Christian monks in Europe around the year 1000 C.E. used polished quartz crystals as reading aids. The crystals were the diameter of ping pong balls and were cut in half They gradually discovered that such a "reading crystal" would be a more powerful magnifier if it would be shallower, with a larger radius, more like a speed hump than a speed bump.
But lenses are known to have been used in the ancient world long before the monks. Sophisticated quartz magnifying lenses were found in Viking settlements dated between 700-1000 C.E. The Romans were known to use a glass bowl filled with water as a "burning lens", which, when exposed to the rays of the sun would cauterize wounds and start fires. Archeologists found reading crystals in Crete and Troy.
French philosopher and mathematician Rene Descarte ("I think, therefore I am") is credited with having discovered the equations that govern the optical properties of lenses in the 17th century, but he could not find anyone with the material skill to pattern lenses after his equations.
In the period shortly after Descarte, there was "crown glass" which was a composite of silicon dioxide, sodium oxide and calcium oxide (silica, soda and lime). By 1758, lens makers also began to use "flint glass" which contains lead oxide. Flint glass is more dense and it disperses light into a spectrum more strongly.
Crown glass and flint glass are the two types of glass used today to create lenses. The main obstacle to manufacturing flint glass in the 18th century was the difficulty of removing bubbles and unevenness in the glass mixture. A Swiss glass maker, Perre Guinand invented a process of stirring the glass while it was molten to remove the bubbles and prevent the unevenness.
Lens making took off around the time that Guinand invented his flint glass manufacturing process. Some of the finest lens makers of the day were Italian. They made lenses that were slightly convex on both sides, so that they looked like lentils. The Latin name for lentil is lens culinaris, and this is the origin of the modern word, "lens".
In addition to crown glass and flint glass, modern day lens makers use quartz crystals and acrylic plastic. These materials may be used in combination. Here is why:
Imagine you are a light beam. Like most other light beams, you are made up of infrared light, ultraviolet light and other wave lengths of light, You are zipping through the universe at, well, the speed of light. You happen to encounter a lens. You decide to go through the lens. But even though the lens material is almost as transparent as air, it is a lot more dense. When you reach the lens, two things happen. First, the density of the glass slows you down, by making you take a slightly different path as you go through the lens. This is called refraction. Second, not all of your wave lengths slow down at the same rate. Blue light travels more slowly in the dense material than red light. This is what causes the lens to work like a prism and create a rainbow effect. It is called dispersion, or chromatic dispersion. Chromatic dispersion is the change of index of refraction with wavelength. When a lens creates too much of a rainbow effect, we say that it creates chromatic aberration. This is a way of saying that we cannot see the true color of the object we are examining because the lens material is making too many rainbows.
Different lens materials have different densities. Common crown glasses have indices of refraction around 1.5-1.6, while extra dense flint glass may have an index as high as 1.75. Crown glass lenses, work well in a magnifier that provides low level magnification because they do not need color correction. The Optivisor is a fine example. Lenses of crown and flint glasses are often used in multi-component lenses because of their complementary properties work better for higher magnification. For example, a strong positive crown lens with its low dispersion may be used in a doublet with a weaker negative lens of flint glass (high dispersion) to correct for chromatic aberration. A Hastings Triplet loupe adds one more lens for an additional purpose. The extra lens is shaped so as to eliminate distortion of the image at the outer edges of the field of vision.