Page:Popular Science Monthly Volume 1.djvu/473
SIGHT AND THE VISUAL ORGAN.
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 nature of the irritation. Squeezing, pinching, pulling, chemical or electric 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.
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.