9 o HERMANN VON HELMHOLTZ 



cites the long roll of Matteucci's painstaking experiments (to 

 which he was obliged to return many years later in another 

 connexion), and finally embarks upon a masterly exposition 

 of du Bois-Reymond's discoveries : ' the fruits of assiduous 

 study, and of ten years' labour consistently concentrated upon 

 one aim, during which the frog and the divisions of the 

 galvanometer were his world a rare example of methodical 

 observation, of rich knowledge, and of that perspicacity of 

 conception which is learned in the school of mathematics/ 

 After communicating the most important results of du Bois' 

 investigations, experiments which he himself demonstrated to 

 his audience by the now familiar method of throwing a beam 

 of light from a mirror connected with an astatic system of 

 magnets upon a graduated scale, he goes on to say that certain 

 physiologists assume what is propagated in the nerve during 

 excitation to be some definite form of motion like the undula- 

 tions that are propagated as sound-waves in the air, and as 

 light-waves in the ether. He submits that electrical phenomena 

 also lead to the idea of such a motion, since the extra- 

 ordinary rapidity of the variations of electromotive force, both 

 in magnitude and direction, makes it probable that this force 

 affects very mobile particles, and that the orientation of these 

 particles is temporarily altered by excitation, from the excited 

 point of the nerve onwards to the muscle, and within the 

 muscle itself. He considers the unexpectedly low rate of 

 propagation in nervous excitation as determined by himself 

 to be incompatible with the older view of an immaterial or 

 imponderable principle as the nervous agent, but quite in 

 harmony with the theory of the motion of material particles 

 in the nerve substance. 



This publication concluded the series of the closely-con- 

 nected physiological investigations which Helmholtz had begun 

 immediately after the publication of his thesis, and he now 

 turned to physiological optics. In this subject he worked 

 out new physical principles, upon which, as physicist, physio- 

 logist, philosopher, and aesthetician, he erected a structure 

 of such extent and security as had never been dreamed of, 

 which to this day arouses wonder and astonishment. 



He had already instituted comprehensive experiments on 

 the law of colour-mixture, in order to correct an error of 



