**Étienne Louis Malus**'s father was Louis Malus de Mitry and was Treasurer of France. Étienne Louis was first educated at home where he was instructed in literature and mathematics. He then attended the engineering school, École Royale de Genie, at Mézières. There he was taught by Monge who realised Malus had special mathematical talents. In 1793 Malus left the school, having been dismissed for political reasons.

On leaving Mézières, Malus joined the army and was posted to Dunkerque. There his abilities were noticed and he was sent to the École Polytechnique as a pupil. Here he was taught by Fourier, and he was perhaps the most able of all of Fourier's pupils. He was to remain associated with École Polytechnique as an examiner all his life.

While studying at the École Polytechnique Malus began to undertake original research, writing papers on the path of light through materials of differing refractive indices.

After graduating from the École Polytechnique Malus rejoined the army, this time taking part in campaigns on the Rhine in 1797. As an army engineer Malus was ordered to accompany Napoleon's invasion of Egypt in 1798. This did not greatly please him since at the time that he received the order he was stationed in the town of Giessen, as part of the occupying force, and he was about to marry the daughter of the Chancellor of the University of Giessen.

While in Cairo, Napoleon's fleet was destroyed in Aboukir Bay and Malus wrote, see [3],

At Napoleon's instigation, while they were in Cairo, the Cairo Institute was set up having 12 mathematical members. As well as Malus these included Monge, Fourier and Napoleon Bonaparte himself. After returning in 1801 Malus held posts in Antwerp, Strasbourg, and Paris.From then on we realised that all our communications with Europe were broken. We began to lose hope of ever seeing our native land again.

His mathematical work was almost entirely concerned with the study of light. This involved him in studying geometrical systems called ray systems, closely connected to Plücker's line complexes. He conducted experiments to verify Huygens' theories of light and rewrote the theory in analytical form. His discovery of the polarisation of light by reflection was published in 1809 and his theory of double refraction of light in crystals in 1810.

In 1811 Malus served, along with Lagrange, Legendre, Laplace and Haüy, on the committee to decide on who to award the prize to for the best work on the propagation of heat in solid bodies. They awarded the prize to Fourier.

Malus received many honours for his work, in particular he was awarded a prize from the Académie des Sciences in 1810 for his memoir on double refraction. In the same year he was elected to the Académie des Sciences and the following year, despite the war between England and France, Malus was awarded the Rumford medal of the Royal Society of London.

**Article by:** *J J O'Connor* and *E F Robertson*