xxi 



smaller man. Nevertheless, while these investigations were progress- 

 ing, he found time to invent the ophthalmoscope (1851), to begin his 

 observations on compound colours (1852) and to write an elaborate 

 essay on the nature of sense impressions (1852). 



Thus, in 1852, at thirty-one years of age, Helmholtz had surveyed 

 a large part of the wide field in which he was to achieve so much. 

 His life work lay mapped out before him. He had already shown 

 the extraordinary many-sidedness which was the most remarkable 

 characteristic of his genius, and had entered upon most of the in- 

 quiries in the prosecution of which his fame was to be won. If an 

 exception is to be made to this statement, it is that in 1852 Helmholtz 

 had hardly given to the world a proof of the mathematical ability 

 which was afterwards to solve the problem of vortex motion. 



It would be impossible in a notice of this kind to enter into any 

 detailed account of his investigations, or to discuss critically the 

 arguments for and against theoretical views which have not in all 

 instances commanded universal assent. The most that can be 

 attempted is to give as briefly as possible a catalogue raisonne of his 

 principal achievements. 



The measurement of the velocity of propagation of nervous action 

 was probably his most important work in the domain of experi- 

 mental physiology. He returned to this subject, and about the year 

 1870 wrote several papers in conjunction with Herr Baxt on questions 

 suggested by it. The original investigation was extended so as to 

 include observations on the living human subject, the measurement 

 of differences in the speed of transmission in different parts of the 

 body, and the effects of temperature. In another memoir published 

 in 1871, Helmholtz discussed experiments, conducted in the labora- 

 tory at Heidelberg by Herr Baxt, on the interval of time which 

 elapses between the presentation of a visible object to the eye and 

 the production of the sense-impression to which it gives rise. 



In 1864 and 1866 he published two papers on the sound which a 

 muscle emits when contracted. In the first of these he showed that 

 if the muscle be stimulated by intermittent electrical shocks the 

 sound given out corresponds in pitch to the frequency of the stimula- 

 tions. In the second he determined the correct pitch of the sound 

 given out by muscles, contracting naturally nnder the influence of 

 volitional impulses, by communicating the vibrations of the muscle 

 to springs or to paper strips. He found that these responded 

 best when their natural period was from 1/18 to 1/20 of a second. 

 This was important as showing that observers who had stated that 

 the natural muscle sound corresponded to from thirty-six to forty 

 vibrations per second had heard the octave of the true fundamental 

 which was itself below the lower limit of audition. 



The investigations of Helmholtz on physical and physiological 



