philosophy of mind: Executives, buffers, and HOTs: Executive theories, such as the theory proposed by the Swiss psychologist Jean Piaget (1896–1980), stress the role of conscious states in deliberation and planning. Many philosophers, however, doubt that all such executive activities are conscious; they suspect instead that conscious states play a more tangential…

executor, in law, person designated by a testator—i.e., a person making a will—to direct the distribution of his estate after his death. The system is found only in countries using Anglo-American law; in civil-law countries the estate goes directly to the heir or heirs. The executor is usually a

exedra, in architecture, semicircular or rectangular niche with a raised seat; more loosely applied, the term also refers to the apse (q.v.) of a church or to a niche therein. In ancient Greece exedrae were commonly found in the parts of major cities that had been reserved for worship, such as the

exegesis, the critical interpretation of the biblical text to discover its intended meaning. Both Jews and Christians have used various exegetical methods throughout their history, and doctrinal and polemical intentions have often influenced interpretive results; a given text may yield a number of

Exekias was a Greek potter and painter who, with the Amasis Painter, is considered the finest and most original of black-figure masters of the mid-6th century bc and is one of the major figures in the history of the art. He signed 13 vases (2 as painter and potter and 11 as potter). The commonest

exemplum, short tale originally incorporated by a medieval preacher into his sermon to emphasize a moral or illustrate a point of doctrine. Fables, folktales, and legends were gathered into collections, such as Exempla (c. 1200) by Jacques de Vitry, for the use of preachers. Such exempla often

Cornelius Nepos: …invention, the universal comparative chronology; Exempla (in at least 5 books), which consisted of anecdotes; possibly a universal geography to match the Chronica; and biographies of the elder Cato and Cicero. There survive only one complete and one partial book from the De viris illustribus; the biographies in these extant…

Uriel Acosta: …after writing a short autobiography, Exemplar Humanae Vitae (1687; “Example of a Human Life”), he shot himself. Acosta’s Exemplar depicted revealed religion as disruptive of natural law and a source of hatred and superstition. In contrast, he advocated a faith based on natural law and reason.

Western philosophy: Bonaventure: …result, Aristotle was ignorant of exemplarism (God’s creation of the world according to ideas in his mind) and also of divine providence and government of the world. This involved Aristotle in a threefold blindness: he taught that the world is eternal, that all men share one agent intellect (the active…

punitive damages, legal damages a judge or a jury may grant a plaintiff to punish and make an example of the defendant. Punitive damages are generally meted out in only the most extreme circumstances, usually in breaches of obligation with significant evidence of oppression, fraud, gross

Miguel de Cervantes: Publication of Don Quixote: The next year, the 12 Exemplary Stories were published. The prologue contains the only known verbal portrait of the author:

exemplum, short tale originally incorporated by a medieval preacher into his sermon to emphasize a moral or illustrate a point of doctrine. Fables, folktales, and legends were gathered into collections, such as Exempla (c. 1200) by Jacques de Vitry, for the use of preachers. Such exempla often

If you are an office, retail, or blue-collar worker, you may have heard the terms “exempt” and “nonexempt” in discussions about job status. These classifications, which are defined by the Fair Labor Standards Act (FLSA), determine whether you’re eligible for overtime pay: Both employees and

property tax: Theory of property taxation: …weakened by a variety of exemptions. In the United States, for example, exemptions apply to about one-third of the land area in the average locality. Most of the land exempted from a property tax comprises streets, schools, parks, and other property of local government, meaning that the application of the…

antidiabetic drug: Other agents: Exenatide is an injectable antihyperglycemic drug that works similarly to incretins, or gastrointestinal hormones, such as gastric inhibitory polypeptide, that stimulate insulin release from the pancreas. Exenatide has a longer duration of action than incretins produced by the body because it is less susceptible to…

Spain: Castile: …Ferdinand claimed the right of exequatur, according to which all papal bulls and breves (authorizing letters) could be published only with his permission. A letter from Ferdinand to his viceroy in Naples, written in 1510, upbraids the viceroy for permitting the pope to publish a brief in Naples, threatens that…

Juan Pablo Forner: …two most important works are Exequias de la lengua castellana (1795; “Exequies of the Castilian Language”), a defense of Castilian literature; and Oración apologética por la España y su mérito literario (1786; “Arguments on Behalf of Spain and Her Literary Merits”), in which he refuted the idea that Spanish literature…

exercise, the training of the body to improve its function and enhance its fitness. The terms exercise and physical activity are often used interchangeably, but this article will distinguish between them. Physical activity is an inclusive term that refers to any expenditure of energy brought about

bicycle: Basic types: …front rider steering; and stationary exercise bicycles.

bicycle: Basic types: …front rider steering; and stationary exercise bicycles.

