I am not really a historian of optics but being a historian of astronomy and a semi-expert on the history of the telescope I somehow couldn’t avoid gaining a certain amount of knowledge on the history of optics a subject sprinkled with a fair number of well known myths a couple of which I intend to deal with here.
The normally excellent Jennifer Ouellette has a rather nice post up at Cocktail Party Physics on pinhole cameras. Unfortunately when she comes to describe their history she manages a couple of slip-ups that I feel I can’t really ignore. After having correctly pointed out that the pinhole camera has its origins in the camera obscura, one of many scientific terms coined by Johannes Kepler, and also correctly saying that the principle behind it was known both in European and Asian antiquity she then moves forward to Ibn al-Haytham and the role he played in its history and the history of optics in general. Here she makes a couple of claims that are simply false.
Ms. Ouellette writes:
Since Aristotle, the common assumption had been that the eye sent out rays of light to scan objects. Alhazen determined that light was reflected into the eye from the things one observed.
The first of these statements is a common myth, which is usually stated that the Greeks believed that… This statement lives from the highly mistaken belief that there is something called Greek science i.e. some sort of unified scientific system, which all or at least the majority of Greek natural philosophers believed in or accepted, this is simply not true. Over the roughly twelve hundred years in which that which we lightly refer to as Greek science was practiced in various parts of the Mediterranean basin there were numerous schools of thought who supported differing and very often contradictory views on the various scientific subjects; optics and the theory of vision were no exception. The ancient Greeks propagated at least five different theories of vision, which can be categorised into three main groups; those which have visual rays going out from the eyes to the objects seen, extromission theories, those which have visual rays going from the objects to the eyes, intromission theories, and those of mixed form. It is somewhat ironic that by implication Ms. Ouellette presents Aristotle as a representative of the extromission theory as he in fact held a mediumistic theory of vision that is normally considered to be an intromission theory.
The oldest theory of vision in Greek science is from the atomists who believed that an image of the object viewed consisting of an outer layer of atoms travelled from the object to the eye, definitely an intromission theory and one with so many problems that I wont discuss it further here. Up next was Plato who is usually credited in popular accounts with an extromission theory i.e. visual rays travel out from the eyes to view the object but in fact whose theory, somewhat confuse and difficult to interpret as with most of his science, was a mixed form with vision being made possible by a combination of visual rays from the eyes and daylight. Aristotle, Plato’s rebellious ex-student, developed a completely different theory in which light created a bridge between the object and the eyes and the image of the object was stamped on the eyes by a vibration along this bridge from object to eye; some historians see an early form of the wave theory of light in this theory but I think this claim is exaggerated. In the Stoic version pneuma issuing from the eyes and activated by sunlight creates a visual instrument that transfers the image along the optic nerve to the brain, a view also held by the physician Galen whose physiology of the eye was adopted by al-Haytham. Last but not least we have the people who actually held a pure extromission theory of vision the mathematicians who created the geometrical theory of optics Euclid, Hero and Ptolemaeus. Now these three hypothesised a cone or pyramid of visual rays emitted from the eyes to encompass the object and were thus able using simple geometry to work out the mathematical rules of optics. In the case of Euclid and Hero it is not totally clear if they actually believed in visual rays emitted by the eyes or whether this was just a useful fiction to aid their mathematical investigations, however with Ptolemaeus it is relatively clear that for him the visual rays are real.
All of the Greek theories sketched above were known to Islamic scholars before al-Haytham and each theory had its defenders, what al-Haytham, who believed in an intromission theory based of light and not visual rays, did was to demonstrated that it was possible to retain the geometrical optics of Euclid et al whilst holding an intromission theory.
In her next sentence Ms Ouellette writes:
He also recorded the laws of reflection and refraction, correctly attributing the effects to the fact that light travels more slowly through denser mediums.
Ibn al-Haytham did in fact record the laws of reflection but as these had already been correctly stated by Euclid et al there is nothing particularly special about that and these laws have nothing to do with the speed of light in media, an unfortunate slip of the pen. He did not however record the laws of refraction, as they weren’t known to him. It is one of the quirks of the history of science that the correct laws of refraction were discovered by another Islamic scholar Ibn Sahl who lived before al-Haytham but this discovery remained unknown and the laws weren’t discovered again until the 17th century when they were discovered independently by four different researchers Harriot, Snel, Descartes and Gregory.