224 HERMANN VON HELMHOLTZ 



' Thomson's experiments, however, did for my new hat. He 

 had thrown a heavy metal disk into very rapid rotation ; and it 

 was revolving on a point. In order to show me how rigid it 

 became in its rotation, he hit it with an iron hammer, but the 

 disk resented this, and it flew off in one direction, and the iron 

 foot on which it was revolving in another, carrying my hat away 

 with it and ripping it up. 



' I got to Manchester on April 4 ; the Roscoes live outside in 

 a charming cottage at the edge of a great park. Roscoe had 

 two friends to dinner Mr. Joule, a brewer and the chief dis- 

 coverer of the conservation of energy, and his colleague, Clifton, 

 a physicist, who were both very pleasant, lively individuals, so 

 we spent a most interesting evening. On Sunday morning we 

 were alone after breakfast, and boldly planned out new ventures 

 in physical chemistry: we discussed the English Universities, 

 and were both of the same mind. 



' Yesterday, in London, I went to see Mr. Graham, the Master 

 of the Mint, one of the first English chemists, who took me 

 round himself and explained everything to me. I was the most 

 interested in Graham's own laboratory, where he showed me 

 a quantity of marvellous new experiments, and presented me 

 with coins, instruments, and chemicals. Then I went with an 

 old Berlin friend to Kensington, to see Prof. Clerk Maxwell, 

 the physicist at King's College, a keen mathematician, who 

 showed me some fine apparatus for the Theory of Colours which 

 I used to work at ; he had invited a colour-blind colleague, on 

 whom we experimented.' 



These many-sided interests, and the absorbing work of 

 preparing his Croonian and other lectures, were darkened by 

 the first shadow of the fatal illness of his son Robert. But his 

 wife worded her letters so as to keep Helmholtz from any 

 immediate return to Heidelberg, and he hoped that his own 

 advice and that of the friendly physicians attending the boy 

 might avert the danger. 



On April 14, 1864, Helmholtz gave his Croonian Lecture to 

 the Royal Society, ' On the Normal Motions of the Human Eye 

 in relation to Binocular Vision,' in which he sketched out his 

 conclusions in regard to the horopter, and the movements of 

 the eye. 



' It was ten before I had finished the first part of my lecture. 



