Hermann Ludwig Ferdinand Helmholtz, a German physicist and physiologist, born in Potsdam, Aug. 31, 1821. At the age of 17 he was admitted to the royal military school in Berlin, and commenced the study of medicine. In 1842, in his graduating thesis entitled De Fabrica Sys-tematis Nervosi Evertebratum, he endeavored to prove the existence of an anatomical connection between the nerves of motion and those of sensation through the intermediate means of ganglion cells, and presented the results of numerous examinations of the delicate nerve fibres of bugs, spiders, crabs, and many lower animals. Until 1843 he was assistant physician at the Charite hospital, and then became military surgeon stationed at Potsdam. During the five years following he practised medicine, contributed a great number of articles on mathematical and physical science to various periodicals, and laid the foundation for his scientific reputation by a finished work "On the Conservation of Force" (Berlin, 1847). He was at once recognized as one of the chief investigators and promoters of the new philosophy of force. A popular lecture on the same subject, delivered some years later in Konigsberg, was translated by Prof. Tyndall of London for the "Philosophical Magazine," and attracted a great deal of attention throughout the world.

In 1843 he had written "Oh the Nature of Putrefaction and Fermentation" (Muller's Archiv fur Anatomic, Physiologie und wissenschaftliche Medicin), proving putrefaction to occur independently of microscopical living beings, but modifiable thereby and then constituting fermentation; in 1845, "On Animal Heat," with especial consideration of the question whether the living animal body gives off as much heat as is produced by the combustion and change of the food it takes in (Berliner encyklopa-disches Worterbuch der medicinischen Wissen-schaften); also, "On the Consumption of Tissue during Muscular Action" (Muller's Archiv, 1845); and "Proof of a Development of Heat during Muscular Action" (Muller's Archiv, 1848). He for the first time proved by actual experiments a difference of chemical composition in the active and in the quiescent muscle. In 1848 he returned to Berlin as prosector of anatomy and tutor in the art academy, and in 1849 became professor of physiology in the university of Konigsberg. Soon afterward he began his celebrated investigations as to the rapidity of propagation of nerve excitation.

The first report of them appeared in the Berichte uber die Verhandlungen der Konig-lichen Preussischen Akademie der Wissemchaf-ten zu Berlin under date of Jan. 21, 1850; and he published two elaborate articles in Muller's Archiv for 1850 and 1852. By means of ingenious methods for ascertaining exceedingly small differences of time, he demonstrated that thought is not instantaneous, that when we touch anything with the hand it takes a definite period to become conscious of the fact, and that when we determine to make a movement a certain measurable time elapses between willing the movement and executing it; he succeeded in accurately measuring these •small fractional portions of a second. He was also the first who gave the correct explanation of the fact that on looking at a person's eye the pupil appears black, and why we cannot under ordinary circumstances see into the interior of the eye; and in 1851 he invented the ophthalmoscope, an instrument by means of which the retina of the living eye can be inspected, and which has completely revolutionized the knowledge of its condition in health and disease, and saved thousands of persons from blindness.

It is said that he was led to this invention by reflecting on the circumstance that while it is impossible during the daytime to see clearly into a room on the opposite side of a street, it can, however, be done at night, when the room is illuminated, and also during the day by throwing into it sunlight reflected by a mirror or a sun glass. His original invention is described in a separate work "On the Ophthalmoscope" (Berlin, 1851), an improvement in the Archiv far physiologische Heilkunde (1852), and the ophthalmometer in the Archiv fur Ophthalmologic (1854). In 1855 Helmholtz became professor of anatomy and physiology in Bonn, in 1858 of physiology in Heidelberg, and since 1871 he has been professor of physics in Berlin. He is the author of two books, each of which forms an era in the branch of science to which it relates. The first, entitled Hand-buch der physiologhchen Optik (Leipsic, 1867), is in very many respects a pioneer labor. In addition to the anatomical description of the eye, it contains three divisions, "Dioptrics of the Eye," of which two sections were first published in 1856, "Doctrine of Visual Sensations" (1860), and "Doctrine of Visual Perceptions," finished in 1866. Helmholtz traces his subject from the remote past, establishes the so-called empirical theory of vision, and not only clearly points out the knowledge attained, but also shows the way in which further advance is to be made.

