THOUGH chiefly celebrated for his discoveries in optics and acoustics, and for his invention of the ophthalmoscope, Von Helmholtz is much esteemed for his essays on scientific and educational topics. His lectures to his classes abound in eloquent passages, but he made beauty of style a minor consideration and the definition of principle his object. He was born August 31st, 1821, at Potsdam, where in 1843 he began his professional life as an army physician. From 1849 to 1855, he was professor of Physiology at Königsberg. He taught Physiology at Heidelberg from 1858 to 1871, and held the chair of Physics at Berlin during the latter part of his life, dying at Berlin, September 8th, 1894. Among his works are “The Conservation of Energy,” “The Doctrine of Tone Sensation,” and “The Manual of Physiological Optics.”