Wednesday, September 20, 2017

The art of seeing

The earliest fossils of the eye, that have been found to date, are from about 540 million years ago. Several millennia of evolution have resulted in the formation of almost two million working parts in the eye, rendering it capable of providing a three dimensional, moving image. However, visual defects have always been exceedingly common, thus making eyeglasses imperative. Furthermore, the eyes have always been sensitive to bright light and UV rays thereby popularizing the use of sunglasses. Today, the combined number of eyeglasses, reading glasses, contact lenses, and sunglasses in the U.S. alone exceeds 235 million pairs annually. How were these glasses developed? When were they invented?

History of eyeglasses

The earliest reference to magnification dates back to 8th century BC. There are Egyptian hieroglyphs which depict simple glass lenses. There are several references to the use of corrective lenses, however most of these are vague. One of the earliest verifiable references is from the 3rd century BC where Seneca the Younger, whose impassive suicide is depicted in several paintings, described the use of a glass globe filled with water as a means to magnify print. In the 8th century reading stones were invented by Abbas Ibn Firnas

Figure 1: A reading stone is a hemispherical lens that is used to magnify text thereby enabling easy reading. Source.

The real breakthrough in understanding the mechanism of vision and corrective glasses came in 1021 when Ibn al-Haytham, a Persian astronomer and mathematician, published a comprehensive study called "Book of Optics". The translation of this book from Arabic to Latin in the 12th century was pivotal to the invention of glasses in Italy in the 13th century. In the book, al-Haytham helped dispel the long-standing and incorrect emission theory of sight. This theory, which was favored by Plato and Euclid, hypothesized that visual perception was due to light rays that were emitted by the eyes. In contrast, the intromission theory, whose main advocates included Aristotle and Galen, stated that vision occurs due to physical forms entering the eye from an object. In his book, al-Haytham further strengthened the intromission theory by describing the phenomenon of afterimage. This is an image that continues to occur in one's vision after exposure to the original image has stopped. He pointed out that after staring at the sun, when a person closes his eyes, he continues to see the solar disk for some time. Furthermore, gazing at the sun causes pain. Both these facts contradicted the emission theory therefore cementing the validity of the intromission theory. 

Figure 2: The front page of the Book of Optics. The illustration depicts many optical phenomena such as rainbows, reflection from mirrors, and perspective effects i.e. the effect the atmosphere has on the appearance of an object when viewed from a distance. Source.

Al-Haytham also discussed the use of a convex lens (shown below) to form a magnified image, which was used to construct the first spectacles in 1286 in Italy. Although it remains unclear who the inventor was, the earliest pictorial evidence of the use of eyeglasses is the portrait of a cardinal reading a scriptorium, made in 1352 by Tommaso da Modena. At the time, spectacles were made based on the empirical knowledge gained from observing the ability of lenses to improve eyesight. It was in 1604 that the German astronomer and mathematician Johannes Kepler published the correct explanations for why convex and concave lenses could be use to correct eyesight. The mechanisms involved in the use of these corrective lenses have been discussed in the following figures.

Figure 3(a): The working of a normal eye. The light enters through the cornea and the convex lens, a lens that curves outwards, in the eye focuses the light rays to bring the images on the retina. In short-sighted and far-sighted individuals, there is a mismatch between the shape of the cornea and the length of the eye resulting in visual defects. Source.

Figure 3(b): The use of concave lenses, lens that curve inwards, to correct myopia (nearsightedness). When a person has myopia, the eye is too long, and the image from far away objects does not fall on the retina. Concave lenses spread out the light before it reaches the eye, therefore letting the image focus on the retina. Source.

Figure 3(c): The use of convex lenses to correct hyperopia (farsightedness). In this case, the eye is shorter than normal, and the image focuses behind the retina if the object is too close. Convex lenses focus the light in addition to the natural lens thereby allowing the image to focus exactly on the retina. Source.

Another class of vision defects called presbyopia occurs in older people; it is characterized by the inability to focus on close objects. This is different from hyperopia because it is caused due to the aging of the eye which results in the hardening of the eye lens. Like in the case of farsightedness, the resulting image focuses behind the retina. It is common for a person to have both presbyopia and myopia in which case they require bifocal lenses. The inventor of bifocals is unknown; there is evidence that they were developed independently by several different people. However, the credit for their invention is usually given to the renowned American polymath Benjamin Franklin, who claimed in the mid-1780s that he was "happy in the invention of double spectacles, which serving for distant objects as well as near ones, make my eyes as useful to me as ever they were". Astigmatic lenses were invented in 1825 by the English mathematician and astronomer, George Airy, who interestingly has both a lunar and a Martian crater named after him. 

Figure 4: Astigmatism is a common eye disorder which is caused due to a irregularly shaped cornea or lens. The cornea has a steeper and flatter curve than normal, which causes light rays to focus unevenly on the retina. The disorder is usually corrected with a cylindrical lens, which is equivalent to the normal curve of the cornea. Source.

