law of diminishing returns. A concept in economics that if one factor of production (number of workers, for example) is increased while other factors (machines and workspace, for example) are held constant, the output per unit of the variable factor will eventually diminish.
History of glass. The history of glassmaking can be traced back to 3500 BCE in Mesopotamia, but Mesopotamians may have created second-rate copies of glass objects from Egypt, where this complex craft actually originated. Other archaeological evidence suggests that the first true glass was made in coastal north Syria, Mesopotamia or Egypt.
Eyeglasses History. The first vision aid, called a reading stone, was invented around 1000 AD. The reading stone was a glass sphere that was laid on top of the reading material to magnify the letters. The first wearable eye glasses were invented around 1284 in Italy. It is thought that Salvino D'Armate was their inventor.