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nomenon it names. An object seen through a doubly refracting body in proper directions appears double. The ray from it is split into two, one of which takes nearly the ordinary course, and is hence called the ordinary ray, whilst the extraordinary ray diverges considerably. If, for instance, through a rhombohedral crystal of Iceland spar, we look at any illuminated point, the image will be duplicated, in all positions of the crystal save one,-namely, when the ray is transmitted along the optic axis of the crystal. This axis is called, strangely enough but by anthority of usage, the axis of double refraction. It is really the axis of no double refraction.
If, now, the two images be viewed through a tourmaline plate an extraordinary result is displayed. The two will appear with different amounts of distinctness, and, as the plate of tourmaline is turned round, one will gradually fade whilst the other grows stronger. The revolution of the plate being continued, presently the fading image wholly disappears, and the other attains a maximum of illumination. Revolve the plate still further, and the order of these changes is reversed; the lost image reappears and grows more and more conspicuous, as the other diminishes in brightness until it, in turn, goes out. It is plain, therefore, that each ray traversing the spar has become endowed with certain properties, which we express by the term polarized. All doubly refracting bodies polarize light; and polarization may also be effected by reflection at certain angles. Theoretically, it is presumed that the molecular arrangement of the spar is such as to separate the series of undulations constituting the ray of common light, into two, in one of which all the transverse vibrations have ceased except those in one plane,- or, we should prefer to say, all the vibrations parallel to some one plane have ceased, while in the other, all have ceased which were perpendicular to the same plane. The tourmaline has such a structure that the plane-polarized ray can penetrate as it were through slits in its substance parallel to its long axis; if the ordinary ray hold this position, the extraordinary vibrating at right angles to the former will be intercepted.
Tourmaline is itself a polarizer. If a second plate of the same substance (in this connection called the analysing plate) be
placed with its long axis perpendicular to that of the former (called the polarizing plate), it will completely intercept a ray passing through the first.
Polarization is also effected, as we have said, by reflection under certain angles. The polarizing angle was found by Sir David Brewster to be connected with the refractive index of the reflecting body through a simple and invariable law,— the latter being the tangent of the former, or, what is easily shown to be the same thing, the polarized ray is perpendicular to the refracted ray. The angle of polarization for water is 53° 11'; for glass, 56° 45'. But these should be called the angles of maximum polarization ; for light falling at other angles upon these media, is more or less partially polarized. There is a great deal of polarized light in the blue light of the sky, and in the glare from the surface of water. If in the latter case the polarized light be intercepted by a plate of tourmaline, the glare will be so diminished that the eye can detect the bottom of a clear stream, or rocks below the surface of the sea, otherwise wholly invisible from the point of observation.
A plate of any crystalline structure, except the tessular, introduced between the polarizing and analysing plates, produces colors, from the interference of the polarized waves; and there are many substances which produce chromatic illumination passing, on the rotation of the body, through all the colors of the spectrum in regular order. On the rotation of the substance under examination from right to left, the changes proceed from red to blue, which is called right-handed, or from blue to red, wbich is called left-handed, circular or elliptical polarization. There is thus afforded, through one of the most delicate and immaterial of natural agents, a means of prying into the obscure molecular arrangement of transparent bodies. Already. useful results have been obtained. Starch, dextrine, and grapesugar, all possess the property of circular polarization; but the two former polarize to the right (whence the name dextrine), the latter towards the left. Thus we can judge through a polarizing apparatus of the changes in an infusion of malt undergoing fermentation. Again, cane-sugar polarizes to the right, and it alone of the sugars has value as an article of commerce; yet it
degenerates rapidly, under certain circumstances, into grapesửga, which will not crystallize. A polarizing apparatus enables us to detect this change, of which neither taste, color, nor specific gravity, would give us warning.
The recondite relations between the mysterious agencies of *nature, were never so finely illustrated as by one of Faraday's surprising developments. He experimented with magnetism upon light. The plane-polarized ray from a lamp was shut off by the analysing plate. In this position he subjected it to the force developed by a current through the coils of an electromagnet, when instantly the ray was partially transmitted through the analyzer and the lamp-flame became visible. He had magnetised the light! "His magnet', says Prof. Tyndall, 'turned the plane of polarization through a certain angle, and thus enabled it to get through the analyser; so that “the magnetisation of light and the illumination of the magnetic lines of force” becomes, when expressed in the language of modern theory, the rotation of the plane of polarization.'
