PROFESSOR AT KONIGSBERG 75 



velocity of nervous impulses. The ophthalmoscope became 

 the most popular of my scientific achievements, but I have 

 already pointed out to the oculists that good fortune had more 

 to do with it than merit. I had to explain the theory of the 

 emission of reflected light from the eye, as discovered by 

 Briicke, to my students. Briicke himself was but a hair's 

 breadth off the discovery of the ophthalmoscope. He had only 

 neglected to ask himself what optical image was formed by the 

 rays reflected from the luminous eye. For his purpose it was 

 not necessary to put this question. Had it occurred to him, 

 he was just the man to answer it as quickly as I, and to invent 

 the ophthalmoscope. I was turning the problem over and over, 

 and pondering the simplest way of making it clear to my 

 audience, when I came on the further issue. 



' The oculist's perplexity in dealing with the condition known 

 at that time as black cataract was familiar to me from my 

 medical studies, and I at once set to work to manufacture the 

 instrument out of spectacle lenses and the cover-glasses used 

 for microscopical objects. At first, however, it was very diffi- 

 cult to use. I might not have persevered save for my convic- 

 tion that it must succeed; but after about eight days I had 

 the great joy of being the first to see a living human retina 

 exposed before me/ 



As a matter of fact the discovery of the ophthalmoscope had 

 not been quite such a simple invention as Helmholtz describes 

 it. The principle underlying the apparatus was difficult to 

 grasp without considerable knowledge of optics, and its intro- 

 duction was therefore a comparatively slow matter, and was 

 delayed until improved mechanical conditions rendered the 

 handling of it much simpler although Bonders, the cele- 

 brated physiologist at Utrecht, held the original form of 

 Helmholtz's instrument to be optically perfect. 



The familiar fact that the eyes of certain animals, such as 

 cats and owls, glisten in the dark, had already been correctly 

 interpreted by Johannes Muller to mean that these so-called 

 'glowing' eyes do not really glow but only reflect light, 

 and that the retina of the eyes that glisten most are pro- 

 vided with a background specially adapted for the reflection 

 of light. Brucke had shown that the eyes of animals seem 

 to glisten most when the beam of a dark-lantern is thrown 



