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H. VON HELMHOLTZ. 



1821-1894. 



BORN at Potsdam, Helmholtz was successively Army Surgeon, 

 Lecturer on Anatomy in Berlin, Professor of Physiology in 

 Konigsberg (1849-56), Bonn (1856-59), Heidelberg (1859-71), 

 Professor of Physics in Berlin from 1871 until his death. In his 

 graduation thesis (1844), he showed that nerve fibres are processes 

 of nerve cells, using for this purpose the ganglia in the leech and 

 crab. In 1843 he contributed an important paper on the fermenta- 

 tion set up by yeast, but his talent and genius lay in his treatment 

 of physiological problems from the physical and mathematical side. 

 By his investigations on animal heat he was ultimately led to lay 

 the foundations of the great doctrine of conservation of energy. 

 By thermo-electrical methods he was able to measure the heat 

 produced during the contraction of an excised bloodless muscle of a 

 frog. He studied the contraction of muscle by means of a myograph 

 recording on a revolving surface, and measured the phases of the con- 

 traction and the duration of each. In 1837, when still only twenty-six 

 years of age, he published his epoch-making essay Die Erhaltung der 

 Kraft ; The Conservation of Force, or, as we now call it, energy, thus 

 applying to energy the doctrine that Lavoisier had applied to matter, 

 its indestructibility. The form of both may be changed ; the amount 

 remains constant. JULIUS ROBERT MAYER (1814-1878) of 

 Heilbronn, about 1842, applied this doctrine to the organic world, 

 and even calculated the mechanical equivalent of heat. J. P. JOULE 

 (b. Salford 1818 ; d. 1889) ascertained experimentally the true 

 equivalent to be 425 kilogramme -metres for 1C. Joule's researches 

 extended over a period of about nine years (1840-49), when the 

 dynamical equivalent of heat was finally determined for mechanical 

 work, electricity, electro-magnetism, and light. Once established in 

 Konigsberg, Helmholtz solved, on a piece of frog's nerve two inches 

 long, a problem that, only a short time before, his great master, 

 J. Miiller, had declared to be incapable of solution, viz. : the rate 

 of propagation of a nervous impulse or the excitatory state in 

 a nerve. In 1851 he invented the ophthalmoscope, the year of 

 our first great International Exhibition "a discovery rather 

 than an invention, a revelation transforming ophthalmology." 

 W. Gumming and Briicke, in 1847, found a method of rendering 

 the normal eye luminous, and came very near the discovery. 

 " The whole world spoke of it ; every one wanted to see the 

 ophthalmoscope, which revived long lost hope." In Bonn he 

 studied physiological optics, and worked out fully the mechanism of 



