92 HERMANN VON HELMHOLTZ 



While Helmholtz was engaged upon his inaugural disserta- 

 tion (the scope of which grew wider and deeper, until it 

 eventually took shape as a searching critique of Brewster's 

 contributions to optics), du Bois-Reymond, who had lately 

 returned in such good spirits from England that he warns 

 Helmholtz 'not to go to England, it spoils one's taste for 

 Germany', advises his friend on June 15, that he had told 

 Sir David Brewster about the ophthalmoscope, and that if 

 a copy of Helmholtz's paper on it were sent, Sir David would 

 be responsible for an English translation. Helmholtz replies 

 in a few days: 



' I should imagine that you would like England under such 

 conditions. I can hardly avail myself of Brewster's proposal 

 to assist in the preparation of an English translation of The 

 Ophthalmoscope, because the second part of the essay on 

 physiological optics which I intend to use for my disserta- 

 tion, and which I shall give as a lecture, and then send to 

 Poggendorff, is intended as a contradiction of Brewster's 

 analysis of solar light, a theory which he has much at heart, 

 and has defended with some heat. His observations on this 

 subject are perfectly correct, but the alteration of the colours 

 of the spectrum by absorbing media depends mostly upon 

 subjective phenomena, contrast and the like, as may be 

 proved convincingly. My treatise is of course written as 

 cautiously as possible, but still I fear that Brewster may 

 take it amiss.' 



On June 28, 1852, Helmholtz delivered his Inaugural Lecture 

 'On the Nature of Human Sense- Perceptions ', in which he 

 not only displayed his unique gift for making the stiffest 

 scientific problems intelligible by a lucid and exceptionally 

 beautiful exposition, but once more opened up new fields 

 of inquiry, lying ' nearer the limits of human knowledge '. It 

 was not until a much later time, after the publication of his 

 Physiological Optics, that the full import of the physical, 

 physiological, and epistemological discoveries which he had 

 even then made could be recognized. It was this lecture 

 that at last won him the complete approval of his critical 

 father: 



'Thanks for the Inaugural Lecture sent me by Dr. Fried- 

 lander, which pleased me greatly by its clearness and its 



