One of the websites that I usually enjoy reading is Wonders & Marvels a collective of historians who post mostly short reports on historical things, oft medical, that they have found fascinating. However, as I recently visited this delightful oasis of historical frivolity I groaned inwardly upon reading the post on Abū Alī al-Hasan ibn al-Hasan ibn al-Haytham (known simply as Ibn al Haytham or in mediaeval Europe as Alhacen or Alhazen) the mediaeval Islamic scholar by Pamela Toler that I found there. I hasten to add that Ms Toler is not solely to blame for the heap of excrement posing as history of science that she has posted there, as she was just regurgitating, probably inaccurately, what she had read in a popular book on Islamic science, about which more later.
After an opening paragraph that gives a somewhat mangled version of a part of Ibn al-Haytham’s biography Ms Toler presents us with the following paragraph:
While confined in his home, Alhazen revolutionized the study of optics and laid the foundation for the scientific method. (Move over, Sir Isaac Newton.) Before Alhazen, vision and light were questions of philosophy. Alhazen considered vision and light in terms of mathematics, physics, physiology, and even psychology. In his Book of Optics, he discussed the nature of light and color. He accurately described the mechanism of sight and the anatomy of the eye. He was concerned with reflection and refraction. He experimented with mirrors and lenses. He discovered that rainbows are caused by refraction and calculated the height of earth’s atmosphere. In his spare time, he built the first camera obscura.
Nearly every single claim in this brief paragraph is wrong and I shall now try, at least in outline, to correct some of the worst errors.
Whilst it is true that Ibn al-Haytham made a major advance in the study of optics I personally, as a gradualist, object strongly to any use of the word revolution in any of its forms when writing about the history of science. Ibn al-Haytham made an important step forward building on the work of others, his work in turn being pushed forward by others. He did not in anyway what so ever invent the scientific method. Put quite simply nobody did. (see next post!)
Before Alhazen, vision and light were questions of philosophy.
Here we start in on the real rubbish. The Islamic scholars inherited their knowledge of optics from the Greeks and to state simply that Greek optics consisted of questions of philosophy is to display a deep ignorance of the history of the subject. The Greek study of optics can be divided into three main areas: philosophical, medical and mathematical. The opinions on vision can be further categorised according to whether the rays that enable vision extrude from the eyes to the object viewed, extramission, from the object viewed to the eyes, intromission, or in both directions mixed.
Philosophical theories of vision were propagated by the Atomists, intromission, Plato, mixed, Aristotle, intromission mediumistic, and the Stoics also mixed. The mathematical theories laid the basis of geometrical optics, and were propagated by Euclid, Hero of Alexandria and Ptolemaeus, all three supporting an
intromission extramission theory although it is not clear if this is purely instrumental in order to facilitate simpler calculations. The principle medical theory transmitted to the Islamic scientists was that of Galen who combined a surprisingly accurate physiological knowledge of the eye with a Stoic philosophy of vision. It is not necessary for our purposes to go into greater details. It should be pointed out that all the theories except that of the atomists involved the presence and active involvement of light although the visual rays were not simply light rays.
Ibn al-Haytham argued philosophically very strongly against extramission and for intromission whilst at the same time demonstrating that an intromission theory could successfully be combined with the geometric optics of Euclid. At the same time he integrated the theory of al-Kindi that light reflects in all directions from all points of a viewed object arguing that only light rays are involved in vision. His theory of vision was thus a clever synthesis of several different theories into a coherent whole and thus a major advance in the understanding of optics.
He accurately described the mechanism of sight and the anatomy of the eye.
Ibn al-Haytham adopted Galen’s description of the anatomy of the eye and added nothing to it. His theory of vision was defective in that he like Galen believed that vision takes place in the lens of the eye and not as Kepler correctly surmised on the retina. Because of this major defect in his theory Ibn al-Haytham was forced to develop an erroneous theory that only rays falling perpendicularly onto the lens of the eye were perceived. Otherwise the eye would perceive multiple images of the object. In reality the lens focusing all the rays no matter from which angle creates just one image on the retina.
