Among the phenomena, all of which constitute the sight, some belong to the domain of physics, and may be submitted to investigation, many may even be demonstrated by experiment; while others, on the contrary, are patent to the observation, but little known as to their cause or their mechanism, and await from the progress of science an explanation which physiology has as yet been unable to give. Even in those phenomena which at first seem purely physical, we must not forget that the refracting media of the eye are organized, and cannot be compared except by approximation to inorganic bodies, on the form and density of which physicists base their calculations. This is necessarily the cause of the differences in the theories promulgated in regard to vision; for although the eye may in some respects be considered as an optical instrument, we can never arrive at exact deductions by comparing organs analogous or even similar in their construction, but different in their nature.
The retina renders the eye sensible of light, and we may therefore consider it as the essential organ of vision. The function of the other portions is to convey the luminous rays to its surface under conditions necessary to a nervous impression, which all combine to insure, but which is accomplished in the retina alone. Other causes besides the contact of luminous waves may excite the retina; thus the pressure of the finger on the eye for example, and the disturbance resulting from a fall or a blow on the head, the action of electricity, and certain affections of the eye and brain, give rise, in the absence of natural or artificial light, to luminous images varying in form and intensity. Light produced under these conditions is called "retinal light"
Like the optic and the other special nerves of the organs of sense, the retina has a special sensibility; it receives the impression from the light and transmits it to the brain, but it is not itself sensitive to touch. No mechanical irritation causes it the slightest pain. In a normal condition, the action of a too brilliant light, and in certain affections of the eye and brain, the least ray will cause a painful sensation, but this pain must be referred either to the encephalon or to the nerves of the iris or of the ciliary circle, independent of the retina and the optic nerve.