Edward L. Thorndike: The law of exercise stated that behaviour is more strongly established through frequent connections of stimulus and response. In 1932 Thorndike determined that the second of his laws was not entirely valid in all cases. He also modified the law of effect to state that rewards…

Edward L. Thorndike: The law of exercise stated that behaviour is more strongly established through frequent connections of stimulus and response. In 1932 Thorndike determined that the second of his laws was not entirely valid in all cases. He also modified the law of effect to state that rewards…

exercise-induced pulmonary hemorrhage, disease condition in horses in which blood appears in the airways during and after strenuous exercise. More than 80 percent of racehorses, including Thoroughbreds, Standardbreds, and American Quarter Horses, are affected to varying degrees. The condition can

Daniel Lieberman: His other books included Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding (2020). He was a member of the American Association for the Advancement of Science and the American Anthropological Association.

Italy: Lombard Italy: …people in arms—the exercitales, or arimanni, who formed the basis of the Lombard army. This concept did not leave much room for Romans, who indeed largely disappear from the evidence, even when documents increase again in the 8th century; it is likely that any Romans who wished to remain politically…

calligraphy: Writing manuals and copybooks (16th to 18th century): …engraved metal plates was the Exercitatio alphabetica (1569; “Alphabet Practice”) by the 17-year-old Clément Perret. Perret’s book contains examples in many different hands chosen to match the language of the text. The beautifully ornate writing in Exercitatio is somewhat overshadowed by the finely drawn cartouches that surround the examples, and…

blood group: Historical background: …et Sanguinis in Animalibus (The Anatomical Exercises Concerning the Motion of the Heart and Blood in Animals) in 1628. His discovery, that blood circulates around the body in a closed system, was an essential prerequisite of the concept of transfusing blood from one animal to another of the same…

Lorenzo Bellini: In Exercitatio anatomica de structura et usu renum (1662; “Anatomical Exercise on the Structure and Function of the Kidney”), published when he was a 19-year-old student at the University of Pisa, Bellini showed for the first time that the kidney consists of an immense number of…

Johann Clauberg: In Exercitationes Centum de Cognitione Dei et Nostri (1656; “One Hundred Exercises on the Knowledge of God and Ourselves”), he proceeded from his proof for the existence of God based on a concept of the infinite to an account of knowledge and being in general. The…

William Harvey: Later life: …Exercitationes de Generatione Animalium (Exercises on the Generation of Animals), it is believed that Harvey attempted to take his own life with laudanum (an alcoholic tincture of opium). However, this attempt failed. On June 3, 1657, at the age of 79, he died of a stroke.

Bonaventura Cavalieri: …to this criticism, Cavalieri wrote Exercitationes Geometricae Sex (1647; “Six Geometrical Exercises”), stating the principle in the more satisfactory form that was widely employed by mathematicians during the 17th century.

Pierre Gassendi: Early life and career: Gassendi’s work Exercitationes paradoxicae adversus Aristoteleos (“Paradoxical Exercises Against the Aristotelians”), the first part of which was published in 1624, contains an attack on Aristotelianism and an early version of his mitigated skepticism. Gassendi thereafter engaged in many scientific studies with his patron, Nicolas-Claude Fabri de Peiresc,…

Graf Spee: …group consisting of the cruisers Exeter, Ajax, and Achilles, commanded by Commodore H. Harwood. At 6:14 am Harwood’s three ships attacked, but in a little more than an hour the Graf Spee had damaged the Exeter and driven off the other two cruisers. The Graf Spee then made off in…

Exeter, city (district), administrative and historic county of Devon, southwestern England. It is located on the River Exe, just above the head of the river’s estuary and about 10 miles (16 km) from the estuary’s entry into the English Channel. Exeter is the county town (seat) of Devon. The

