II. But the true life of a man of science or literature is not constituted by its outward incidents; it is made up of the deeds he has done; i.e., of the powers of thought he has manifested, and the results they have evolved. Sir David Brewster's acts are of two descriptions. To the first, which have offered in so many directions benefit to the arts, and which may be termed his "Inventions"—we have no space to allude except in the most cursory manner. Many now living must recollect the sensation originally produced by the ingenious and beautiful kaleidoscope; and many of these must have joined in the then universal regret, that through defects of our miserable and incongruous law of Patents, that benefit was withheld from the discoverer, which is due to every one whose genius augments through material forms the comforts or pleasures of society. Multitudes innumerable of these kaleidoscopes were made and run after in Great Britain and through Europe; but the ingenuity of Sir David Brewster received little or no pecuniary reward. Next in order we might refer to the lenticular stereoscope. The discovery of the principle of the stereoscope is due to Wheatstone; but to Sir David is unquestionably due the claim, that in his hands—chiefly through the skilful application of semi-lenses—it started into an applicable instrument. Higher than these in pure scientific merit, are his improvements of microscopes and telescopes; his initiation of the Bude light; and highest of all, that early proposal for the use of dioptric lenses, and of zones in lighthouses. Fresnel subsequently appropriated this discovery without knowing that he did our countryman a wrong; but the verdict of the scientific world has been just.—We hasten, however, from details whose very superabundance oppresses one, to those discoveries of loftier reach, and of primary theoretic bearing, with which Sir David enriched Optical science. Here, too, one might easily lose oneself as in a wilderness. No one had ever a quicker eye towards the new, than this acute Inquirer. It is an attribute belonging only to a rare class of exercised minds, which detects and separates novelty from amidst the common, and discerns at once the presence of something unexplained. Eminently possessed of this peculiar gift, Brewster, throughout his whole life, followed memoir by memoir, demanding attention to facts as yet unexplained by theory; and in this way he became instrumental in obtaining the correction of too general conclusions, and so of aiding the progress even of theories he had not seen reason to accept, to an extent which, amid our scientific contests, has failed to be recognized and adequately acknowledged. An impartial history of science—written when our times can be calmly surveyed—will not overlook such benefits; but no history, wherever and whenever written, can fail to record and appreciate the achievements to which we shall now advert.—Previous to the beginning of this century, Newton's Optics contained nearly all we knew concerning light, with the exception of those signal discoveries of Huyghens. But new and comparatively evanescent qualities of the energy producing that sensation, came to be discerned. We shall not speak of inflection—that subject which, not having escaped Newton, was yet of so fresh an interest, that it provoked the attention of Brougham, and the early efforts of Brewster. More subtle and strange than that, light was found to be affected by modifications, scarcely recognized, and never investigated until after the first decade of this century Nor are these modifications accidental. On the contrary, they are found to enter as prime essentials among the elements of any theory of light. These modifications are chiefly two: first, a ray of light is variously and apparently fantastically turned out of its path, by some action on the part of a class of diaphanous bodies into which it enters; and, secondly, the ray so disturbed, as also when reflected in a certain manner, acquires peculiar characteristics. With regard to the first order of subjects, we claim for Sir David Brewster the merit of having completely surveyed all the great phenomena, and laid down their general laws. The fact of ordinary refraction had indeed long been known; and Snell gave the formula that indicates the new course of the ray. As to double refraction, as produced by a certain class of crystals, Huyghens had announced a formula giving the direction of both rays: but in neither case had any connection been established between the phenomena and the nature of the body producing them. Although not earliest in order of time, we mention first in the course of our narrative, Sir David's discovery of the effects of pressure, traction, &c., in producing the double-refracting power. This series of facts, now no longer isolated, distinctly points, as the cause of such irregularity in refraction, to internal molecular irregularities in the body producing it; and our countryman completed the theory of the subject in several masterly memoirs, in which he connects all these phenomena with the mathematical form of crystals: their refracting qualities depending on the question, whether, like the cube, they have three mathematical axes; one, as in the case of the rhombohedral and pyramidal systems; or, two, as with all prismatic systems. The paper in which he unfolds the law of double refraction in the latter class of crystals, is as fine a specimen of generalization as modern inquiry has brought forth.—The triumphs of our countryman, in reference to the second great class of new optical phenomena, have been quite as great. The phenomena in question are the aspects and conditions of what is called Polarized light. We do not and cannot refer here to the multitude of unlooked-for facts detected by Brewster, in illustration of this most brilliant portion of modern experimental science. They are so numerous, and they came upon him and on the world so rapidly, that, as already stated, their real importance was often overlooked, because of the feeling of distraction occasioned by their variety. But it is necessary to remark that here, too, we owe to Sir David our first statement of fundamental laws. When Malus discovered the phenomenon of polarization, he deemed it isolated, or unconnected with known properties of the substances occasioning it. This isolation disappeared in Brewster's hands; and his first achievement was the statement of the formal laws of polarization, alike by reflection and refraction. No sooner had the undulatory theory of light begun to yield consequences, than these formal laws were explained by it; and the fortunate deduction went very far to place that theory in credit. But still more; these laws after all were only general approximations,—something akin to the theorem of the elliptical orbits of the planets. They, too, were found subject to minute "perturbations," which did not long escape the keen eye of Brewster; and he did much to track out the inquiry recently perfected by Jamin—an inquiry productive of results demanding those serious modifications in the old formulæ and processes of Theory, which were obtained from the subtle analysis of Cauchy.—It were needless to pursue our narrative, for, in the best form in which we can put it, it must be incomplete. One general remark, however, must be made. The speciality of Brewster's genius, viewing him as a Philosopher and not as a mere Observer, consisted in his power to detect and deal with that class of laws which are termed Empirical—laws which express the general and formal relations of multitudes of determinate facts. More than any other man perhaps of his day, he was entitled to be held the Kepler of physical optics. The achievements we have been illustrating, and others akin to them, stand towards an ultimate and absolute theory of Light, precisely as the three laws of the great German are related to the final discovery of Newton. has the Newton of physical optics yet arrived? Does a theory already exist in that science worthy of a place analogous to that of Gravitation? Sir David doubted it: nor need it be concealed that his successes in his own direction, and the special aptitude of his faculties for the accomplishment of his peculiar task, may have rendered him at times insensible to the actual and surprising grasp of the doctrine of Undulations. But on such remote matters, and the speculations connected with them, we have no disposition to dwell. Whatever new phases optical science may yet assume, Brewster's name will ever be proudly associated with its history.
III. On glancing over what we have written, we feel keenly its utter inadequacy. Of some of Brewster's brilliant discoveries even in Optics we have said nothing;—e.g., those elaborate investigations concerning absorption, &c. Nor have we been able to refer to his services to practical meteorology, or to his contributions towards the cosmical theory of the Temperature of the globe. Space equally fails us as we touch the literary labours of this most active spirit. His writings would fill a multitude of volumes. Witness that arduous work the Encyclopædia, and the dissertations in it that proceeded from his own pen. Witness those editorial labours concerned with our Scottish scientific journal,—the edition of Fergusson,—the treatises on "New Philosophical Instruments," on Optics, on the Kaleidoscope, the Stereoscope, &c. &c. Witness the most interesting "Life of Newton;" the "Martyrs of Science;" the "Treatise on Natural Magic;" and the "More Worlds than One." To the close,
