<P> Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia . The so - called Nimrud lens is a rock crystal artifact dated to the 7th century BC which may or may not have been used as a magnifying glass, or a burning glass . Others have suggested that certain Egyptian hieroglyphs depict "simple glass meniscal lenses". </P> <P> The oldest certain reference to the use of lenses is from Aristophanes' play The Clouds (424 BC) mentioning a burning - glass . Pliny the Elder (1st century) confirms that burning - glasses were known in the Roman period . Pliny also has the earliest known reference to the use of a corrective lens when he mentions that Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for nearsightedness, though the reference is vague). Both Pliny and Seneca the Younger (3 BC--65 AD) described the magnifying effect of a glass globe filled with water . </P> <P> Ptolemy (2nd century) wrote a book on Optics, which however survives only in the Latin translation of an incomplete and very poor Arabic translation . The book was, however, received, by medieval scholars in the Islamic world, and commented upon by Ibn Sahl (10th century), who was in turn improved upon by Alhazen (Book of Optics, 11th century). The Arabic word for "lens", عدسة ʿadasa ("lentil") is a direct loan translation of Latin lens, lenticula . The Arabic translation of Ptolemy's Optics became available in Latin translation in the 12th century (Eugenius of Palermo 1154). Between the 11th and 13th century "reading stones" were invented . These were primitive plano - convex lenses initially made by cutting a glass sphere in half . The medieval (11th or 12th century) rock cystal Visby lenses may or may not have been intended for use as burning glasses . </P> <P> Spectacles were invented as an improvement of the "reading stones" of the high medieval period in Northern Italy in the second half of the 13th century . This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the late 13th century, and later in the spectacle - making centres in both the Netherlands and Germany . Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses (probably without the knowledge of the rudimentary optical theory of the day). The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in 1608, both of which appeared in the spectacle - making centres in the Netherlands . </P>

The focal length of a convex lens for various colours of light is