Phillips Exeter Academy, private, coeducational, college-preparatory school (grades 9–12) in Exeter, N.H., U.S. It was founded as a boys’ school in 1781 by John Phillips, a local merchant and uncle of Samuel Phillips, the founder three years earlier of Phillips Academy in Andover, Mass. Exeter’s

Exeter, town (township), seat of Rockingham county, southeastern New Hampshire, U.S., on the Exeter River at the falls of the Squamscott River (tidal), southwest of Portsmouth. The town was founded in 1638 by John Wheelwright and a group of religious exiles from the Massachusetts Bay colony. During

Exeter Book, the largest extant collection of Old English poetry. Copied c. 975, the manuscript was given to Exeter Cathedral by Bishop Leofric (died 1072). It begins with some long religious poems: the Christ, in three parts; two poems on St. Guthlac; the fragmentary “Azarius”; and the allegorical

exfoliation, separation of successive thin shells, or spalls, from massive rock such as granite or basalt; it is common in regions that have moderate rainfall. The thickness of individual sheet or plate may be from a few millimetres to a few metres. Some geologists believe that exfoliation results

biopsy: In exfoliative cytology, cells shed from body surfaces, such as the inside of the mouth, are collected and examined. This technique is useful only for the examination of surface cells and often requires additional cytological analysis to confirm the results.

exfoliative dermatitis, generalized redness and scaling of the skin that usually arises as a complication of a preexisting skin disease or of an allergy. More rarely, it may be indicative of a systemic disease, such as cancer of the lymphoid tissue. The onset of exfoliative dermatitis is gradual;

bivalve: Internal features: …pores also open into the exhalant chamber so that all waste products exit the animal in the exhalant stream. The paired labial palps in the mantle cavity are used in feeding. The outer palp on each side bears a long, extensible proboscis with a ciliated groove that collects organic material,…

speech: Respiratory mechanisms: …inhalation (inspiration) and exhalation (expiration). Inspiration and expiration are equally long, equally deep, and transport the same amount of air during the same period of time, approximately half a litre (one pint) of air per breath at rest in most adults. Recordings (made with a device called a pneumograph)…

muffler: exhaust gases from an internal-combustion engine are passed to attenuate (reduce) the airborne noise of the engine. To be efficient as a sound reducer, a muffler must decrease the velocity of the exhaust gases and either absorb sound waves or cancel them by interference with…

emission control system: The exhaust pipe discharges burned and unburned hydrocarbons, carbon monoxide, oxides of nitrogen and sulfur, and traces of various acids, alcohols, and phenols. The crankcase is a secondary source of unburned hydrocarbons and, to a lesser extent, carbon

avionics: …craft), torquemeter (in turboprops), and exhaust pressure ratio indicator (in turbojets). Performance instruments include the altimeter, Machmeter, turn and slip indicator, and varied devices that show airspeed, vertical velocity, and angle of attack. Communications instruments include two-way radios allowing direct voice communication between the aircraft and the ground as well…

diesel engine: Fuel-injection technology: …to be solved: the engine exhaust contained an excessive amount of smoke, even at outputs well within the horsepower rating of the engine and even though there was enough air in the cylinder to burn the fuel charge without leaving a discoloured exhaust that normally indicated overload. Engineers finally realized…

diesel engine: Two-stroke and four-stroke engines: …four-stroke-cycle engine, the intake and exhaust valves and the fuel-injection nozzle are located in the cylinder head (see figure). Often, dual valve arrangements—two intake and two exhaust valves—are employed.

automobile: Emission controls: Exhaust-gas recirculation is a technique to control oxides of nitrogen, which are formed by the chemical reaction of nitrogen and oxygen at high temperatures during combustion. Either reducing the concentrations of these elements or lowering peak cycle temperatures will reduce the amount of nitrogen oxides…

food preservation: Presterilization procedures: …lids by a process called exhausting. Exhausting is accomplished using steam exhaust hoods or by creation of a vacuum.

