Favorite tidbits:
The occasional rapid eye movements that accompany our dreams have at least one very practical purpose: they feed the front of the eye.
Daylight on Earth is a million times stronger than dim starlight, and night rapidly follows day. Eyes must not only feed on light; they must respond to extremes of glut and starvation. ... Our pupils, evolved for daylight living, dilate to four times their constricted width when the light grows dim, granting a 16x increase in their light-gathering powers. ... Although we lack certain exotic adaptations that can aid nocturnal vision, we can still detect light that is a billionth the strength of daylight. This is sensitive enough that, in good conditions, we could see the flame of a single candle 17 miles away.
A camouflaged object in dappled light is immediately noticed by a color-blind person: its distinctive sheen gives it away.
The grass IS always greener on the other side of the fence, because individual blades of grass are more clearly seen from the side than from above.
The eye exists to detect movement. Any image, perfectly stabilized on the retina, vanishes. Our eyes cannot see stationary objects, and must tremble constantly to bring them into view. ... Just 1 degree away from the very center of my vision, I see things only 1/2 as well. 5 degrees off center, my "visual acuity" is quartered. Beyond that 5 degree radius, I can no longer be sure of what I am seeing. At 20 degrees from the foveal center, my visual acuity falls below the common legal standard for blindness. ... Were every photoreceptor connected to the brain by a nerve fiber, the optic nerve would be as thick as the eyeball, making it impossible to move. ... There are more than 40 different types of nerve cell in the retina, each with their own "field of view." There are cells which edit the responses of photoreceptors and bipolar cells that compress that information by factors of 12-1000, depending on the light level; no one knows how this adjustment is made. Bipolar cells pass their information on to the 6 layers of ganglion cells, each layer connected with the others by at least 30 different types of amacrine cell. There, the information is once again savagely edited, until only a million signals remain to be carried along the optic nerve to the brain. The lateral geniculate nucleus treats the information it receives much as the horseshoe crab's retina treats light. Contrast is everything. I compares signals from cones & enhances the differences.
Evolution cannot un-invent. The basic building blocks of complex structures like the eye were available long before they were recruited for sophisticated tasks like vision. Human lenses are derived from proteins that protect bacteria from rapid rises in temperature. The flexible guanine mirrors which make a cat's eye glow in the dark also gas-proof the swim bladders of fish. The red & yellow pigments in the eyes of insects also color the wings of butterflies. ... And is it not ironic, that in 538 million years of natural selection, eyesight should evolve from a simple light-detecting cell, pass through numerous variations and generate countless different ways of seeing, and come at last to serve as the dominant sense of the planet's dominant species - an animal who sees only what it wants to see?
We are the products of editing rather than authorship.
Many women produce monsters: they just never know it. Human infants whose body plan is in disarray rarely make it out of the womb alive; really gross errors crop up so early in the development cycle that the fetus is spontaneously aborted long before the mother is aware of being pregnant. Over 1/2 of all pregnancies go disastrously wrong - we just never notice them.
The eyes of men converse as much as their tongues. ... How can we fail to shudder at an eye that cannot weep, turn, or even blink?
A hedgehog can attempt to seduce a toilet brush, and its long-term breeding chances are not much affected. For a human being, a mistake of this scale would be catastrophic. ... We are the moodiest animal on the planet, deploying, with great finesse and subtlety, every waking moment, behaviors that in other animals are crude, last-ditch, and above all, short-lived bursts of anomalous behavior.
Every light is a shade, compared to the higher lights, till you come to the sun; and every shade is a light, compared to the deeper shades, till you come to the night. ... The most isolated societies do little more than distinguish light from dark. As societies emerge into the wider world, red is the first "true" color they identify, followed by a greenish-yellow. Green & yellow are invariably distinguished before blue is distinguished from black. The rest follow in no particular order.