78 HERMANN VON HELMHOLTZ 



to any sensation, but that it must fall on a nerve-ending in 

 the retina before it can be perceived. He then returned to 

 the work that had been so happily interrupted, on the stimula- 

 tion of nerve. In his earlier experiments he had assumed the 

 induction shock by which the nerve was excited to be instan- 

 taneous, and he now proceeded, in order to justify this view 

 for the minute time-intervals involved, to answer the question 

 previously raised as to the exact time at which an induction 

 current produces its physiological effect. It was important 

 not to ascribe to some action in the nerve a loss of time 

 which might have occurred in the electrical apparatus. This 

 is especially true of experiments on man, when the currents 

 are of such a strength that serious errors might creep in. In 

 the middle of April he sent du Bois a short note on this 

 physical preliminary to his further work on nerve, for the 

 Academy, together with the longer paper for Poggendorjfs 

 Annalen. Du Bois-Reymond replies: 



' My brain reels at your appalling industry and encyclopaedic 

 knowledge. How can you get up new lectures and still carry 

 on all this work ? All the same I am not quite satisfied with 

 your exposition. I have read your essay and the abstract 

 several times without understanding what you did, or how 

 you did it. At last I discovered the method for myself, and 

 then I saw what you are driving at. Don't be vexed if I say 

 that you must take more pains to get away from your own 

 standpoint, and place yourself on the level of those who do 

 not yet know what it is all about, and what you want to 

 tell them/ 



This reproach, however, was unjustified, for in the nature 

 of the case the treatise presupposed that its readers would 

 be trained physicists and mathematicians. Both then and 

 later Helmholtz was in the habit of writing and re- writing 

 many parts of his papers four and even six times, altering 

 the arrangement before he was satisfied, and never holding 

 an investigation to be finished till it presented itself to him 

 in logical completeness, correctly formulated. Accordingly he 

 replies : ' As regards the form of the essay, I took particular 

 pains with it, and finally satisfied myself. But it is true that 

 the more one elaborates a thing the harder it often becomes 

 to understand. It is a very difficult subject to deal with.' 