fatigue, specific form of human inadequacy in which the individual experiences an aversion to exertion and feels unable to carry on. Such feelings may be generated by muscular effort; exhaustion of the energy supply to the muscles of the body, however, is not an invariable precursor. Feelings of

method of exhaustion, in mathematics, technique invented by the classical Greeks to prove propositions regarding the areas and volumes of geometric figures. Although it was a forerunner of the integral calculus, the method of exhaustion used neither limits nor arguments about infinitesimal

strategy: Medieval strategy: …both types of strategy—overthrow and exhaustion. The Crusader states of the Middle East were gradually exhausted and overwhelmed by constant raiding warfare and the weight of numbers. On the other hand, one or two decisive battles, most notably the ruinous disaster at the Battle of Ḥaṭṭīn (1187), doomed the Crusader…

exedra, in architecture, semicircular or rectangular niche with a raised seat; more loosely applied, the term also refers to the apse (q.v.) of a church or to a niche therein. In ancient Greece exedrae were commonly found in the parts of major cities that had been reserved for worship, such as the

museum: Exhibition: Many museums have abandoned the traditional view of exhibition, by which storage and display are ends in themselves, in favour of an approach that enhances the setting of the object or collection. To this end museums use the expertise of a number of specialists—designers,…

Pietro Longhi: , The Dancing Master and Exhibition of a Rhinoceros at Venice. Popular for their charm and seeming naivete, his paintings have a Rococo sense of the intimate and manifest the interest in social observation characteristic of the Enlightenment. His works, like those of Antoine Watteau, were based on carefully observed…

world’s fair: The Great Exhibition and its legacy: the golden age of fairs: The Exhibition of the Industry of All Nations, more commonly known as the New York Crystal Palace Exhibition, was held in 1853–54 in an iron-and-glass structure in Bryant Park. It showcased the same types of displays as its London counterpart but also included an especially impressive…

Canadian National Exhibition: The fair is staged at Exhibition Place, a venue that covers about 200 acres (80 hectares) west of downtown Toronto, on the shore of Lake Ontario. One of the largest fairs in North America, it features concerts, ice and stunt shows, parades, shopping, carnival attractions, agricultural displays, talent competitions, a…

exhibitionism, derivation of sexual gratification through compulsive display of one’s genitals. Like voyeurism (q.v.), sexual display is almost universal as a prelude to sexual activity in animals, including humans; it is regarded as deviant behaviour when it takes place outside the context of

Christianity: Characteristics of Christian myth and legend: , in his Protreptikos [“Exhortation”]) and other Church Fathers roundly condemned the belief that Greek myths might be autonomous sources of truth. In spite of its ambiguous use of mythic symbols and themes, the history of Christian doctrine, from its origins to the present day, testifies to the…

Origen: Life: …he addressed to Ambrose his Exhortation to Martyrdom. During this period falls the “Discussion with Heracleides,” a papyrus partially transcribing a debate at a church council (probably in Arabia) where a local bishop was suspected of denying the preexistence of the divine Word and where obscure controversies raged over Christological…

Sophist: Writings: …discussion of law in the Protrepticus, or “Exhortation to Philosophy,” by the 3rd-century-ce Syrian Neoplatonist Iamblichus, and the so-called Dissoi logoi found in the manuscripts of Sextus Empiricus (3rd century ce). This evidence suggests that while most later writers took their

Qu Qiubai: …Soviet life were published as Exiang jicheng (1921;“Journey to the Land of Hunger”). That book made a considerable impression on Chinese intellectuals, as did his second book, Chidu xinshi (1924; “Impressions of the Red Capital”).

JPEG: …exchangeable image file format (EXIF) information that stores details about when a picture was taken and even settings such as exposure and shutter speed. JPEG files typically end in the extension “.jpeg” or “.jpg.”

Judaism: Babylonia (200–650): …in approximately 100 ce an exilarch, or “head of the [Jews in] exile”—who claimed more direct Davidic descent than the Palestinian patriarch—to rule over the Jews as a quasi-prince. About 220, two Babylonian disciples of Judah ha-Nasi, Abba Arika (known as Rav) and Samuel bar Abba, began to propagate the…

exile and banishment, prolonged absence from one’s country imposed by vested authority as a punitive measure. It most likely originated among early civilizations from the practice of designating an offender an outcast and depriving him of the comfort and protection of his group. Exile was practiced

Liz Phair: Exile in Guyville: Eventually, her tapes were reviewed in fan magazines, and Phair was offered a $3,000 advance from the independent label Matador Records, which she used to record the 18 songs on Exile in Guyville. When the album was cut, producer and drummer Brad…

the Rolling Stones: Sticky Fingers and Exile on Main St.: …Flash” and the double album Exile on Main St. (1972) remains the Stones’ creative and iconic peak. The studio albums Let It Bleed (1969) and Sticky Fingers (1971) plus the in-concert Get Yer Ya-Ya’s Out! (1970) gave them the repertoire and image that still defines them and on which they…

