SIGHT AND THE VISUAL ORGAN. 459 



sole source of conscious impressions. It is the same irritation coming in 

 contact with the organ of consciousness, which we observe in the 

 nerves of touch, with this difference that, in the above case, the quality 

 of feeling differs ; it is luminous or colored. The mere producing of 

 this sensation of the luminous does not in any way depend on the na- 

 ture of the irritation. Squeezing, pinching, pulling, chemical or elec- 

 tric irritations which give rise to the sensations of warmth or pain in a 

 nerve of touch, call forth in the optic apparatus, by virtue of its specific 

 sensitiveness, only a feeling of light, accompanied by neither pain nor 

 heat. 



Fro. 1. 



A, Brain ; B, Visual Nerve ; C, Retina ; X, Boot of the Optic or Visual Nerve. 



You ask how men have arrived at the knowledge of these things, 

 seeing the mechanism in question is almost entirely removed from any 

 direct investigation. First, then, the umbellar expansion of the optic 

 nerve, the retina, enables us to make experiments ; this retina being in 

 such close contact with the eye, the optical part of the visual organ, 

 that it is accessible to every sort of mechanical irritation. You have 

 yourselves, consciously or unconsciously, often made such experiments, 

 when you watched the circles and sparks of fire and light, which become 

 visible on rubbing or pressing your eyes through their lids, or striking 

 them with a hard substance. Here the eye, as an optical apparatus, 

 remains passive. Just as a man, who sees, is aware of the phenomena 

 even in the deepest darkness, so is also a blind man, as long as the 

 retina is endowed with its specific sensitiveness or sensory power, by 

 which it responds to every irritation with a luminous sensation. Even 

 after blindness, this dancing of sparks of fire and light may be kept 

 up by continual irritation in the eye to such a tormenting extent, that, in 

 order to prevent it, we usually cut the optic nerve just behind the eye, 

 when the sparkling and scintillating cease. 



