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 HELMHOLTZ 623 ie military surgeon stationed at Potsdam. During the five years following he practised medicine, contributed a great number of arti- cles 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 at- tention throughout the world. In 1843 he had written " On the Nature of Putrefaction and Fermentation" (Miiller's Archiv fur Anatomie, Physiologic und wissenschaftliche Mediciri), proving putrefaction to occur inde- pendently of microscopical living beings, but modifiable thereby and then constituting fer- mentation ; 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 encyklopd- disches Worterbuch' der medicinischen Wissen- schaften) ; also, " On the Consumption of Tis- sue during Muscular Action " (Mtiller's Archiv, 1845) ; and "Proof of a Development of Heat during Muscular Action" (Miiller's Archiv, 1848). He for the first time proved by actual experiments a difference of chemical compo- sition 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 liber die Verhandlungen der Kbnig- lichen Preussischen Akademie der Wissenschaf- ten zu Berlin under date of Jan. 21, 1850; and he published two elaborate articles in Mtiller's Archiv for 1850 and 1852. By means of in- genious 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 defi- nite period to become conscious of the fact, and that when we determine to make a move- ment a certain measurable time elapses be- tween 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 in- terior 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 revolu- tionized the knowledge of its condition in health and disease, and saved thousands of 400 VOL. vin. 40 persons from blindness. It is said that he was led to this invention by reflecting on the cir- cumstance 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 sepa- rate work " On the Ophthalmoscope " (Berlin, 1851), an improvement in the Archiv fur physiologische EeilTcunde (1852), and the oph- thalmometer in the Archiv fur Ophthalmolo- gie (1854). In 1855 Helmholtz became profes- sor of anatomy and physiology in Bonn, in 1858 of physiology in Heidelberg, and since 1871 he has been professor of physics in Ber- lin. 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 Eand- lucli der physiologischen 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 Sensa- tions " (I860), and " Doctrine of Visual Per- ceptions," 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 at- tained, but also shows the way in which fur- ther 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 analy- sis, combination, and appreciation of colors. His second important work is entitled Die Lehre von den Tonempfindungen, als phy- siologische Grundlage fur die Theorie der Musik (Brunswick, 1862 ; 3d ed., 1870). In this he throws the light of natural science upon the inmost principles of music and ros- thetics. In vain had philosophers and mu- sicians 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 fur- nishing us a means of acquiring knowledge until then unthought of. The invention essen- tially consists in the use of hollow bodies, called resonators, the volume of air in which vibrates in the presence of a previously deter- mined sound. Investigation by means of res- onators may in the future lead to discoveries more important still than those hitherto at- tained; but it has already solved many puz- zling questions of the past. Thtas it had long been known that the note of many musical in- struments is accompanied by its octave and the fifth of its octave ; but by the employment of resonators it has been found that every scrund, as it generally occurs in nature, and as it is produced by most of our musical instruments