Malcolm Cowley: His Exile’s Return: A Narrative of Ideas (1934; rev. ed. published 1951 under the subtitle A Literary Odyssey of the 1920’s) is an important social and literary history of the expatriate American writers of the 1920s. In it he signaled the importance of their rediscovery of…

James Joyce: Legacy of James Joyce: …Poems, 1936) and a play, Exiles (1918)—though competently written, added little to his international stature.

Donald Hall: The poems collected in Exiles and Marriages (1955) exhibit the influence of Hall’s academic training: their style and structure are rigorously formal. In The Dark Houses (1958) he shows a richer emotional range, presaging the intuitive, anecdotal works for which he has become best known—e.g., A Roof of Tiger…

Djibouti: Transportation and telecommunications: Financed largely by the Export-Import Bank of China, the $3.4 billion project was completed in October 2016. Capable of accommodating freight trains at speeds of up to 75 miles (120 km) per hour and passenger trains at speeds of up to 100 miles (160 km) per hour, the electrified…

Export-Import Bank of the United States, one of the principal agencies of the U.S. government in international finance, originally incorporated as the Export-Import Bank of Washington on February 12, 1934, to assist in financing the export of American-made goods and services. Its name was changed

Francisco Suárez was a Spanish theologian and philosopher, a founder of international law, often considered the most prominent Scholastic philosopher after St. Thomas Aquinas, and the major theologian of the Roman Catholic order, the Society of Jesus (Jesuits). The son of a wealthy lawyer, Suárez

pollen: …and an outer layer, the exine. The intine consists, at least in part, of cellulose or hemicellulose. The outer and most durable layer, the exine, is very resistant to disintegration; treatment with intense heat, strong acids, or strong bases has little effect upon it. The constituents of the exine have…

coal: Macerals: …groups are generally recognized: vitrinite, liptinite (formerly called exinite), and inertinite. The vitrinite group is the most abundant, constituting as much as 50 to 90 percent of many North American coals. Vitrinites are derived primarily from cell walls and woody tissues. They show a wide range of reflectance values (how…

existence, in metaphysics, that which applies neutrally to all and only those things that are real. Metaphysicians have had a great deal to say about the existence or nonexistence of various things or categories of things, such as God, the soul, a mind-independent or external world, abstract or

existential crisis, a period of inner conflict during which a person is distraught over questions about identity, meaning, and purpose. Although the defining characteristics of an existential crisis vary among psychologists, most agree that it is at heart a period of anxiety and conflict about

existential import, in syllogistic, the logical implication by a universal proposition (i.e., a proposition of the form “All S is P” or “No S is P”) of the corresponding particular statement (i.e., “Some S is P” or “Some S is not P,” respectively). The validity of some syllogistic figures (see

mental disorder: Humanistic and existential psychotherapies: In contrast to dynamic psychotherapy, humanistic and existential psychotherapies focus on the current experience of the patient in resolving problems. Humanistic therapy is represented primarily by the person-centered approach of American psychologist Carl Rogers, who held that the essential features of therapy are…

quantification: The existential quantifier, symbolized (∃-), expresses that the formula following holds for some (at least one) value of that quantified variable.

the Mekons: …Ancient & Modern (2011), and Existentialism (2017), the last of which captures a live experimental performance in Brooklyn featuring just-written politically charged songs.

existentialism, any of various philosophies, most influential in continental Europe from about 1930 to the mid-20th century, that have in common an interpretation of human existence in the world that stresses its concreteness and its problematic character. According to existentialism: (1) Existence

ethics: Existentialism: …one work, a pamphlet entitled Existentialism Is a Humanism (1946), Sartre backed away from so radical a subjectivism by suggesting a version of Kant’s idea that moral judgments be applied universally. He does not reconcile this view with conflicting statements elsewhere in his writings, and it is doubtful whether it…

ethics: Existentialism: …one work, a pamphlet entitled Existentialism Is a Humanism (1946), Sartre backed away from so radical a subjectivism by suggesting a version of Kant’s idea that moral judgments be applied universally. He does not reconcile this view with conflicting statements elsewhere in his writings, and it is doubtful whether it…

David Cronenberg: Rabid, The Fly, and Crash: …(albeit similar commercial reception) for eXistenZ (1999), a kinetic virtual-reality adventure that he wrote, and Spider (2002), a harrowing look into the mind of a schizophrenic man (played by Ralph Fiennes).

