MODERN BIOLOGY 409 



into a strait waistcoat. By degrees, however, he recovered his health and 

 in his old age had the satisfaction of being universally recognized as the one 

 who had first laid down the principle of the conservation of energy. 



Of Mayer's rivals Joule belongs entirely to the history of physics. Helm- 

 holtz, on the other hand, worked both as a physicist and as a biologist 

 and therefore deserves further mention in this place. Hermann Ludwig Fer- 

 dinand Helmholtz was born in i8ii at Potsdam, where his father was a 

 teacher in the gymnasium. He studied medicine in Berlin, where he was one 

 of J. Miiller's pupils, and became first of all an army doctor, afterwards being 

 appointed professor of physiology at Konigsberg (in 1849), ^^'^ later holding 

 the same appointment at Bonn and Heidelberg. In 1871, however, he was 

 made professor of physics at the University of Berlin, and somewhat later 

 he became director of a newly-founded physico-technical institute at Char- 

 lottenburg. These two posts he held until his death, in 1894. Being univer- 

 sally regarded as one of the foremost scientists of his day, he was the recipient 

 of innumerable honours both at home and abroad. His research activities 

 were also as multifarious as any that natural science has had to record in 

 recent times. As his career testifies, he was an expert in both biology and 

 physics; besides this, he was not only a mathematician and a philosopher 

 of high standing, but also an excellent stylist and an eloquent speaker. As 

 his doctor's dissertation he published a valuable account of the nerve-cells 

 in ganglia and the nervous ramifications emanating therefrom in different 

 animal forms. His measurement of the rapidity of the reproduction of im- 

 pressions through nerve-fibres was of fundamental importance. Of still 

 greater significance, however, was his work as a sense-physiologist. Modern 

 physiological optics in particular were in all essentials founded by him. He 

 invented the ophthalmoscope, by the aid of which it has become possible for 

 doctors to examine the retina of the eye; he further explained the mechanism 

 of lens-accommodation and also founded the theory of colours and colour- 

 perceptions that has been adopted in modern times. Physiological acoustics 

 were likewise founded by him; he explained the connexion and mechanical 

 action of the bones of the ear, as also the part played by the organ of Corti 

 in the perception of tone quality. Again, from the point of view of purely 

 theoretical science, he worked out a theory of the sense-perceptions, in 

 which he dealt with such abstract and complicated questions as the rela- 

 tion of the geometrical quantities to the conception of sense, and the justi- 

 fication of the geometrical principles based thereon. His purely physical 

 and mathematical work naturally falls outside the scope of this history. 



Helmholtz works out the theory of indestructibility 

 The above-mentioned paper tjher die Erhaltung der Kraft (On the Conservation 

 of Forced, nevertheless, deserves further mention here. As a result of it, the 

 law of the conservation of energy was given the theoretical formula that 



