powder and Selective laser sintering processes – see Mykita Mylon (The frames can be 3-D printed by Fused Filament Fabrication for pennies of ABS, PLA or nylon) Nylon Metal Various metals and alloys may be used to make glasses, such as gold, silver, aluminum, beryllium, stainless steel, titanium, monel, and nickel titanium. Natural material Natural materials such as wood, bone, ivory, leather and semi-precious or precious stones may also be used. Corrective lens shape Corrective lenses can be produced in many different shapes from a circular lens called a lens blank. Lens blanks are cut to fit the shape of the frame that will hold them. Frame styles vary and fashion trends change over time, resulting in a multitude of lens shapes. For lower power lenses, there are few restrictions, allowing for many trendy and fashionable shapes. Higher power lenses can distort peripheral vision and may become thick and heavy if a large lens shape is used. However, if the lens is too small, it can drastically reduce the field of view. Bifocal, trifocal, and progressive lenses generally require a taller lens shape to leave room for the different segments while preserving an adequate field of view through each segment. Frames with rounded edges are the most efficient for correcting myopic prescriptions, with perfectly round frames being the most efficient. Before the advent of eyeglasses as a fashion item, when frames were constructed with only functionality in mind, virtually all eyeglasses were either round, oval, rectangular or octagonal. It was not until glasses began to be seen as an accessory that different shapes were introduced to be more aesthetically pleasing than functional. History Precursors Scattered evidence exists for use of visual aid devices in Greek and Roman times, most prominently the use of an emerald by Emperor Nero as mentioned by Pliny the Elder. The use of a convex lens to form an enlarged/magnified image was most likely described in Ptolemy's Optics (which survives only in a poor Arabic translation). Ptolemy's description of lenses was commented upon and improved by Ibn Sahl (10th century) and most notably by Alhazen (Book of Optics, c. 1021). Latin translations of Ptolemy's Optics and of Alhazen became available in Europe in the 12th century, coinciding with the development of "reading stones". There are claims that single lens magnifying glasses were being used in China during the Northern Song Dynasty (960-1127). Robert Grosseteste's treatise De