1669 Birefringence

The book of science

Tom Sharp

Rasmus Bartholin optics


Iceland spar

Double vision

Light’s lack of a longitudinal vibration (vibration along its direction of travel) is eventually explained by Albert Einstein—the speed of light is constant.

Many frequencies are inherent in normal, incoherent light in the range perceptible to human eyes, and each of those is a blur of ordinary and extraordinary transverse waves. Some people see ghosts, some see auras, some say they have second sight.

See also in The book of science:

Readings on wikipedia: