HERMANN VON HELMHOLTZ 



does not pass by the side of this body. To receive a 

 part of this light, it is necessary that the eye of the 

 observer be placed between the luminous body and the 

 illuminated eye ; this evidently cannot be done with- 

 out intercepting the light which goes to the illumin- 

 ated eye, unless we employ a special device. An 

 observer, moreover, cannot see the light returned from 

 one eye into the other if the last is exactly accommo- 

 dated for the pupil of the observer. In these circum- 

 stances, there is formed on the retina of the eye observed 

 an exact but dim image of the pupil of the observer. 

 Conversely, the media of the eye under observation 

 forms precisely on the pupil of the observer an exact 

 image of its retinal image, and, consequently, the 

 observer can only see in the eye of the other the 

 reflection of his own black pupil. This explains how, 

 in ordinary circumstances, we cannot see the fundus 

 of the eye we look at, and how we cannot distinguish 

 even the parts which reflect the light the most strongly, 

 such as the point of entrance, generally white, of the 

 optic nerve or the blood vessels. The pupil appears 

 black even in albinos (subjects in whom the choroid 

 has no pigment), if we take the precaution to inter- 

 pose a black card, having a hole in it the diameter 

 of the pupil, and thus prevent the light from 

 penetrating into the eye through the sclerotic, an 

 observation first made by Donders. It is, in reality, 

 the light which passes through the sclerotic which 

 gives to the pupil of albinos its well-known red colour. 

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