it is made lighter by the white of the vesicles of fog; when we ascend in the atmosphere, we leave a great portion of the vesicles of vapour beneath us. So that, while rays reach the eye in less proportion, and, the sky being covered with a lesser number of particles reflecting its light, its colour becomes of a deeper blue. For the same reason, the blue in the neighbourhood of the horizon is less intense than at the zenith. If the sky is paler in the open sea, and in high latitudes, than in the interior of the continents, and in the neighbourhood of the equator, it must be attributed to the vesicles of fog.*

ON THE POLARISATION OF SERENE AIR.-A luminous ray is completely polarised, when, even under perpendicular incidence, it cannot pass through

TWILIGHT. During a serene day, as the sun approaches the horizon, the neighbouring portion of the sky is coloured yellow or red. The rays, which have been traversing a great thickness of the atmosphere, lose, on their way, a great portion of the blue rays, and we receive only the red rays.

a thin film of tourmaline cut so as to contain the axis of this crystal, and placed in a situation perpendicular to this radius. The plane passing through this radius and this axis is named the plane of polarisation. The polarised ray also possesses other characters, by which it is defined; but this is sufficient.

If the plate of tourmaline is turned 90°, its surface remaining perpendicular to the luminous rays, these rays pass through the plate; the plane of polarisation is then perpendicular to the axis of the crystal. So that the polarised ray can no longer be considered as symmetrical with reference to exterior space: hence its name.

Light may be partially polarised; it may then be regarded as constituted of natural light not polarised, and light completely polarised. All possible intermediate conditions may be observed between these two states of light.

The act of reflection polarises luminous rays; the plane of the polarisation of a reflected ray is that in which the reflection takes place; but, if the incidence is normal, the light is not polarised.

From these principles we may explain the polarisation of the light of a clear sky. If we imagine a plane passing through the sun and the observer, the light coming from the sun, which reaches the eye of the latter, along a certain line, situated in this plane, has been reflected by the aerial molecules situated in the course of this line. This light would, therefore, be polarised in a plane passing through the sun; now, this is actually what we observe. If M. ARAGO's chromatic polariscope, or SAVART'S fringe polariscope, (instruments which are described in most treatises on Natural Philosophy) is directed toward the sky, we recognise that the intensity of the polarisation is very great near the zenith, that it goes on increasing until about 90° degrees from the sun; after which, it progressively diminishes to a distance of 150° from this body, at least, if the place of this latter is a little above or a little below the horizon. At this place the polarisation is insensible. This point, which is situated in the sun's vertical, has been usually designated the neutral point. Beyond this, the polarisation begins to increase; but the plane of the polarisation, instead of coinciding with the vertical of the sun, has become perpendicular to it. MM. ARAGO, BIOT, and FORBES, attribute this latter phenomenon to the secondary reflections that occur between the aerial molecules taken two and two. It is evident that, with respect to one molecule of air, the rest of the atmosphere plays the part of an illuminating body in the form of a horizontal ring surrounding it on all sides. The portion of light that the molecule borrows from this source must, therefore, be polarised in a horizontal plane, or, at least, in a plane but little inclined to the horizon; and in all cases, on account of its symmetry, this plane must be perpendicular to the vertical of the sun, if the molecule is itself situated in this plane. In proportion as we examine the parts of the sky that are nearer to the point of the horizon opposite to the sun, polarisation in a horizontal direction must go on increasing, and end by predominating over vertical polarisation, which, on the contrary, continues diminishing, until the incidence of the solar rays approaches more and more toward being normal with the same molecules.

M. BABINET found a second neutral point, in the neighbourhood of the sun; its height is 17° 30′ above the centre of the sun. Mr. FORBES explains it by the preceding theory, namely, the predominance of reciprocal horizontal reflections.

The angles 159° and 17° 30′ are the mean result of numerous observations made by M. BRAVAIS, on the summit of the Faulhorn, in 1842; we may believe that, in the plain, the situation of these neutral points is somewhat different. The presence of clouds in the sky is even sufficient to displace them from their natural positions.-M.