i 7 2 HERMANN VON HELMHOLTZ 



after-image that appears on a dark ground into a negative 

 image, diminishes with the time. 



After confirming Fechner's theory in detail, Helmholtz 

 produced after-images of pure prismatic colours in his eye, 

 observed them upon a field covered with another prismatic 

 colour, and found that the phenomena in no way differed 

 from those which arise on regarding the colours of natural 

 bodies and pigments. There was, however, one interesting 

 case, in which he looked at a round spot brightly illuminated 

 by a spectral colour, then observed its after-image upon a field 

 covered with the complementary colour, and completely purified 

 from diffuse white light. The complementary colour then 

 appeared purer and more saturated within the after-image, 

 than around it. Helmholtz thence concluded that although 

 the prismatic colours are the purest and most saturated, that 

 is the most free from a mixture of white, presented to us in 

 nature, yet, by the above means, the sensation of even more 

 highly saturated colours may be excited, in comparison with 

 which the purest prismatic colours will appear whitish. 



Towards the end of August Helmholtz moved with his 

 family to Heidelberg, where he received a great ovation. The 

 newly-formed Ophthalmological Society presented him with 

 a cup, inscribed 'To the Creator of Modern Science, the 

 Benefactor of Mankind, in grateful remembrance of the dis- 

 covery of the Ophthalmoscope/ 



In September he attended the meeting of the British 

 Association at Aberdeen, and the Naturforscher-Versammlung 

 at Carlsruhe, the capital of the State to which he was to 

 consecrate his mighty energies for the next thirteen years. 

 His address 'On After-images ' gave a summary of the 

 experiments and conclusions described above, and another 

 discourse 'On the Physical Causes of Harmony and Dis- 

 sonance* was the epitome of a lecture on 'The Physiological 

 Causes of Harmony in Music', delivered to a large and 

 enthusiastic audience the year before at Bonn Beethoven's 

 birthplace. 



'Of all the subjects at which I have worked,' he says forty 

 years later, ' I have chiefly felt myself a dilettante in Music. 

 Art and Science are essentially distinct in their external 

 aspects and technique ; but I am none the less convinced 



