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That a change in the germ, which influences the formation of the retina, may affect at the same time also the formation of the cornea, the iris, the lens, the visual centres, etc., I admit, if necessary, although they are formations that differ much more from one another in their original nature than do probably hair and teeth.
But when I say that the eye "makes use of" light, I do not merely mean that the eye is capable of seeing; I allude to the very precise relations that exist between this organ and the apparatus of locomotion. The retina of vertebrates is prolonged in an optic nerve, which, again, is continued by cerebral centres connected with motor mechanisms.
A subjective visual sensation, arising from certain conditions in the retina and connected portions of the optic nerve, may by chance resemble a familiar impression, and so be at once interpreted as an effect of a particular external object. More frequently, however, the automatic activity of the centres must be regarded, either in part or altogether, as the physiological cause of the phenomenon.
When man is not endowed with inner perception, he is not informed of these important mysteries. The retina of outer vision, though sensitive and delicate, may, nevertheless, be a hindrance to the inner eye which alone can perceive.
For the same reason, the outstretched legs of the horse going up to their limit and at once returning give in very quick succession, near their extreme limit, an ascending and a descending phase which are not strictly but sensibly alike, and so doubly impress the retina, and obtain for the legs "attention" when in that extreme position.
Make with ink on white paper a very black spot about half an inch in diameter, with a tail about an inch in length, so as to resemble a tadpole, as in Plate II.; look steadfastly for a minute on the center of this spot, and, on moving the eye a little, the figure of the tadpole will be seen on the white part of the paper; which figure of the tadpole will appear more luminous than the other part of the white paper; which can only be explained by supposing that a part of the retina, on which the tadpole was delineated, to have become more sensible to light than the other parts of it, which were exposed to the white paper; and not from any idea of mechanical impression or chemical combination of light with the retina.
Everything is confused in my wearied retina. PROMETHEUS. But nothing pained you personally; and what about me, from time immemorial suffering the wrath of the gods? AHASVERUS. You? PROMETHEUS. My name is Prometheus. AHASVERUS. You! Prometheus! PROMETHEUS. And what was my crime? Out of clay and water I made the first men, and afterwards, seized with compassion, I stole for them fire from the sky.
'Thank God! said the old man: 'your journey shall be prosperous such is the will of Heaven! The experiment was tried on many other occasions by the young lady, and always with similar success, although never without a certain degree of trepidation, even after she had learned that the spectral appearance in the heavens was nothing more than the picture retained on the retina of the eye.
As he observes, the apparent rays of light, light-patches, mists of light, and so on, due to changes of blood-pressure in the retina, only manifest themselves clearly when the eyes are closed and the more powerful effect of the external stimulus cut off.
Of these are all the irritative ideas already explained, as when the retina of the eye imitates by its action or configuration the tree or the bench, which I shun in walking past without attending to them.
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