MALUS, ETIENNE LOUIS, the discoverer of the laws of the polarization of light by reflection, born at Paris on the 23rd of June 1775, was the son of Anne Louis Malus du Mitry, and of Louise Charlotte Desboves, his wife.

His father had a place in the Treasury of France, and gave him at home an excellent education in mathematics and in the fine arts, as well as in classical literature, with which he rendered himself so familiar as to retain many passages of the Iliad in his memory throughout life. At seventeen he was admitted, after a severe examination, as a pupil of the School of Military Engineers; and about the same time he amused himself with writing a regular tragedy in verse on the death of Cato. He soon distinguished himself in his military studies, and he was about to obtain a commission as an officer, when an order of the minister Bouchotte imputed to him the offence of being a suspected person, probably on account of the situation held by his father, and he was dismissed from the school. He was then obliged to enter the army as a private soldier in the fifth battalion of Paris, and he was employed in this capacity upon the fortifications of Dunkirk. Here he was soon distinguished by M. Lepère, the director of the works, as superior to his accidental situation; and he was selected as one of the young men who were to constitute the members of the Ecole Polytechnique, then to be established upon the recommendation and under the direction of Monge, who immediately chose him, from a previous knowledge of his merit, as one of the twenty who were to be made instructors of the rest. This body constituted at that moment the only refuge of the sciences in France, and the enthusiasm of its members was proportionate to the advantages which they enjoyed, and to the importance of the trust committed to them. In the three years which he passed in this institution he was much employed, amongst other applications of the higher geometry, in pursuing the mathematical theory of optics, a department of science in which he was afterwards so eminently to distinguish himself by experimental discoveries. He was then, however, obliged to abandon for a time the pursuit of scientific investigations, and he was admitted into the corps of engineers; with the seniority of his former rank in the school. He served in the army of the Sambre and Meuse; he was present at the passage of the Rhine in 1797, and at the affairs of Ulm and Aalenkirch. Whilst he was in Germany, he formed an attachment with Miss Koch, the daughter of the chancellor of the university of Giessen; and he was on the point of marrying her, when he was obliged to join the Egyptian expedition. He was present in that campaign at the battles of Chabreps and of the Pyramids; he was also present at the affair of Sabish, at the siege of El Arish, and at that of Jaffa. After the surrender of that place, he was employed in the repairs of the fortifications, and in the establishment of military hospitals. Here he was attacked by the plague, and fortunately recovered from it without any medical assistance. He was then sent to fortify Damietta; he was afterwards at the battle of Heliopolis, at the affair of Ceram, and at the siege of Cairo. After the capitulation with the English, he embarked on board the transport "Castor," and arrived in France on the 26th of October 1801. His health was exhausted, and his spirits were broken by fatigue and anxiety; but his attachment to his betrothed bride was undiminished, and he hastened to Germany to fulfil his engagement. His fidelity was rewarded, during the eleven years that he survived, by the most constant and affectionate attention on the part of his wife; and she died a year or two after him, a victim to the same disease which had been fatal to her husband.

He had, however, enough of strength and vigour of constitution remaining to enable him, besides the official superintendence of the works carrying on at Antwerp and at Strasbourg, to pursue the study of his favourite sciences; and upon occasion of a prize question proposed by the Institute, he undertook the investigation of the extraordinary refraction of Iceland crystal, which the experiments of Dr. Wollaston had previously shown to agree very accurately with the laws laid down by Huygens; and besides completely confirming all Dr. Wollaston's results, he had the good fortune greatly to extend the Huygenian discovery of the peculiar modification of light produced by the action of such crystals, which Newton had distinguished by the name "Polarity," and which Malus now found to be produced in a variety of circumstances, independently of the action of crystallized bodies. It seems natural to suppose that the investigation of the laws of the internal reflection of light at the second surface of the crystals, must have led him to the discovery of the effects of oblique reflection in other circumstances; but according to Biot, there was more of accident in his actual progress; for he informs us that Malus had been looking through a piece of crystal at the image of the sun, reflected from the windows of the Luxembourg, to the house in the Rue d'Infer, where he lived, and that he was much surprised to find one of the double images disappear in a certain position of the crystal, although the next day, at a different hour, he could no longer observe the phenomenon, from the alteration of the angle of incidence. The merit of his discovery was soon acknowledged by his election as a member of the Institute, as well as by the adjudication of a biennial medal from the Royal Society of London, on the foundation of Count Rumford. It has been thought creditable to the Royal Society to have conferred this distinction in the time of a war between the two countries; but if any credit were due for only doing justice conscientiously, it would attach, on this occasion, to those members of the council who saw their own optical speculations in great danger from the new mass of evidence which appeared likely to overthrow them, at least in the public opinion, and who were still the most active in offering this tribute of applause to the more fortunate labours of a rival.

