HELMHOLTZ IN KONIGSBERG 



essentially new method for illuminating and examining 

 the retina. 



Of the light which strikes on the retina, one part is 

 absorbed, and that principally by the black pigment of 

 the choroid, and the other part is diffusely reflected, 

 and returns from the eye through the pupil. In ordin- 

 ary circumstances, we do not perceive the light which 

 issues from the pupil ; this opening, on the contrary, 

 appears jet black. We must look for the explanation 

 in the particular conditions of refraction in the eye, 

 and we must also remember that, owing to the pig- 

 mentation of the eye, only a small amount of light is 

 returned from it. In all systems of refractive surfaces 

 (lenses), which form an image of a luminous point, 

 the rays may be traced from the image to the lumin- 

 ous point, traversing exactly the same path as they 

 followed in passingfrom the luminous point totheimage. 

 In other words, if we put the luminous point in the 

 position first occupied by the image, the image will 

 now be formed in the place previously occupied by the 

 luminous point. 



The result is as follows : When the human eye is 

 exactly accommodated for a luminous point, and forms 

 an exact image of the point on the retina, if we con- 

 sider the illuminated part of the retina as a second 

 luminous object, the image formed by the media of the 

 eye coincides exactly with the body given ; so, in 

 front of the eye, all the light which returns from the 

 retina is directed towards the luminous body, and it 

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