ing to this, Theoretical Optics is now in the deductive period of its history. 4. Remarkable relations between light, heat, and electricity have been discovered, showing that they are everywhere inseparably connected; and that, if three distinct principles, they form a wondrous trinity in unity, sustaining whatever is permanent, and changing whatever changes throughout the whole of inanimate nature. These relations, however, point to the conclusion, hereafter to be realized, that all the phenomena of radiant heat, light, and electricity are but different effects of vibratory movements of the ether which pervades all space and fills the interstices between the particles of all ponderable bodies communicated and attended by corresponding movements of the particles of bodies. The correspondences detected between certain phenomena of light and radiant heat, especially, seem to leave little room to doubt the existence of a close correspondence in the essential nature of these two principles. Electricity and heat also, under certain circunstances, comport themselves very much alike: A striking evidence of a similarity in the nature of these two subtil agents, or, at least, of the existence of a subtil link of connection between them, has recently been furnished by a remarkable experiment made by the great English physicist, Michael Faraday. On the 2d of November, 1815, he announced at a meeting of the Council of the Royal Institution, the remarkable discovery, that "a beam of polarized light is deflected by the electric current, so that it may be made to rotate between the poles of a magnet." This effect was produced upon a beam of polarized light transmitted through a piece of flint glass, or of borate of lead glass, placed between the poles of an electro-magnet. "Already had Faraday proved the identity of machine, chemical, magnetic, and animal electricity; and now advancing a step higher in the inquiry, he finds the most ethereal principle with which we are acquainted capable of producing phenomena which have hitherto been regarded as the exclusive property of ponderable bodies only. Light, the subtile agent of vision, the source of all the beauty of color, is now shown to have some close relation with electricity, to which has long been referred many of the vital functions. As life and organization exist only where there is light, this discovery of Mr. Faraday's would appear to advance us towards some knowledge of those physiological phenomena which are the most recondite subjects of science." Such is the announcement of this wonderful discovery;-certainly one of the most remarkable discoveries of the age. What more surprising than

that a beam of light should be put in motion,-should be set in rotation by a magnet! It almost seemed to be the spiritual controlled by the material. Our joyous surprise at the sudden appearance of this wondrous truth was greatly heightened by the persuasion that it was but the beautiful herald of a glorious host, destined, ere long, to emerge from the depths of the dark unknown. We now know, indeed, that the action of the magnet upon the beam was not direct, but through the intervention of the piece of glass which the beam traversed; still the discovery opens up a new grand vista into that mysterious region which seems to separate the material from the spiritual world.

The physical theories of heat and electricity, upon the undulatory hypothesis, have not as yet been established and elaborated, like the physical theory of light, and the time may yet be far distant when they will be: but there is good reason to believe that there is a time, sealed up in the womb of futurity, when another Newton is destined to arise, and astonish the world by the discovery of a still vaster unity than that of universal gravitation:-a unity which will not only link the earth to the heavens, but embrace within its scope all the multiplicity of Inanimate Nature.

The optical discoveries of the present century are, for the most part, of a character too recondite to be intelligibly explained in a popular article. They are, besides, so numerous as to form a considerable portion of modern treatises on optics; and to attempt even to classify them would be no inconsiderable undertaking. We will confine ourselves to the mention of two or three of the most important. The first that we will specify, is the great truth called the doctrine of the Interference of Light. It is, that two rays of light, from the same source, arriving at the same point by different routes, will strengthen each other, or neutralize each other and produce absolute darkness, according to the difference in the lengths of their paths. This doctrine was propounded by Dr. Young, in the year 1800, and comprehends a multitude of curious phenomena. According to the undulatory theory of light, it is but the counterpart to the well-known fact, that waves of water, on meeting, increase or neutralize each other and produce smooth water, according as they meet in the same or in opposite states; as well as to the similar fact that two sonorous waves of air, when proceeding from two organ pipes pitched nearly in unison, will strengthen each other, or destroy each other and produce silence, according as they meet

in similar or opposing states. The second great discovery that we would signalize, is that of the polarization of light. It is, in a few words, that a ray of light is capable of being so modified as to exhibit different properties on different sides;as, for example, to be strongly reflected when a glass-plate is placed against one of its sides, and feebly reflected when the plate is presented under the same angle to another side. This modification may be communicated to light, or in other words, light may be polarized in various ways: viz., by passing it through a doubly-refracting crystal, in which case both of the emergent beams are polarized, or by reflection from glass or some other substance at a certain angle, or by transmitting the ray through a bundle of glass-plates, or through a piece of agate or tourmaline. The discovery of one form of polarized light, that by double refraction, was made by the celebrated Dutch philosopher, Huyghens, about the year 1680; but little notice was taken of the phenomenon until after the year 1808, when a French philosopher, by the name of Malus, accidentally discovered the polarization of light by reflection. He happened to be observing the light of the setting sun, reflected from the windows of the Luxembourg palace, through a doublyrefracting crystal of Iceland spar; and he observed that in turning round the crystal, the two images varied in their intensity. Polarized light often comports itself differently from common light under the same circumstances; and is the fertile source of a large class of curious and brilliant optical phenomena, discovered during the last forty years by Brewster, Biot, and other natural philosophers. It has also lit up the dark labyrinth of the interior of ponderable bodies, and in part disclosed to view its plan. What is yet more remarkable, like a divine spirit, it will dart from point to point, in this labyrinth, weigh each hidden particle, as it sparkles in the light of its presence, count their whole number, and come forth bearing an open scroll upon which is inscribed the weight of the whole in letters of light.

