144 HERMANN VON HELMHOLTZ 



holtz's anatomical observations as reported to him by du Bois. 

 In Berlin, for instance, the young surgeon had amused himself, 

 in the intervals of keen mental activity, by watching the move- 

 ments of the people going in and out through the Branden- 

 burg Gate, with a telescope, from his little laboratory in a tower 

 at the corner of the Dorotheen-strasse and Sommer-strasse, 

 comparing them with the descriptions and figures given by 

 Weber in his work on the human locomotor apparatus. In this 

 way he discovered, as du Bois relates, that there was an error 

 of some practical importance in Weber's figures, in consequence 

 of which thousands of recruits had been compelled into an 

 unnatural position of the foot during their march on parade. 

 His observations were long afterwards confirmed by instan- 

 taneous photography. 



On March 27, his appointment as Professor of Anatomy and 

 Physiology in Bonn, from Michaelmas, 1855, was announced. 



During the summer Helmholtz devoted himself almost ex- 

 clusively to his Handbook of Physiological Optics, which was 

 to be given to the printers early in the winter, and he writes 

 of the finished portion to Ludwig : ' The only really new bit 

 of mathematics in Part I of Physiological Optics is the proof 

 of Gauss's laws of principal points and nodal points by 

 means of an accessory theorem (p. 50), which finds a useful 

 application in the theory of the ophthalmoscope also/ 



During his last days at Konigsberg he received an invitation 

 from William Thomson, now Lord Kelvin, from Kreuznach, 

 to attend the British Association in September. Thomson 

 wrote that his presence would be one of the most interesting 

 events of the meeting, so that he hoped to see him on this 

 ground, but also looked forward with the greatest pleasure 

 to such an opportunity of making his acquaintance, as he had 

 desired this ever since the ( Conservation of Energy ' had come 

 into his hands ; he ended with expressing his deep regret that 

 he had not been present at the Hull Meeting, having only 

 heard later that Helmholtz had been there. 



On July 29, Helmholtz left Konigsberg, and went, after a short 

 visit to his relations in Dahlem and Potsdam, to Bonn, where 

 he found a suitable and healthy dwelling for his family in the 

 building that had formerly been the summer residence of 

 the Ecclesiastical Elector of Cologne, and was accordingly 