It is a favorite figure of speech with some writers and speakers, who have a weakness for bathos, to inform their readers or hearers what emotion is suitable to certain emergencies in their discourse. When we are told that the heart which has no tears to shed at the recital of this moving story, must be hard indeed,' we make it a point of honor not to weep. When we are assured that it is the best evidence of an indurated bosom and a seared conscience, not to be ready to go into flagrant indignation at the outrage,' we endeavor to be unusually calm; and when it is emphatically announced to us that 'nò man with the least sense of beauty or sentiment of character could fail to love her,' we confess to such a revolt at the superb impudence which dares to guage our emotions by its own, that we enter at once upon a hearty hatred of her. Notwithstanding our feeling upon this subject, we venture to affirm that every mind must approach the study of the stupendous forces of nature revealed to us by the phenomena of light, with something of awe. For we tread consciously near the outer boundaries of the material and close upon the invisible threshold of the spiritual.
Knowing the velocity of sound, and that of light, we can readily calculate the increased clastic force with which it would be necessary to endow the air, in order to make the velocity of the former equal to that of the latter. This enables us, in the next place, to deduce the bursting power of the ether when so much of it is enclosed in a cube of an inch in the side, as is equal in quantity of matter to that existing in a cubic inch of air. It will be found to be more than twelve trillion pounds on each face of the cube. In dealing with the phenomena of light, 'we cannot escape', says Sir John Herschel, 'from the conception of enormous force in perpetual exertion at every point through all the immensity of space. If we trace the vibrations of the ethereal molecules to their source, it is scarcely possible to suppose that the material particle, which gives rise by combustion or otherwise to these vibrations, does not itself undergo the same phases of undulation. Now, if the force necessary to drive this particle through its total excursion from its point of repose, within the brief period of (one-fourth of) an undulation, be calculated, — by assigning as the smallest length of such an excursion under which the retina may still be sensible to the vibration, only one quintillionth of an inch,- it is found to exceed the force of gravity more than thirty-five thousand millions to one! Thus, light, in the length of its waves and the rapidity of their transmission, in the excursions of the ethereal particles necessary to propagate it and in the force requisite to generate these excursions, in the minuteness of its penetration and the vastness of its dispersion,- stretches almost across the finite, and links the infinitely little with the infinitely great. Bacon complains that the manner in which Light and its causes are handled in Physics, is somewhat superstitious, as if it were a thing half-way between things divine and things natural;' and the manner probably remains to this day. But Bacon himself confesses that light'hath a relation and correspondence in nature and corporal things, to knowledge in spirits and incorporal things.'
ART. III.-1. History of the Life of Arthur, Duke of Wel
lington. From the French of M. Brialmont, Captain of the Staff of the Belgian Army; with Emendations and Additions by the Rev. G. R. Gleig, M. A., Captain-General to the Forces and Prebendary of St. Paul's. In Four Volumes. London: Longman, Green, Longman, and Brothers. 1860.
The work of General Brialmont has hardly received, from the press of this country, the degree of attention to which it is fairly entitled as the most authentic account of a man who occupied so large a space, for so long a time, in the eyes of Europe, as the Duke of Wellington. It was published at a time not at all favorable to a large American circulation. It was on the eve of the war, when the public mind was too deeply absorbed in contemplation of the approaching crisis, to be diverted into other channels of less immediate interest. A part, moreover, from the fact that the exploits of Wellington had been performed in support of a cause which is peculiarly distasteful to Americans of all classes, and every shade of political opinion, the most striking portion of them had already, many years since, been narrated by a military writer of surpassing ability, whose fascinating pages find no rival in the work of de Brialmont. We allude, of course, to Colonel Napier, and his great history of the Peninsular War. Though doubtless possessed of great talents, and many of those high qualities which are always found associated with brilliant achievement, there seems to have been nothing in the character or conduct of Wellington calculated to excite the admiration or enthusiasm of a people supposed to be peculiarly attached to a republican form of government. He was the most haughty noble of an age in which haughtiness and nobility were far more closely allied than they are at present, and he seems to have had quite as great a contempt for the vulgar herd as Coriolanus, or any other Roman of them all. The earlier years of his military service had been devoted to the overthrow of ancient thrones, and the extinction of ancient dynasties, in India, where he had already become a highly useful