The author's original researches in this work include every portion of the subject, from the investigation of the limits of human power of perception to that of the details of vision, and the analysis, combination, and appreciation of colors. His second important work is entitled Die Lehre. von den Tonempfindungen, als phy-siologische Grundlage fur die Theoric der Musik (Brunswick, 1862; 3d ed., 1870). In this he throws the light of natural science upon the inmost principles of music and aesthetics. In vain had philosophers and musicians for more than 2,000 years endeavored to explain musical harmony; the discovery of its cause, an achievement of the most recent times, is mainly due to Helmholtz. He invented the method of analyzing sound, thereby furnishing us a means of acquiring knowledge until then unthought of. The invention essentially consists in the use of hollow bodies, called resonators, the volume of air in which vibrates in the presence of a previously determined sound. Investigation by means of resonators may in the future lead to discoveries more important still than those hitherto attained; but it has already solved many puzzling questions of the past.

Thus it had long been known that the note of many musical instruments is accompanied by its octave and the fifth of its octave; but by the employment of resonators it has been found that every sound, as it generally occurs in nature, and as it is produced by most of our musical instruments or the human voice, is not a simple single sound, but a compound of several tones of different intensity and pitch, all of which different tones combined are heard as one. Helm-holtz discovered that the difference of quality or timbre of the sounds of different musical instruments resides in the different composition of these sounds, i. e., is due to the fact that different compound sounds may contain the same fundamental tone, but differently mixed with other tones. He also discovered the acoustic cause of the vowel sounds of human speech, and not only analyzed or decomposed them into their constituent elementary tones, but also imitated or rather artificially produced vowel sounds from elementary tones of tuning forks. Helmholtz has propounded a hypothesis of the manner of excitation of the acoustic nerve which in its physiological aspect has been generally accepted, although its anatomical basis is not yet sufficiently established.

He has demonstrated that beats occasion dissonance, and that the perception of the relationship of two musical sounds is due to the sensation of one or more common simple sounds in them, and the perception of that of two chords to the sensation of one or more common compound sounds; shown how beats and foreign ingredients in sounds produce an intermittent excitation of certain fibres of the auditory nerve, and thereby become unpleasant; and suggested a reasonable explanation of the adaptability of music for emotional expression. A list, probably incomplete, of his contributions to various periodicals, published lectures, etc, enumer-ates more than 70 articles besides those already mentioned. Some of the more important are: "Course and Duration of certain Electric Currents," " Theorem on the Distribution of Electric Currents," "Analysis of Sunlight," " On the Theory of Composite Colors," "On the Accommodation of the Eye," "Comparison of the Luminousness of Different Colors," "Explanation of the Stereoscopic Phenomena of Lustre," "On Combination Tones," " On the Persistence of Visual Impressions," "On the Notes of Various Musical Instruments," "On the Normal Movements of the Human Eye," "Intermittent Movements of Liquids," "On the Friction of Liquids," " The Mechanism of the Ear Ossicles and Tympanic Membrane," "On a Theorem concerning geometrically similar Motions of Fluid Bodies, applied to the problem of guiding aerial balloons," and " On the Limits of the Power of the Microscope." Helmholtz's readiness and ability to present to the public at large in intelligible language the results of his researches, sometimes the most abstruse, have contributed largely to his fame.

He has been delivering popular scientific lectures for more than 25 years. One on human vision was delivered in Konigsberg in aid of Kant's monument (Leipsic, 1855), and in May, 1853, appeared one on Goethe's labors in natural science. This, together with three others, was issued in book form (Brunswick, 1865; second series, 1871). All these have been translated by Prof. E. Atkinson, under the title of " Popular Lectures on Scientific Subjects," with an introduction by Prof. Tyndall (London and New York, 1873). The French academy admitted him to foreign membership in 1870, after a discussion during which a well known author said: "You will place yourselves in the worst light before the world if, for any reason, you refuse to admit Helmholtz, the foremost and greatest naturalist of the age;" adding, " Nothing is wanting to his glory, but he is wanting to ours."

Helmet Shell.