HELMHOLTZ IN BERLIN 



appeared during the last ten years of his busy life. 

 Further, he was cheered by the social brightness of a 

 happy home life. In 1861, Helmholtz entered upon 

 a second marriage with Miss Anna von Mohl, of a 

 Wiirtemberg family of high social position. This 

 lady became the worthy helpmate of so great a man ; 

 by her side many of his immortal works were created, 

 and their home was the centre of a brilliant social, 

 artistic, and intellectual circle. Two children were 

 born of this marriage. The elder son, Robert, died 

 in 1889 at a comparatively early age, after he had 

 given promise of having inherited some of the apti- 

 tude for mathematical and physical research which 

 so distinguished his father ; while the daughter knit 

 together the houses of Siemens and Helmholtz by 

 marrying the son of Werner von Siemens. The loss 

 of his son was deeply felt, and it is said that Helm- 

 holtz never recovered from the blow. Thus the 

 brilliancy of his career was dimmed by his experience 

 of sad events that come to all alike. 



During the last twenty-three years of his life, Helm- 

 holtz devoted his energies entirely to the investigation 

 of physical problems. The only exception to this 

 statement is, that questions of a philosophical nature, 

 which will be dealt with in a future chapter, also 

 engaged his attention. To these questions he was 

 led by the consideration of his theory of knowledge, 

 founded on the thorough examination he had made 

 into the nature of sensation and perception. In 

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