4o6 HERMANN VON HELMHOLTZ 



the last few weeks by Dr. Lenard, my Assistant. He covered 

 some Geissler's Tubes with excessively thin plates of alumi- 

 nium, and succeeded in obtaining plates of such a thickness 

 that they are completely air-tight, and are yet so thin that 

 a perceptible portion of the kathode rays that excite phosphor- 

 escence can traverse them. He then found that these rays, 

 once generated, can be propagated in spaces filled with gas 

 with greater or less ease in different gases, which opens up 

 a whole new field of research, since the production of these 

 rays can now be entirely separated from their observation. 

 I have advised him to mark the importance of his results by 

 sending a short report to the Berlin Academy. I hope it 

 may be considered worthy of acceptance for the Proceedings.' 



Helmholtz was greatly distressed by the conviction that 

 Hertz was rapidly approaching his end. 



On December 6, 1892, a heavy blow descended on him 

 in the death of his faithful friend, Werner von Siemens. The 

 noble character and sympathy of this distinguished man (who 

 rightly stated in his Reminiscences, that he had never under- 

 taken any work with the sole object of enriching himself, but 

 had always kept the common good in view) had been an abiding 

 stimulus to Helmholtz. His immense energy, and invariable 

 success in bringing high and ideal aims into the realities of 

 practical life, had supported Helmholtz throughout his whole 

 life, and had but recently prepared the ground for his present 

 labours. The irreparable loss of this gifted and practical 

 friend brought isolation to Helmholtz in many a department 

 of intellectual work, as well as in the intercourse of daily life. 



Close on this followed anxiety for the health of his son 

 Fritz, whose recurrent bodily suffering paralysed his energy, 

 and permanently obstructed his mental development. Helmholtz 

 was once more driven to find calm and resignation to his lot in 

 arduous intellectual labour. 



He took the keenest interest in all new discoveries and 

 researches. Thus on November 20 he writes to his old 

 Heidelberg pupil, Lippmann, who had sent his colour-photo- 

 graphs from Paris : 



'I had not previously seen any proofs of your famous 

 invention, and am amazed at the saturation and depth of these 

 colours. . . . The principles of your theoretical account of the 