Nor was the remuneration of Malus confined to empty honours only; from the liberality of the French government, he obtained promotion in his own profession as an military man; and this not for services performed in the field, nor even in a difficult and dangerous expedition to unknown regions, but for experiments made with safety and tranquillity in his own closet. That government had not carried the refined principle of the division of labour so far as to resolve that all public encouragement should be limited to the precise department in which a public service had been performed; and hence a mark of distinction, which a gentleman could accept without degradation, was not deemed an incommensurate remuneration for a discovery in abstract science. Such a refinement, which has been practically introduced into our own country, might appear, to a man who had a heart, something worse than sordid; he might fancy that a great nation, as well as a great individual, should treat its dependents, "not according to their deserts, but after its own honour and dignity." If, however, a person in office happened to have anything like a heart about him, the outcry of an indiscriminating opposition would soon teach him to silence its dictates.

1. M. Malus's first publication appears to have been a paper "On an Unknown Branch of the Nile," in the first volume of the Décade Egyptienne. 2. He presented to the Institute a mathematical "Traité d'Optique," before the completion of his experiments on double refraction; it was published in the Mémoires présents à l'Institut, vol. ii., Paris, 1810. 3. His more important discoveries were first made known in the second volume of the Mémoires d'Arcueil, Paris, 1809, 8vo; and again, 4, in the "Theory of Double Refraction," Mem. Prés. à l'Inst., vol. ii., a paper which obtained a prize on the 2d of January 1810. 5. In a short "Essay on the Measurement of the Refractive Force of Opaque Bodies," contained in the same volume, he employs the method, before made known by Dr Wollaston, for conducting the experiment, and computes the forces concerned upon the Newtonian hypothesis, applied, however, in a manner somewhat arbitrary to the circumstances of the problem. 6. Remarks on some "New Optical Phenomena," Mem. Inst. Sc., 1810, p. 105, Paris, 1814, read 11th March 1811. This paper is principally intended to prove that two portions of light are always polarized together in opposite directions, and that no part of the light concerned is destroyed, "as Dr Young had been inclined to suspect." The author found that light transmitted obliquely through a number of parallel glasses at a proper angle, becomes at last completely polarized. M. Arago had discovered a case which appeared to be an exception to the general law of the polarization of transmitted light; but it was afterwards readily explained from the theory of the production of colours by interference, as applied to transmitted light. A letter containing the substance of this paper was published in Thomson's Annals, iii. 257, April 1814, on occasion of some discoveries of Sir David Brewster, which had been supposed to be wholly new. 7. "On Phenomena accompanying Refraction and Reflection," p. 112, read 27th May, showing the universality of polarization at a proper angle, and examining the effect of a metallic surface. 8. "On the Axis of Refraction of Crystals," p. 142; describing an apparatus for finding the properties of bodies with respect to polarized light, applied to the determination of the axis of crystals, and to the examination of the structure of organized bodies, which appear in general to have certain axes of polarization, as well as those which are manifestly crystallized.

The zeal and energy of Malus supported him to the last, not only in the continuance of these interesting investigations, but also in his duties as an examiner at the École Polytechnique. He died on the 24th of February 1812, universally regretted by the lovers of science in all countries, and deeply lamented by his colleagues, who said of him, as Newton did of Cotes, that if his life had been prolonged, we should at last "have known something" of the laws of nature. (For a full account of Malus's discoveries see the Sixth Preliminary Dissertation, chap. v., § 2. See also Delambre, M. Inst. 1816, p. xxvii.; Biot, in Biographie Universelle, xxvi., Paris, 1830.)