Description
Usually consisting of a pair of lenses held in a frame, spectacles may have originated in the ancient civilizations of China and the Mediterranean, but early forms were crude. It was only in the 18th century that the grinding of lenses became sufficiently advanced to make then really effective for correcting sight defects. In order to correct for near sightedness, the lenses must be concave, which diverges the rays of light (angles them apart), and for long sightedness they must be convex, to angle the rays of light together.

Spectacles are not solely used for defective eyesight; the Inuit and the Yupik use wooden spectacles with very small slits in them to reduce the glare from snow and ice.