76 HERMANN VON HELMHOLTZ 



into the eye that is to be examined, the observer looking 

 past it. All eyes can be made to glisten, both in animals and 

 in man. The first human eye purposely made to shine in the 

 dark was du Bois-Reymond's, illuminated by Briicke. Briicke's 

 subsequent attempts at constructing an instrument for the 

 illumination of the retina failed (according to Graefe) on account 

 of the mode of illumination he adopted. 



Helmholtz asked himself in the first place, as he related 

 in the monograph published in Berlin, 1851, why all that we 

 can see of the background of the uninjured eye appears 

 absolutely dark; he ascribed this to the refractive media of 

 the eye, which under normal circumstances prevent us from 

 seeing illuminated points of the retina behind the pupil. The 

 first requisite therefore was to find a source of illumination 

 by which just that portion of the retina that we see through 

 the pupil may be adequately illuminated. By means of a little 

 camera obscura blackened inside he proved by calculation and 

 experiment that for any system of refractive surfaces, the 

 reflected rays, even when they have passed through the re- 

 fracting media and left the eye, must be wholly congruent 

 with the incident rays, and that they all return ultimately to 

 the original point of illumination. 



Since the observer's eye cannot be brought into line with 

 the reflected light without intercepting the incident rays, no 

 light can be returned to his pupil from the recesses of the 

 observed eye that has not emanated from his own. Only 

 those points of the retina will accordingly be visible on which 

 the dark image of his own eye is projected. Further, if the 

 observed eye is not a perfect refracting system, part of the 

 light reflected from it will indeed return to the luminous 

 point, but part passes by it, and it becomes possible for an 

 observer, placed as nearly as possible in the line of the incident 

 light, to perceive a little of the light that emerges ; and this 

 produces the luminosity of the human eye as discovered by 

 Briicke. In order to obtain an exact image, some method is 

 required which makes it possible to look into the eye not only 

 in the approximate, but in the exact line of the incident light, 

 and Helmholtz found, in trying to make the image as bright 

 as possible, that this can be done by superposing three parallel 

 glass plates, in which the light from a source of illumination 



