PROFESSOR AT HEIDELBERG 235 



and in this he was not deceived. His wife too, although she 

 was of South German extraction, embraced the cause of Prussia 

 with enthusiasm : on July 12 she writes to her mother, ' Every 

 right-minded person is Prussian, since Austria has made this 

 French alliance.' 



Helmholtz's dearest wishes were fulfilled sooner than could 

 have been anticipated, and he returned with fresh zest and 

 courage to his great work on visual perception, which formed 

 part of the Physiological Optics. 



His earliest observations in optics and acoustics had taught 

 him that besides the sensations of the nervous apparatus there 

 enters into our sense-perceptions the further factor of a specific 

 psychical activity, which co-operates in the representation of 

 the external object that has excited our sensation. In his 

 lecture on Kant he had already, in agreement with Lotze, 

 treated the impressions made upon our sensory nerves as 

 being merely the signs of certain external objects ; holding that 

 correct inferences from the sensations to the corresponding 

 objects had arisen through experience. His observations on the 

 blind spot in the eye, on over-tones, &c., now brought in a new 

 point the recognition of a law that is valid for all our sense- 

 perceptions, viz. that we attend to our sensations only in so 

 far as they enable us to recognize external objects, while we 

 do not analyse such sensations as have no direct relation with 

 external objects, until we begin to investigate our impressions 

 scientifically. Helmholtz now went on to the difficult problem 

 of the nature of the correspondence between the percept and 

 its object, in other words, what kind of truth are we to ascribe 

 to our ideas and perceptions ? 



Just as the sensations of the eye, ear, and tactile sensibility 

 are intrinsically so different that no comparison in regard to 

 quality and intensity can be made between those of different 

 senses (this is called a difference in the mode of the sensation, 

 while the disparity between homogeneous sensations is 

 described as one of quality) : so the same is the case if a com- 

 parison between the percepts of psychical states (which Kant 

 refers to a special sense, the innate or intuitive) and those 

 of the eye or ear be attempted. Yet in spite of many differences 

 they have one thing in common, that the percepts of the 

 internal as of the external senses are arranged in time-sequence 