Michael Rumaker: Exit 3, and Other Stories (1966; U.S. title, Gringos and Other Stories) contains short fictions rife with marginal characters and random violence. A Day and a Night at the Baths (1979) and My First Satyrnalia (1981) are semiautobiographical accounts of initiation into New York’s homosexual…

Stephen Colbert: Education and early career: …created the award-winning sketch show Exit 57 (1995–96) and the bizarre sitcom Strangers with Candy (1999–2000), both on the Comedy Central cable network. Colbert worked on several other television projects before joining in 1997 Comedy Central’s The Daily Show, which was hosted by Jon Stewart. For eight years he was…

Philip Roth: Exit Ghost (2007) revisits Zuckerman, who has been reawoken to life’s possibilities after more than a decade of self-imposed exile in the Berkshire Mountains. Indignation (2008; film 2016) is narrated from the afterlife by a man who died at age 19. The Humbling (2009; film…

exit interview, typically a survey given by an employer to a departing employee, though exit interviews can also involve people leaving other types of organizations or institutions, such as an educational facility. The purpose of exit interviews is to understand why talent is leaving, what might

Ian Rankin: In 2007 Rankin published Exit Music, in which Rebus retires. Though Rankin maintained at the time that it was to be the last novel in the series, the superannuated Rebus was on the case again in Standing in Another Man’s Grave (2012), Saints of the Shadow Bible (2013), Even…

the Chemical Brothers: …the duo’s 1995 debut album, Exit Planet Dust, but it also sired an entire genre, big beat. The Chemicals’ 1997 follow-up, Dig Your Own Hole, kept them ahead of a growing legion of imitators by expanding their sonic spectrum, which ranged from the crude adrenal inrush of “Block Rockin’ Beats”…

Voter News Service: …aid in the reporting of exit-poll numbers during national elections. The consortium was created in 1990 by media companies ABC, CBS, CNN, Fox News, NBC, and the Associated Press under the direction of political scientist Murray Edelman.

Banksy: Exit Through the Gift Shop and books: Banksy directed the 2010 film Exit Through the Gift Shop, a documentary that ostensibly profiled the lives and work of the world’s most talented graffiti artists. Critics were divided on the film, though, as some chose to accept it at face value while others saw it as a satire, with…

Anne Rice: Erotic novels and the Mayfair witches: …Rampling, Exit to Eden (1985; film 1994) and Belinda (1986).

Exmoor, high moorland in northwestern Somerset and northern Devon, England, that forms a national park 268 square miles (693 square km) in extent. It borders the Bristol Channel on the north and has a beautiful coastline of rugged headlands interspersed with narrow, wooded valleys, or coombs.

Exmoor, high moorland in northwestern Somerset and northern Devon, England, that forms a national park 268 square miles (693 square km) in extent. It borders the Bristol Channel on the north and has a beautiful coastline of rugged headlands interspersed with narrow, wooded valleys, or coombs.

Devon: …in the north, part of Exmoor National Park. Dartmoor, with shallow marshy valleys, thin infertile soils, and a vegetation of coarse grasses, heather, and bracken, is a granite plateau rising to above 2,000 feet (600 metres), the crests capped by granite tors (isolated weathered rocks); the moor is used for…

Exmouth, town (parish), East Devon district, administrative and historic county of Devon, southwestern England. It is situated on the east side of the mouth of the River Exe estuary on the English Channel. Its fort, commanding the estuary to the north, was captured by the Parliamentarians in 1646

Exmouth Gulf, inlet of the Indian Ocean in Western Australia, between North West Cape and the mainland. It is 55 miles (90 km) long north to south and 30 miles across the mouth and has a maximum depth of 72 feet (22 metres). The west coast was charted by the Dutch navigator Abel Janszoon Tasman in

photoreception: Image formation: In the 1890s Austrian physiologist Sigmund Exner was the first to show that lens cylinders can be used to form images in the eye. He discovered this during his studies of the ommatidia of the horseshoe crab Limulus.