EICHENFRIED WALTER VON, an ingenious mathematician and natural philosopher, was descended from a noble family, and was born at Kieslingswalde, in Upper Lusatia, on the 13th of April 1651. After spending some years in study at the University of Leyden, in 1672 he entered the Dutch army, where he remained till the autumn of 1674. Several of the subsequent years of his life he spent in travelling in various parts of Europe, particularly England, France, and Italy; and during that time he was diligent in collecting everything of interest regarding natural philosophy. It was while sojourning abroad that he discovered the curves to which the reflected rays of light are tangents, and which have been called the "caustic curves of Tschirnhausen." In a paper which he read before the Académie des Sciences of Paris in 1682, he showed that the caustic formed by parallel rays when reflected from the concave surface of a hemisphere is an epicycloid; but it remained for MM. De la Hire and Bernoulli accurately to investigate the properties of this curve. On his return to his native place, he took to fashioning burning-glasses. (See Burning-Glasses.) He likewise discovered the principle of Chinese porcelain, from which he may well be regarded as the founder of the famous Dresden porcelain manufacture. The only works which he published separately were the *Medicina Corporis*, 1686, and the *Medicina Mentis*, 1687. Tschirnhausen died in October 1708, and King Augustus of Poland had him buried with great pomp.