First of all, the distinction of white and black, with their mean point in grey, is referred to the activity or inactivity of the total retina in the graduated presence or absence of full light.
Thus light, pressure, or mechanical stimulation acting on the retina and optic nerve invariably produces luminous impressions.
He seems to have been well acquainted with the projection of images of objects through small apertures, and to have been the first to show that the arrival of the image of an object at the concave surface of the common nerve - or the retina - corresponds with the passage of light from an object through an aperture in a darkened place, from which it falls upon a surface facing the aperture.
In the retina the cones prevail in numbers over the rods, as in the mammals, and their tips contain, as in other Sauropsida, coloured drops of oil, mostly red or yellow.
In hypermetropia the retina is in front of the principal focus of the eye.
The sense-organs of medusae are of two classes: (1) pigment spots, sensitive to light, termed ocelli, which may become elaborated into eye-like structures with lens, retina and vitreous body; (2) organs of the sense of balance or orientation, commonly termed otocysts or statocysts.
One result of this among the Vertebrata is that the eyeball is pink in colour, since the cornea, iris and retina being transparent, the red blood contained in the capillaries is unmasked by the absence of pigmentary material.
They are at the same time both optic nerve-end cells, that is to say, retina cells, and corneagen cells or secretors of the chitinous lens-like cornea.
This theory of complementary colours as due to the polarity in the qualitative action of the retina is followed by some criticism of Newton and the seven colours, by an attempt to explain some facts noted by Goethe, and by some reference to the external stimuli which cause colour.
When, for instance, the axons of the ganglion cells of the retina are severed by section of the optic nerve, and thus their influence upon the nerve cells of the visual cerebral centres is set aside, the nerve cells of those centres undergo secondary atrophy (Gadden's atrophy).