We have already remarked that it is the prevailing opinion among men of science, that the great period of induction in Optics is past. In the history of the close of this period, the two philosophers, Young and Fresnel, hold by far the most prominent place. They have distinguished themselves as the great champions of the undulatory theory of light. Professor Whewell, in tracing the analogies between the progress of astronomical and of optical science, remarks, that Young and Fresnel combined make up the Newton of optical science.

They "both possessed, in an eminent degree, the leading characteristics of the discoverer's mind; perfect clearness of view, rich fertility of invention, and intense love of knowledge." They were both alike skilful experimentalists and profound mathematicians. But Fresnel, who had received his mathematical education at the Polytechnic School in Paris, was more dexterous in the use of the modern analysis, and more remarkable for "the inventiveness and sagacity with which he devised experiments and applied mathematical reasoning to them." Dr. Young appeared first on the field, and has the honor of having been the first to revive the undulatory theory of light, which, since the time of Huyghens, had fallen into comparative obscurity, and to develop the principal evidences of its truth. Fresnel passed over the same ground after him, and almost independently of him; and labored with the most distinguished success in the later work of confirming his own and Dr. Young's inductions. According to Whewell," Dr. Young died in 1820, when he had scarcely completed his fiftysixth year. Fresnel was snatched from science still more prematurely, dying in 1827, at the early age of thirty-nine."

As to the discoveries in the science of Heat, or in Thermotics, we will only remark that very many of the newly-discovered phenomena of light are found to have their counterparts in certain phenomena of heat; as reflection, refraction, polarization, &c. Every one knows that heat is reflected, like light, when it falls upon the surface of polished tin; and that the heat from the sun may be concentrated, along with the light, into a focus by a burning-glass. But not only may heat be reflected and refracted, as these facts show, like light, but it can also be polarized, and made to exhibit a variety of phenomena analogous to those exhibited by polarized light. It has also been ascertained that heat has the property of exciting an electrical current; in fact, that all changes of temperature are attended with disturbances of the electric equilibrium. If a piece of bismuth be soldered to a piece of antimony, and the free ends be connected by a copper wire, passing round from one to the other, heat applied to the point of junction will immediately set an electric current in circulation along the wire and through the metals from end to end. If a compass-needle be placed over or under the current circulating along the wire, it will at once place itself crosswise to the current. A bundle of such soldered bars of bismuth and antimony, with the two ends connected by a copper wire, which is coiled repeatedly around a rectangular frame of wood, in the centre of which a

magnetic needle is poised, forms what is called Melloni's Thermomultiplier. To this instrument are we indebted for most of the modern discoveries in relation to heat. It is a new thermometer of wonderful sensibility. So sensitive is it, that the heat of the body, as one approaches it, sets the needle in motion at the distance of thirty feet; and Professor Henry, Secretary of the Smithsonian Institution, has found that, placed in the focus of a telescope, "the heat of the smallest cloud on the verge of the horizon was instantaneously perceptible, and that that of a breeze four or five miles off could be readily perceived." It is an interesting fact, quite recently ascertained, that the principle of interference obtains in the case of the meeting of two rays of heat, no less than in the meeting of two rays of light, and of two waves of sound. Professor Henry has succeeded in producing cold by the interference of two rays of heat;-a phenomenon which is the counterpart to the production of darkness by the interference of two rays of light, and of silence by the interference of two sonorous waves; and which lends a powerful support to the undulatory theory of radiant heat.

(To be continued.)

THE LATE REV. SYLVESTER JUDD.

1. Margaret; a Tale of the Real and Ideal, Blight and Bloom: including Sketches of a Place not before described, called Mons Christi. Boston: Jordan and Wiley. 1845. 2. Philo; an Evangeliad. By the Author of "Margaret; a Tale of the Real and Ideal." Boston: Phillips, Samson and Company. 1850.

3. Richard Edney and the Governor's Family. A Rus-urban Tale, Simple and Popular, yet Cultured and Noble, of Morals, Sentiment and Life, Practically Treated and Pleasantly Illustrated: Containing, also, Hints on Being Good and Doing Good. By the Author of " Margaret" and "Philo," "Margaret, a Tale of the Real and the Ideal," and "Philo, an Evangeliad." Boston: Phillips, Samson and Company. 1850.

Ir is pleasant to have found at last in America a man who