138 HERMANN VON HELMHOLTZ 



these points of view, when you receive it, as should I think 

 be the case shortly/ And du Bois-Reymond judged it as 

 follows : ' Never had any one blended the fullest knowledge of 

 physical and mathematical optics with such a vivid and exact 

 idea of the anatomical conditions of vision as did Helmholtz/ 



Just as Helmholtz's treatise on the Law of the Conservation 

 of Energy had been epoch-making in the development of 

 the physical sciences, so his experiments on Accommodation in 

 conjunction with the Ophthalmoscope brought about a com- 

 plete revolution in ophthalmology. As his great lecture on 

 ' The Interaction of Natural Forces ' had made the principles of 

 the colossal work of his youth accessible to the scientific world 

 as a whole, so now the opportunity presented itself of bringing 

 the physiological-optical discoveries that had occupied him in 

 the past year before wider circles. On February 27, 1855, 

 at KGnigsberg, he gave a popular scientific lecture in aid of 

 the Kant Memorial, which treated of the subjectivity of the 

 sensations, and of their analogy with Kant's theory, and the 

 psychical processes that underlie the interpretation of our 

 sensations. ' Last Tuesday/ he writes to his father, ' I gave 

 another lecture upon " Human Vision ", in which I tried to 

 put forward the correspondence between the empirical facts 

 of the physiology of the sense-organs and the philosophical 

 attitude of Kant, and also of Fichte, although I was somewhat 

 hindered in my philosophical exposition by the need of making 

 it popular/ 



He sends Ludwig the following interesting account of the 

 philosophical views which then prevailed in Konigsberg : 



* In the early years of my stay, " nature-philosophy " was 

 still rampant among the students, and the scientific circles of 

 the city often took up the cudgels against my attitude. I never 

 set myself aggressively in opposition to Rosenkrantz, who had 

 once been the demi-god of the city, though now he has only 

 a very limited and half-incredulous public ; but left the weight 

 of facts to speak for itself. . . . The more intelligent portion of 

 the scientific public will only attend, as a rule, to speculative 

 investigations when they issue from men whose sound and 

 original experimental work has proved them to be firmly 

 grounded on the rock of facts/ 



But while he believed that philosophy, when it has been 



