PROFESSOR AT HEIDELBERG 231 



till now I had no suspicion of the effects that could be produced 

 by such an instrument, in regard to mass and power, as well as 

 to variety of timbre/ 



Part III of Physiological Optics was to appear in the next 

 year, 1866, and Helmholtz was constrained to hasten the publica- 

 tion of this last portion, in order not again to omit a mass of 

 new results by other workers, in connexion with his own 

 researches, as had occurred with the first two separately 

 published sections. Great inconvenience had arisen from the 

 fact of the work being published as a part of Karsten's Allgemeine 

 Encydopddie der Physik. 



Du Bois writes : ' I never open your Optics without getting 

 angry at your having let yourself in for fathering the still-born 

 projects of Karsten, which in the first place damaged the 

 circulation of the book, and in the second, compelled you to use 

 a form that by no means makes it more lucid, or easier to under- 

 stand. The colossal pages of the closest print, crammed with 

 the most abstruse matter, give one no resting-place, and any- 

 thing you have written hardly needs to appear in small print.' 



It was not until the work had appeared independently as the 

 Handbook of Physiological Optics that du Bois was able to write 

 to him on April 25, 1867 : 



' The book will only produce the greatest part of its effect 

 now, when it conies into the market freely, as a whole. In my 

 own laboratory, for example, the young people like Rosenthal 

 and Hermann hardly know it at all, since it is by no means the 

 sort of book one can work through in the time for which one 

 can decently borrow it/ 



Helmholtz found it a severe task to incorporate the new 

 matter, and to utilize it for Part III, and for his full Bibliography. 



' How delightful the state of a learned theologian, jurist, or 

 historian must be, who spends his whole life in bringing out 

 new editions of the same book with minute alterations, while we 

 poor men of science cannot get one work ready before the 

 beginning of it is already out of date/ he complains to Bonders: 

 but he does not falter ; and most of the facts and theories that 

 had become known were submitted to a searching criticism. 



The whole of the year 1865 was thus devoted to the pre- 

 paration of Part III of Physiological Optics, a. gigantic task that 

 tried his health severely. His persistent attacks of migraine 



