Devices When Oedipus was walking to the city of Thebes, he met the Sphinx who

4.3.3 Devices When Oedipus was walking to the city of Thebes, he met the Sphinx who

demanded an answer to her riddle: “What walks on four feet in the morning, two in the afternoon, and three at night?” Oedipus answered: “Man—as an infant crawls on all fours, an adult walks on two legs, and an old man uses a walking stick.” This is one of the oldest references to prostheses, devices that help us over- come missing or defective body parts. War, mining, and transportation have been major causes of injuries that require artificial organs or limbs. Some devices are used to augment organs that are functioning below expectations, such as hearing aids or eyeglasses. Other devices, such as the kidney dialysis machine and the artificial heart, attempt to replace the failing function of an organ. New research in tissue regeneration is exploring the possibility of growing complete artificial organs, one cell at a time, on a structure called a scaffold. It is important to use materials that are compatible with the human body, and that do not cause negative immune responses.

Eyeglasses or spectacles for vision correction and eye protection have been in use for a long time. The earliest mention was in an Egyptian hieroglyph in the eighth century BCE, which depicted “simple glass meniscus lenses.” In the first century, the Roman Emperor Nero is said to have watched gladiatorial games using an emerald as a corrective lens. Seneca the Younger wrote “Letters, how- ever small and indistinct, are seen enlarged and more clearly through a globe or glass filled with water.” Corrective lenses were said to be used by Abbas Ibn Firnas of Cordoba in the ninth century, who found a way to produce a very clear glass. This glass was shaped and polished into round shapes for viewing and was called a reading stone. Around 1284, Salvino D’Armate of Florence was credited with inventing the first wearable eyeglasses. The earliest image for the use of eye- glasses is the Tomaso da Modena’s 1352 portrait of the cardinal Hugh de Pro- vence reading in a scriptorium. Other clerics in Italy, such as Fra Alessandro da Spina of Pisa, were credited with making glasses. The development of glasses, both of concave lenses for distance vision to help near-sighted people (myopia), as well as of convex lenses for close reading to help farsightedness (hyperopia) and vision in old age (presbyopia), are related to the development of telescopes and microscopes. Benjamin Franklin suffered from both myopia and presbyopia, and is credited with inventing bifocals in 1784 to avoid having to switch between two pairs of glasses.

139 SCIENCE OF READING GLASSES Light bends when it enters from air to a denser medium, and the refractive index n is the

4.3 THERAPY

ratio of the speed of light in the medium divided by the speed in air. The refractive index of air is 1 and water is 1.33. Glass has a refractive index of 1.54–1.61 and plastic poly- carbonate is 1.58. Diamond has the much higher refractive index of 2.42. The power of a corrective lens is measured in diopters. The bending of light follows the Snell Law

n 1 sin u 1 ¼ n 2 sin u 2

where n 1 and n 2 are the refractive indexes of the two media, and u 1 and u 2 are the angles between the vertical and the beams. So for a glass with refractive index 1.50, the entering beam bends to a smaller angle of the leaving beam: