426 HERMANN VON HELMHOLTZ 



velocities. If, however, the pondero-motive forces can be 

 derived from the above principle, this would, according to 

 Helmholtz's statement, involve certain variations with respect 

 to the co-ordinates and to the corresponding components of 

 velocity, resulting in very complex calculations. This work, 

 which remained unfinished, aimed at proposing a more simple 

 and obvious method of solving the problem, and was fitted to 

 eliminate these purely formal difficulties, while its results were, 

 as he stated, confirmatory of his earlier conclusions. 



On Sunday, July 9, the corrected proof of his Preface to 

 Hertz's Mechanics was read aloud to him at his daughter's 

 house at the Wannsee, and later Frau von Siemens (after search- 

 ing for him in vain through the house to take him out for 

 a walk) found him sitting by a quiet window, his little notebook 

 and pencil in his hand, lost in thought. * His eyes were bright, 

 and his whole bearing was wonderfully happy/ He explained 

 that he had been fortunate enough that day to discover some- 

 thing which he, and for a long time many before him, had been 

 seeking; but he would have no time before Wednesday to 

 work out his ideas, which he hoped to lay before the Academy 

 on Thursday. By the loth he was hesitating again whether 

 there might not be some fallacy in his argument but to the 

 day of his death he remained convinced that all the phenomena 

 of Nature were comprised in the Principle of Least Action, 

 under the universal form in which he expressed it a view 

 which Hertz had adopted from him, and on the universal validity 

 of which the future only can pronounce. 



During the summer a contribution from Helmholtz appeared 

 in the Zeitschriftf. Psych, u. Physiol. d. Sinnesorg., entitled ' On 

 the Origin of the Correct Interpretation of our Sense-impres- 

 sions ', subsequently rewritten by himself for the second edition 

 of his Physiological Optics, then being issued; the concluding 

 parts only came out in the following year. This, his last con- 

 tribution to Physiological Psychology, has a peculiar interest, 

 because it re-states and re-establishes the very same opinions 

 which Helmholtz put forward in his first lectures at Kdnigs- 

 berg ; and because he thus proclaims himself, at the close of 

 his long and fruitful career, to be the faithful adherent of 

 empiricism, without denying that the nativistic position is 

 tenable under certain limitations. In all directions whatsoever, 



