io8 HERMANN VON HELMHOLTZ 



that Helmholtz writes to Bonders after his autumn journey 

 of 1853:- 



'I have not yet received Dr. Cramer's treatise on the Ac- 

 commodation of the Eye, and confess that I am very curious 

 to see how much room he has left me for my own observations. 

 Your letter was the first I had heard of his work, and though 

 I regret the time lost on investigations that turn out to be 

 the property of another, I am of course only too glad to clear 

 the way as much as I can for a young man who makes his 

 debut in science with such a striking piece of work, and to 

 help him towards recognition. I had equally arrived on theo- 

 retical grounds at the idea of a simultaneous tension of the 

 radial and circular fibres of the iris, but am inclined to ascribe 

 a considerable part to the tensor choroidae also. 1 



Helmholtz employed the enforced leisure of summer to con- 

 tinue his experiments on the mixture of homogeneous colours 

 by other methods than those hitherto employed. He found, in 

 agreement with Grassmann, that besides indigo-blue and yellow, 

 another pair of complementary colours existed in the spectrum, 

 although he had not previously been able to demonstrate 

 them, and further, that all colours, with the exception of 

 green, yield simple complementary colours. In order to deter- 

 mine the breadth of the colours in the spectrum, he defined 

 white as the sensation due to the sum of the light-components 

 simultaneously perceived by the eye, together with the vivid 

 memory of such as were perceived immediately before. 



He also carried on his experiments on the time-relations 

 of excitation in man throughout the summer, arriving at the 

 conclusion that the rate of the nervous impulse in man is 

 about three times as great as in the frog. As regards the 

 initiation of electrical processes during the excitation of nerve 

 and muscle, he was able to determine 'that the electrotonic 

 condition of the nerve begins with the entry of the primary 

 current, whereas the negative variation of the muscle begins 

 appreciably later than the excitation, but precedes the first 

 trace of contraction*. 



At the beginning of August, Helmholtz left KSnigsberg, and 

 took his wife and the two children to his mother- and sister- 

 in-law at Dahlem. Du Bois-Reymond was not in Berlin, but 

 he had the satisfaction of seeing Johannes Muller, who was 



