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Home , Book of Optics, Dioptrics, Emission theory (vision), Hunayn ibn Ishaq, Optics (Ptolemy), Theodoric of Freiberg

BOOKS & ARTS COMMENT IN RETROSPECT Book of Jim Al-Khalili revisits Ibn al-Haytham’s hugely influential study on its millennium.

he greatest physicist of the medieval era vision and the associated physiology of the led a life as remarkable as his discover- and the psychology of perception; and ies were prodigious, spending a decade Books IV to VII, covering traditional physi- Tin prison and at one point possibly feigning cal optics. The work’s most celebrated contri- mental illness to get out of a tight spot. Abu bution to is its explanation of vision. Ali al-Hassan ibn al-Haytham (Latinized to At that time, scholars’ understanding of Alhazen) was born in Basra, now in southern the phenomenon was a mess. The Greeks Iraq, in ad 965. His greatest and most famous had several theories. In the fifth century bc, work, the seven-volume (Kitab had argued that a special light al-Manathir) hugely influenced thinking shone out of the eye until it hit an object, across disciplines from the theory of visual thereby making it visible. This became perception to the nature of in known as the of vision. It medieval art, in both the East and the West, was ‘refined’ by , who explained that for more than 600 years. Many later European you also need external light to see. Plato’s stu- scholars and fellow polymaths, from Robert dent suggested that rather than the CAMBRIDGE OF TRINITY COLLEGE, AND FELLOWS THE MASTER Grosseteste and Leonardo da Vinci to Galileo eye emitting light, objects would ‘perturb’ Galilei, René Descartes, Johannes Kepler and the air between them and the eye, triggering , were in his debt. Indeed, the sight. Other philosophers around this time, influence of Ibn al-Haytham’s Optics ranks including Epicurus, attempted a form of alongside that of Newton’s work of the same ‘intromission theory’ of vision (light enter- title, published 700 years later. ing the eye from outside), but it was Plato’s Interest in optics began in antiquity. The theory that was given a mathematical basis Babylonians, Egyptians and Assyrians all used by , who described light rays emerg- polished quartz . The basic principles ing in a cone from the eye. Several centuries of geometric optics were laid down by Plato later, expanded on this idea. and Euclid. They included ideas such as the Early Islamic scholars such as al-Kindi and propagation of light in straight lines, and sim- Hunayn favoured a combined emis- ple laws of reflection from plain mirrors. The sion–intromission theory. They posited that earliest serious contribution from the Islamic the eye sends out light to the observed object, world came from ninth-century Arab scholar Ibn al-Haytham’s work long pre-dated Newton’s. which then reflects the light back into the eye. Ya’qub ibn Ishaq al-Kindi. It took the genius of Ibn al-Haytham to As a young man, Ibn al-Haytham received famous experiments on the nature of light. finally resolve the issue. He argued that if we an excellent education and was widely noted For example, using a , he see because rays of light are emitted from the as a mathematical and scientific prodigy. proved that light travels in straight lines; he eye onto an object (Plato and Euclid’s ‘sight Frustrated by his administrative duties work- also mathematized the fields of rays’), then either the object sends back a sig- ing in a government post in the vast Islamic (reflection of light by mirrors) and dioptrics nal to the eye or it does not. If it does not, how Empire — which at the time stretched from (refraction of light through lenses). This huge can the eye perceive what its rays have fallen India to Spain — he was sacked owing to real body of experiment and theory culminated on? Light must be coming back to the eye, and or, as some speculate, faked mental illness. in his Book of Optics. this is how we see. But if so, what use is there Sometime during the first decade of the This treatise can be regarded as a science for the original rays emitted by the eye? The new millennium, he proposed an ambitious textbook. In it, Ibn al-Haytham gives light could come directly from the object if it project to dam the Nile. He was invited to detailed descriptions of his experiments, is luminous or, if it is not, could be reflected Egypt by the Fatimid caliph al-Hakim bi’amr such as exploring how light rays are reflected from the object after being emitted by another Illah. However, on seeing the scale of the off plain and curved surfaces. He includes source. Rays from the eye, decided Ibn al- task, Ibn al-Haytham quickly realized that it the apparatus he used, the way he set it up, Haytham, are an unnecessary complication. was beyond him. He was promptly impris- the measurements and his results. He then He also went further than anyone before oned in Cairo for wasting the caliph’s time. uses these observations to justify his theo- in trying to understand the underlying phys- Far from cowing him, the decade of ries, which he develops with geometrical ics of refraction. He argued that the speed of imprisonment granted Ibn al-Haytham the models. He even urges others to repeat light was finite and varied in different media, seclusion to think and write, particularly on his experiments to verify his conclusions. and he used the idea of resolving the path optics. After his release Many historians of science consider Ibn Book of Optics around the year 1020, (Kitab Al-Manazir) al-Haytham to be the first true proponent of LIGHT he began working at IBN AL-HAYTHAM the modern . A Nature special issue a prolific rate, car- (ALHAZEN) The work can be roughly divided into nature.com/light205 rying out a series of ad 1011–21 Books I, II and III, devoted to the theory of

