HELMHOLTZ 293 



conservation of energy, inasmuch as the cutting of lines of 

 force, which is necessary to induction, always involves the 

 expenditure of work, which is proportional to the induced 

 current, from which it is seen that the heat of the current is 

 the equivalent of the work done by induction (and not de- 

 rived from the production of cold in some other place); Joule 

 at the time had also shown that this was the fact. Helmholtz 

 also shows in this paper that the law of conservation of 

 energy may be deduced as a consequence of the exclusive 

 existence of attractive and repulsive forces, the strength of 

 which depends upon the distance apart of the bodies which 

 act on one another - a proof, however, which has since lost 

 the importance at that time ascribed to it, since it has become 

 more and more clear that quite other kinds of force occur in 

 nature, as Faraday already was aware. 



In 1848, Helmholtz received a position as a teacher of 

 anatomy in the Berlin Art Academy, and a year later a pro- 

 fessorship of physiology at the University of Konigsberg, 

 whereupon he married. He there invented the ophthalmo- 

 scope, which enables the retina of the living eye to be seen. 

 In the year 1855 he went as professor of anatomy and phy- 

 siology to Bonn, and in 1858 as professor of physiology to 

 Heidelberg, where he married for the second time after the 

 early death of his first wife. In the year 1 871, he was called 

 to Berlin as the first professor of physics, and in 1888, at the 

 age of sixty-seven, he became president of the newly founded 

 Physikalisch-Technische Reichsanstalt,^ which post he re- 

 tained until his death at the age of seventy-three. 



Helmholtz's scientific individuality lay in his wide ranging 

 capability of easily making himself at home in all domains of 

 knowledge; the essentials were soon grasped by him, and 

 were then ready to hand for further work. All the great 

 connections within exact knowledge of all kinds had to be 

 familiar to him in order to render possible his numerous and 

 1 Corresponding to the English National Physical Laboratory. 



