HELMHOLTZ IN HEIDELBERG 



duced when one stereoscopic picture is white and the 

 other grey, shows also that the impressions on the two 

 retinas are not combined, in this case, into one sensa- 

 tion. There is a rivalry of the fields of visions, best 

 seen in the rivalry of colours, when one stereoscopic 

 picture is red and the other blue. A true combina- 

 tion purple is not produced, but there is a peculiar 

 'sheen,' and the red at one moment has the pre- 

 dominance, and the instant after it is the blue. 



The whole of this beautiful research is a good 

 illustration of the method of Helmholtz. Complicated 

 as the movements of the eyeball apparently are, they 

 become simple when we consider that they are just 

 the movements that are necessary to see single objects 

 with two eyes. It was this simple principle that 

 guided Helmholtz. Only those, however, who have 

 read the chapters on the subject in his Physiological 

 Optics can form a conception of the amount of work 

 expended upon it. The bibliography alone is a model 

 of literary research. 



In 1869, he investigated the cause of hay-fever, a 

 troublesome affection to which he was susceptible, 

 and which interfered much with his pleasure during 

 holiday time. He announced the discovery in a letter 

 to Binz, who secured its insertion in Virchow's Archiv 

 fur pathologische Anatomie. Helmholtz found in the 

 mucus of the nasal secretions of persons affected by 

 this disease micro-organisms of a vegetable nature, 

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