364 HERMANN VON HELMHOLTZ 



1889, Parts VI and VII in 1892, Part VIII in 1894, and the 

 conclusion not till 1895, after his death. During this time he 

 evolved many developments and improvements of his earlier 

 theories, and again exchanged a lively correspondence with 

 many learned scientific men. On March 2, 1885, he writes to 

 Lord Rayleigh : 



1 1 have never doubted that our colour-system depended on 

 three variables, and no more. In regard to colour-blindness, 

 the recent observations of Bonders and of my assistant 

 Dr. A. Koenig show that this defect cannot be referred simply 

 to the lack of one of the fundamental colours, but that two of 

 the primaries (red and green) appear to acquire a more even 

 distribution in the spectrum, so that now one and now the 

 other makes a more vigorous impression ; in other words, the 

 resulting curve approximates now more to the red, and now to 

 the normal green sensation. In addition to this we have every 

 shade of lessened power of discrimination. Consequently 

 different individuals require very different mixtures of lithium 

 and thallium light, in order to make up sodium light. ... I am 

 much excited over the electrochemical equivalent of silver, 

 having occupied myself during the last winter with the attempt 

 to construct good methods of absolute measurement for galvanic 

 currents. ... I confess that I am getting heartily sick of giving 

 lectures. It is possible that we may be going to have a scientific 

 Physical Observatory here, as a gift from Dr. Werner Siemens, 

 with no teaching attached to it, the Direction of which has been 

 offered to me. But the matter moves all too slowly for my age, 

 which is sixty-three/ 



The close of the year 1885 brought great joy to the Helm- 

 holtz family. After long anxiety over the health of their son 

 Robert, he was able, on December 23, to take his doctor's 

 degree in Berlin, with a highly commended thesis on the ' In- 

 vestigation of Vapour and Fog, particularly those of Solutions '. 

 His essay on ' The Alterations of the Freezing-point as 

 calculated from the Vapour Tension of Ice ' was published the 

 next year, followed a few months later by his ' Experiments 

 with a Steam-Jet', published in the Annalen d. Physik u. 

 Chemie, which was very well received in the scientific world. 



Helmholtz also wrote an interesting ' Report on Sir William 

 Thomson's Mathematical and Physical Papers ' for Nature, in 



