HELMHOLTZ IN KONIGSBERG 



In the same way the objective of a camera appears 

 black in a dark room when we throw on it the 

 image of a single flame. If the eye observed is, 

 on the contrary, neither exactly accommodated for 

 the luminous body, nor for the pupil of the observer, 

 it is possible that we may perceive a little of the light 

 which emerges from the observed eye, and the pupil 

 may then appear to be luminous. It is easy to see 

 that the observer may receive light coming from all 

 points of the retina of the eye under observation, on 

 which falls the diffusive image of his own pupil. If 

 we substitute a luminous disc for the pupil of the 

 observer, the image of diffusion formed of this disc 

 in the eye observed will coincide exactly with that of 

 the pupil of the observer, for luminous rays will radiate 

 from one or many points of the disc to each point of 

 its image of diffusion ; then, conversely, the rays given 

 off from each point of the circle of diffusion will reach 

 one or many points of the luminous disc, that is to 

 say, the pupil of the observer. The eye observed 

 appears luminous when the image of diffusion of the 

 pupil of the observer coincides with that of a luminous 

 object in the eye observed. If then we look at an 

 eye by a light from a flame from which we have shut 

 off, by an opaque screen, the rays that dazzle us, 

 whether the eye observed is accommodated for a distant 

 or near object, its pupil will appear illuminated in red. 

 During the experiment accommodation must be 

 at rest, if the observer is far away, because the 

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