He experimented with mirrors and lenses
Ibn al-Haytham describes a limited number of experiments to demonstrate that light in propgated in straight lines, something that nobody had doubted since Euclid at the latest. To what extent these were actual experiments or just thought experiments is disputed amongst the experts.
In his spare time, he built the first camera obscura.
The principle of the camera obscura was already known to Aristotle and even earlier to the Chinese scholar Mo-Ti in the fifth century BCE who referred to it as the locked treasure room.
Ms Toler discovered her enthusiasm for Ibn al-Haytham as she tells us through reading a popular book on Islamic science.
Modern physicist Jim al-Khalili, in his excellent The House of Wisdom: How Arabic Science Saved Ancient Knowledge and Gave Us the Renaissance, calls Alhazen the greatest physicist of the medieval world, and possibly the greatest in the 2000 years between Archimedes and Sir Isaac Newton. His Book of Optics was first translated into Latin in the late twelfth or early thirteen century. It had an enormous impact on the work of western scientists from Roger Bacon (c. 1214-1292) to Isaac Newton (1642-1727).
By pure chance I stumbled across a video of a public lecture that Jim al-Khalili gave earlier this year on the subject of his book. I’m not going to give a detailed analysis of this lecture as it contains enough errors to keep me in blog posts for at least a year. He seems to be of the opinion that because he is a physicist and was born in Baghdad that this qualifies him to write a book about the history of Islamic science. In the lecture he proudly tells us that he devoted all of eighteen months of his research time to researching this book. A. Mark Smith took fourteen years to research and write his annotated translation of the first three books of the Latin edition of Ibn al-Haytham’s Book of Optics but he is a mere historian and not a physicist. In his lecture al-Khahili gives a clear and explicit commitment to a Whig interpretation of the history of science and strong implied commitment to the great man theory; both of these have been rejected by real historians of science long ago.
A typical example of al-Khalili’s arrogant ignorance occurs in his lecture when he talks about the first use of place value decimal fractions in Arabic mathematics. First of all he apparently doesn’t know the difference between the decimal point and decimal fractions. He also doesn’t appear to know that the Babylonians were using place value fractions, albeit sexagesimal not decimal, a thousand years earlier. He then goes on to say that The Chinese developed decimal fractions about the same time as Arabic mathematicians “but we don’t know as much about them”! I hope he doesn’t make this statement within reach of the Needham Research Insitute. Al-Khalili appears to confuse his own ignorance of the history of science with the general state of the art.
Returning to the blog post we have the following statement:
Jim al-Khalili […] calls Alhazen the greatest physicist of the medieval world, and possibly the greatest in the 2000 years between Archimedes and Sir Isaac Newton.
None of the three scholars named above was a physicist in the modern sense of the word and as I’ve said in the past there is no such thing as “the greatest”. Even if we ignore these criticisms al-Khalili’s statement would be a hopeless exaggeration. A much more sensible assessment of Ibn al-Haytham’s achievements is given by somebody who knows what he is talking about, historian of optics David C. Lindberg:
Alhazen was undoubtedly the most significant figure in the history of optics between antiquity and the seventeenth century.
There is a substantial difference between the two claims most important, historically, Ibn al-Haytham’s influence stops with the new model of optics developed by Kepler.
As far as I can see without having read his book al-Khalili is a typical example of a scientist thinking because I’m an expert in my subject I can also write about its history without doing the groundwork. Writing history requires a different form of expertise to doing modern science and writing about the history of science requires a wide range of expertise that cannot be thrown together in ones spare time in eighteenth months if one is trying to write a survey of the scientific activity of a major culture over a period of something approaching a thousand years. To do so without first acquiring the necessary expertise results not in history but in a collection of anecdotes and clichés most of them inaccurate and many simply false.
 What is the collective noun for historians? A heap? A huddle? A hysteria? I somehow feel it should be alliterative.
 This quote is taken from David C. Lindberg, Theories of Vision from al-Kindi to Kepler, which is a good source for anybody wishing to fill in on the history of optics that I only sketch above.