164 | NATURE | VOL 518 | 12 FEBRUARY 2015 © 2015 Macmillan Publishers Limited. All rights reserved BOOKS & ARTS COMMENT of a light ray into its vertical and horizontal components of velocities. He carried out all his work geometrically, and introduced many Books in brief new ideas, such as the study of how the atmos- phere refracts light from celestial bodies. Hell and Good Company: The Spanish Civil War and the World it Made Later Islamic scholars, including the Richard Rhodes Simon and Schuster (2015) thirteenth-century Persians Qutb al-Din al- His 1986 The Making of the Atomic Bomb (Simon and Schuster) is Shirazi and Kamal al-Din al-Farisi, extended a towering chronicle of modernity. Now historian Richard Rhodes the Optics. Al-Farisi, who wrote The Revi- examines the “little world war”, Spain’s hellish 1936–39 civil sion of the Optics (Tanqih al-Manazir), conflagration (see Nature 494, 34; 2013). As he shows, it was a used to arrive at the first correct testing ground for medical and technological advances — in blood mathematical explanation of the transfusion on one hand, and on the other in airborne warfare, which (at the same time as, but independently of, led to the bombing of Guernica immortalized in Pablo Picasso’s the German scholar ). great painting. Luminaries drawn to the war, he shows, ranged from The Book of Optics was first translated geneticist J. B. S. Haldane to George Orwell. into Latin in the late twelfth or early thir- teenth century, as De Aspectibus. The English philosopher and empiricist How to Fly a Horse: The Secret History of Creation, Invention, then wrote a summary of it, as did his Pol- and Discovery ish contemporary Witelo. It was soon being Kevin Ashton Doubleday (2015) cited across Europe. Among the many ideas This study of creativity by Kevin Ashton — the technical pioneer behind taken up by Ibn al-Haytham’s Latin-reading the ‘Internet of Things’ — is a testament to Thomas Edison’s definition disciples was that pure light was not visible, of genius (1% inspiration, 99% perspiration). Science, Ashton argues, and that its job was simply to allow us to see is less a thing of “eureka shrieks” than of hard work, small steps and colour. Even Kepler, understanding of adversity. His case studies compel, from Réunion “Many who studied Ibn al- slave Edmond Albius’s 1841 breakthrough in vanilla pollination to historians of Haytham’s work, surgeon Judah Folkman’s “series of repetitive failures” that led to the science consider thought this; it took discovery of angiogenesis, now key to cancer treatments. Ibn al-Haytham Newton to describe to be the first light as itself being true proponent made up of differ- Future Arctic: Field Notes from a World on the Edge of the modern ent colours. (Other Edward Struzik Island (2015) scientific erroneous ideas In September 2014, dwindling sea ice forced some 35,000 walruses method.” in Optics include onto the Alaskan coast — just one ecological event in a multitude a repetition of besetting the ‘climate-changed’ Arctic. Journalist and explorer Ptolemy’s mistaken law of refraction, and Edward Struzik cogently analyses the environmental and policy an incorrect understanding of reflection as a challenges, drawing on research into past extinctions and present more intense form of refraction.) disruptions such as tar-sand exploitation, military territorialism and Ibn al-Haytham’s work decisively influ- tundra fires. As he ticks off the costs to indigenous peoples, ocean enced the theory of perspective that flowered biodiversity, caribou habitat and more, the case for an Arctic treaty in Renaissance European science and art. and serious conservation efforts becomes ever clearer. De Aspectibus was translated into Italian in the fourteenth century, making it accessible to practitioners such as the Florentine art Words Onscreen: The Fate of Reading in a Digital World theorist and architect Leon Battista Alberti, Naomi S. Baron Oxford University Press (2015) author of the 1435 treatise On Painting (Della For every digital devotee clutching an e-reader, there is an old-school pittura), the sculptor Lorenzo Ghiberti and bibliophile brandishing a physical book. But which works best for the geometer-artist Piero della Francesca. reading comprehension? In this thoughtful study, linguist Naomi They harnessed Ibn al-Haytham’s discussions Baron investigates each platform in the light of recent research, on perspective to help to create the illusion and surveys US, Japanese and German reading habits. E-readers, of three-dimensional depth on canvas and in she finds, democratize access and offer easy storage, but can also friezes. These revolutionary artists strove to discourage tackling more involved texts or rereading, and encourage understand both the objective world and the “power browsing” rather than perusal. She recommends allowing visual system that determined its appearance. room for both options — letting “form follow function”. Today, as we use laser beams to manipu- late atoms, stimulate neurons with light or convey information in entangled photons, What Nature Does For Britain it is worth recalling that the foundations of Tony Juniper Profile (2015) this field were laid down around 1,000 years Part research round-up, part manifesto, this treatise on Britain’s ago by Ibn al-Haytham. ■ ‘natural capital’ is a model of pragmatism. As environmentalist Tony Juniper shows, UK ecosystems were valued at £1.6 trillion (US$2.4 Jim Al-Khalili is a professor of at the trillion) in 2011 by the Office for National Statistics. Yet poor practices University of Surrey in Guildford, UK, and such as overfishing and soil degradation are breaking nature’s bank. the author of Pathfinders: the Golden Age Juniper offers smart policy action points for switching to sustainability, of Science. and ingenious case studies — from ‘woodland system’ farming to e-mail: [email protected] reintroduced beavers that help with riverine flood control. Barbara Kiser

12 FEBRUARY 2015 | VOL 518 | NATURE | 165 © 2015 Macmillan Publishers Limited. All rights reserved