PROFESSOR AT KONIGSBERG 131 



Johannes Schulze of Bonn, whom Helmholtz consulted, 

 replied that the Ministry intended to appoint an anatomist to 

 Bonn, since the existing physiological lectures appeared to 

 give satisfaction, but that Helmholtz should have his support 

 if he would promise to give most of his time, for the present 

 at any rate, to anatomy. His wife's health appeared to him 

 such a serious consideration that he again expressed his wish 

 to undertake the post on these terms, with certain reserva- 

 tions, but the answer was so long delayed that he gave up all 

 hopes of the appointment. 



During the summer of 1854, Helmholtz, who now devoted 

 himself almost entirely to physiological optics, had sent a 

 paper to Poggendorff, which was published in the following 

 year with the title ' On the Composition of Spectral Colours '. 

 In this he returns to the observation (erroneously stated in his 

 earlier work on compound colours, and subsequently corrected) 

 that indigo and yellow are the only complementary colours 

 in the spectrum, an assertion legitimately attacked by Grass- 

 mann in favour of Newton's earlier theories of colour-mixture. 

 The special physiological properties of the human eye, to which 

 the erroneous conclusion was due, were now submitted by 

 Helmholtz to a thorough analysis. 



Owing to the dispersion of colours in the eye, it cannot 

 be simultaneously accommodated for two kinds of rays ; if a 

 luminous point sends out red light and blue light at the same 

 time, and if the eye is accommodated for the distance of the 

 point with red illumination, blue light gives a diffusion- 

 circle, and there is either a red point in a blue circle, or 

 with reversed accomm dation a blue point in a red circle. 

 The eye can indeed be accommodated so that red and blue 

 light form diffusion-circles of equal magnitude, and there is 

 a minute speck of the mixed colour, yet it is scarcely 

 possible to fix this position of the eye when there is any 

 considerable difference in the refrangibility of the two kinds 

 of light ; whereas in the complementary colours previously ex- 

 amined by Helmholtz the difference of refrangibility is minimal, 

 and accommodation accordingly is more easily fixed. Find- 

 ing that his earlier methods only resulted in a minute area 

 covered with the mixed colour, he adopted an arrangement 

 similar to that of Foucault, in which now one and now the 



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