The closer the better: development of contact lenses

Leonardo da Vinci has been credited as being the first to come up with the notion of contact lenses. His ideas included submerging the head in a bowl full of water or wearing a water-filled glass hemisphere over the eye. However, his proposals were not based on vision correction; he wanted to learn about the mechanism of the eye by altering corneal power.

Figure 5: Leonardo da Vinci's contact lens drawing from 1508. The lens had a funnel on one side so that water could be poured into it. Unsurprisingly, his apparatus was immediately discarded due to its impracticality. Source.

After reading da Vinci's work, the French mathematician René Descartes proposed another prototype in 1636- a glass tube filled with liquid placed in direct contact with the cornea. The protruding end of the tube was composed of clear glass, which would be shaped to correct vision. However, this was impractical because it made blinking impossible. Based on Descartes' model, Thomas Young, an English polymath, made a pair of basic contact lenses in 1801. To do so, he used wax to affix the water-filled lenses to his eyes. Like da Vinci, his lenses were not intended for vision correction. In 1845, Sir John Herschel, another English polymath, became the first to recommend lens fitting. He suggested grinding the inside curvature of the glass lens such that it conformed to the irregular surface of the cornea. Furthermore, the outer curvature of the same lens was to recreate a normal cornea. He also advocated using an actual mold of the eye and using a gel filling between the cornea and the lens. 

In 1888, Adolf Fick, a German ophthalmologist, constructed the first successful model of a scleral contact lens from heavy brown glass. He had tested the lens first on rabbits, then himself, before trying them on volunteers. He also filled the empty space between the cornea and the glass with dextrose solution. 

Figure 6: Scleral lenses vs conventional lenses. Scleral lenses have a larger diameter and cover the entire cornea as well as a large portion of the "white" of the eye aka sclera. Scleral lenses are still used today for people who have irregular corneas, dry eyes, or those who cannot wear the smaller, conventional lenses. Source.

Fick's compatriot August Müller worked on eyeglasses and corneal lenses in his doctoral thesis. Instead of using the unwieldy brown glass like Fick, he made scleral lenses in 1888, from blown glass which was lighter and better shaped to match the cornea. Furthermore, he suggested the lenses would remain in place because of lubrication by the tear film that usually coats the eye. However, even these lenses were not very successful because they had to be inserted underwater to avoid trapping air bubbles and required the use of cocaine to anesthetize the eye. Using contact lenses became convenient when plastic lenses were developed. In 1936 William Feinbloom, an American optometrist, introduced plastic in lenses, making them lighter. Further enhancements were made by synthesizing corneal lenses in 1949. These were much smaller and could be worn for longer periods of time. The final improvement in lens development came with the use of gas permeable materials, which allowed oxygen to get through to the cornea, and the development of hydrogel lenses (also known as soft lenses) which allowed for greater comfort and high oxygen permeability.

Sunglasses: allowing unabashed staring for centuries

The earliest prototypes of sunglasses dates back to 1200 and were developed by the Inuit. Also known as snow goggles, they were constructed with driftwood, bone, or ivory which were pierced with slits. During late winter and spring, strong ultraviolet light reflects from the snow and ice, and can burn the retina causing painful snow blindness. Therefore, snow goggles were created to reduce the amount of light entering into the eyes without compromising visual acuity. Furthermore, the width of the slits also broadened the field of view. However, neither of the glasses were able to protect the wearers against harmful UV rays.

Figure 6: Inuit snow goggles, a far cry from modern sunglasses. Source.

Another version of sunglasses were invented in 12th century China, where they were made out of flat panes of smoke colored quartz. Chinese judges would hold these crystals in front of their faces in order to mask their expressions during interrogation of witnesses. However, there was no interest in developing sunglasses till the early 20th century. A previous attempt had been made by by James Ayscough, an English optician, in 1752 when he tried to develop tinted lenses in spectacles. He was not concerned with protecting the eyes from sun rays, but instead believed that the blue- or green-tinted glasses could correct for specific vision impairments. The modern sunglasses were made famous by Hollywood stars in the 1920s who used them to shield their eyes from the bright studio lights. The use of UV filters in these glasses also helped reduce the discomfort caused by the lights. The mass production of sunglasses by the company Foster Grant began in 1929. Interestingly, the company was in trouble because the actresses started wearing shorter hairstyles, which caused a collapse in the market of combs. This lead them to develop mass market celluloid sunglasses. In 1936  polaroid filters had been developed by the American inventor Edwin Land. Thee incorporation of these filters into sunglasses allowed them to protect against glare, which causes corneal damage, thus sealing their necessity. 

Figure 7: Mechanism of polaroid glasses. Light emitted by the sun, lamps, or candles flames are unpolarized- the light scatters in all directions. However, when this light reflects from a horizontal surface, it becomes polarized. Polarized light travels in a more uniform direction resulting in glare formation. Polaroid lenses contain a special filter that only transmits one type of polarized light. In the example above, vertically polarized light, but not horizontally polarized light, is transmitted thereby reducing glare. Source.