xxiv 



tion at the surfaces of the cornea and the crystalline lens. This 

 instrument could not easily be applied to the faint image formed by 

 the front surface of the crystalline lens, and he devised ingenious 

 methods of indirect measurement by comparing the image with that 

 produced at the surface of the cornea by a weaker source of light. 



These observations are classic and placed the explanation of the 

 mechanism of accommodation on a firm basis. They were followed 

 by many others described either in memoirs or in the great work on 

 * Physiologische Optik ' which appeared in parts between the years 

 1856 and 1866. The geometry of the movements of the eye was dis- 

 cussed |in 1863. Three papers were devoted to the form of the 

 Horopter, or locus of points the images of which are formed on 

 " corresponding points " of the two retinae. Helmholtz also devoted 

 much time to the study of after images, to contrast phenomena, to 

 the theory of binocular vision, and to the relations between the con- 

 vergence of the eyes and the sense of distance. 



A masterly memoir, published in 1874, contained by far the most 

 satisfactory explanation of anomalous dispersion that had, up to that 

 time, been given. 



Attempts had been made by Sellmeier and Ketteler, to solve the 

 problem on the hypothesis that material particles are set in motion 

 by the ether. No retarding or frictional forces were introduced and 

 thus difficulties arose when the periods of the material and ethereal 

 vibrations were identical. On the other hand, O. E. Meyer had 

 postulated a viscous ether, but had not introduced the material par- 

 ticles. Helmholtz combined the two hypotheses by assuming a 

 frictional resistance to the motions of atomic matter, which was 

 supposed to be mingled with the ether. The agreement of 

 the results with experiment was sufficiently good to prove that 

 whatever changes may be made in our conception of the mechanical 

 relations between the ether and matter, the equations deduced from 

 the assumptions of von Helmholtz are a close approximation to a 

 satisfactory mathematical expression of the phenomena. 



The acoustical work of Helmholtz is more accessible to the 

 English reader than his researches on optics, for, whereas the 

 ' Physiologische Optik * has not been translated, the great com- 

 panion book ' Die Lehre von den Tonempfindungen ' was done into 

 English by the late Alexander Ellis, F.R.S., and was enriched by the 

 notes and appendices added by the accomplished translator with the 

 author's consent. 



The first part of the * Tonempfindungen ' was chiefly devoted to 

 harmonic upper partials. It was shown that they " are not, as was 

 hitherto thought, isolated phenomena of small importance, but that, 

 with very few exceptions, they determine the qualities of tone of 

 almost all instruments, and are of the greatest importance for those 



