adjacent parts. Somehow, in a way that is not fully understood, the brain processes this information to deduce
the color of the object, largely independent of the color of the light source. This phenomenon is called the
Land effect, and you can see it for yourself: check the color of some object under fluorescent light indoors,
and compare it with the color when seen outdoors in sunlight. Its color will not appear to have changed, even
though fluorescent lights are tinted somewhat blue and sunlight is somewhat redder.
The human eye is capable ofaccommodation, meaning that it able to use muscles to automatically focus
images. Sometimes this doesn’t work properly, though. If the image is formedbeforereaching the retina, the
person has a condition known asmyopia,ornearsightedness. For people with this condition, closeup objects
appear in focus, but distant images are fuzzy and out of focus. This condition may be corrected by placing
diverging (concave) lenses in front of the eye, either with eyeglasses or contact lenses.
If the image has not yet formed when light reaches the retina, then the person has a condition called
hyperopia,orfarsightedness. For people with this condition, distance objects are clear, but closeup objects
are out of focus. This condition may be correced by placing converging (convex) lenses in front of the eye.
A condition of perfect vision (neither myopia nor hyperopia) is calledemmetropia.
Many people upon reaching their 40s have difficulty focusing on closeup objects because the eye’s accom-
modation abilities are not as robust as they were during youth — a condition calledpresbyopia. Older people
often require converging lenses (reading glasses) to see close objects. People who have presbyopiaandeither
myopia or hyperopia often wear either reading glasses with contact lenses, or eyeglasses withbifocallenses,
which are shaped so that looking through the top half of the lens corrects for distance vision, while looking
through the lower half corrects for close-up vision. Somewhat less common aretrifocallenses, which correct
for distant, mid-range, and close-up vision when looking through the top, middle, and bottom of the lenses,
respectively. A recent innovation being offered by ophthalmologists and optometrists iscomputer glasses,
which are similar to reading glasses, but designed to help the wearer focus clearly at the typical distance of a
computer monitor.
49.3 The Trilobite Eye
Trilobitesare an extinct class of arthropods that were among the first living organisms on Earth. Trilobites
pre-dated the dinosaurs; they lived from the early Cambrian period (about 550 million years ago) until the
great Permian extinction of 250 million years ago, which almost wiped out all life on Earth.^1 ;^2 (See Figure
49.3.) Trilobite fossils can be found in great numbers, and range in size from 1 millimeter to as much as 2
feet long.
Most species of trilobites had a pair ofcompound eyes, similar to those found on many species of insects
today. A compound eye consists of a grid of a large number of very small lenses, all spaced very closely
together. Unlike the flexible lens of the human eye, though, trilobite eyes had rigid lenses composed of the
crystalline mineral calcite, and thus lacked the ability of accommodation that human eyes have.
To minimize the effect of optical aberrations, trilobites developed an eye lens in a shape that tended
to minimize spherical aberrations. These shapes bear a remarkable resemblance to minimum-aberration
lens designs developed by French mathematician and philosopher Ren ́e Descartes (Fig. 49.4) and by Dutch
mathematician and physicist Christiaan Huygens (Fig. 49.5).^3
(^1) The cause of the Permian extinction is not known.
(^2) Some paleontologists believe there may be some small chance that trilobites may still be alive even today, in some unexplored
depths of the oceans.
(^3) The Descartes and Huygens minimum-aberration lens designs are based on a mathematical curve now called theoval of Descartes.
See E.N.C. Clarkson and R. Levi-Setti, “Trilobite eyes and the optics of Des Cartes and Huygens”;Nature, 254 , 663–667 (1975).