[Islam-Online- Psychology]

Latin manuscripts of Ibn al-Haytham's The Book of Optics had rendered the author's name in various forms, for example, Alhacen filius Alhaycan, Hacen filius Hucayn filius Haycen, and Achen filius Hucayn filius Aycen.39 In the twelfth century, The Book of Optics was translated into Latin with the name Perspectiva. In 1572, Risner produced the Opticae Thesaurus. This was the first printed version of Alhazen's book with added titles and annotations from Vitello's Perspectiva.40 Risner found in the two manuscripts he used for his edition that the author's name was rendered as Alhazen, which obviously corresponds to Ibn al-Haytham's first name, al-Hasan. In the early 1900s, copies of The Book of Optics were found in manuscript collections in Istanbul. Krause published a complete list of these manuscript copies in 1936. In 1942, Mustafa Nazif published the first substantial study of Ibn al-Haytham's optical work based directly on original manuscripts. The first volume, which has the Arabic text of Books I-III, was published in 1983 by the National Council for Culture, Arts, and Letters in Kuwait. The Book of Optics was translated with an introduction, commentary, glossaries, concordance, and indices by Sabra of Harvard University in 1989. 41 All citations in the present study are taken from Sabra's excellent translation.
According to Ibn Abi Usaybi'a's Tabaqat, Ibn al-Haytham's writings include fourteen works wholly devoted to the subject of light and vision, a clear indication of their author's strong and sustained interest in optical matters.42 These writings are (1) Kitab lakhkhastu fihi 'ilm al-manazir min kitabay Uqlidis wa Batlamyus, wa tammamtuhu bi ma'ani al-maqala al-ula al-mafquda min kitab Batlamyus (A book in which I have summarized the science of optics from the two books of Euclid and Ptolemy, to which I have added the notions of the first discourse which is missing from Ptolemy's book); (2) Maqala fi al-maraya al-muhriq, mufrada 'amma dhakartuhu min dhalik fi talkis kitabay Uqlidis wa Batlamyus fi al-manazir (Treatise on burning mirrors, which is separate from what I have stated on this subject in the summary of the two books of Euclid and Ptolemy on optics); (3) Kitab al-manazir (Book of optics); (4) Maqala fi daw al-qamar (Treatise on the light of the moon); (5) Maqala fi qwa' quzah wa al-hala (Treatise on the rainbow and the halo); (6) Maqala fi ru'yat al-kawakib (Treatise on the appearance of the stars, or On seeing the stars); (7) Maqala fi al-maraya al-muhriqa bi al-dawa'ir (Treatise on spherical burning mirrors); (8) Maqala fi al-manazir 'ala tariqat Batlamyus (Treatise on optics according to Ptolemy's method); (9) Maqala fi kayfiyyat al-azlal (Treatise on the quality of shadows); (10) Maqala fi adwa' al-kawakib (Treatise on the light of the stars); (11) Maqala fi al-athar alladhi fi wajh al-qamar (Treatise on the mark on the face of the Moon); (12) Maqala (or qawl) fi al-daw (Discourse on light); (13) Maqala fi al-kura al-muhriqa (Treatise on the burning sphere); and (14) Maqala fi surat al-kusuf (Treatise on the form of the eclipse).
These publications show clearly his expertise in this domain of science. He was quite familiar with earlier works on the subject, such as those of Euclid and Ptolemy. But he was not a translator or commentator on the previous literature; he made an original contribution to the field. His writings also show how psychophysics and experimental psychology relied on the solid base of investigations in astronomy. This result shows that psychophysics borrowed its structure from physics and its spirit from psychology in the eleventh century. 43
The Book of Optics of Ibn Al-Haytham is divided into seven books. Book I shows the manner of vision generally, Book II details the visible properties, Book III examines the errors of vision and their causes, Book IV studies visual perception by refraction from smooth bodies, Book V shows the positions of images, Book VI examines visual errors by reflection and their causes, and Book VII studies the manner of visual perception by refraction through transparent bodies whose transparency differs from that of air. Book I is titled "On the Manner of Vision in General" and consists of eight chapters: "Preface to the book," "Inquiry into the Properties of Sight," "Inquiry into the Properties of Lights and into the Manner of Radiation of Lights," "On the Effect of Light upon Sight," "On the Structure of the Eye," "On the Manner of Vision," "On the Utilities of the Instruments of Sight," "On the Reasons for the Conditions Without the Combination of Which Vision Is Not Effected." In the introduction to Book I, he summarizes and reviews the literature on previous research in optics. He notes:

Early investigators diligently pursued the inquiry into the manner of visual sensation and applied their thoughts and effort to it, eventually reaching the limit to which their investigation had led, and gaining as much knowledge of this matter as their inquiry and judgment had yielded. Nevertheless, their views on the nature of vision are divergent and their doctrines regarding the manner of sensation not concordant. Thus, perplexity prevails, certainty is hard to come by, and there is no assurance of attaining the object of inquiry. How strong, in addition to all this, is the excuse for the truth to be confused, and how manifest is the proof that certainty is difficult to achieve! For the truths are obscure, the end hidden, the doubts manifold, the minds turbid, the reasoning various; the premises are gleaned from the senses, and the senses (which are our tools) are not immune from error.44

He concluded Book I with the following remarks:
We have shown the reasons on account of which the eye cannot perceive any visible object unless the object combines the conditions stated. The preceding chapters and the explanations we have given in them are what we intended to make manifest in this book. 45 Ibn al-Haytham distinguishes two main approaches to the study of vision, which he ascribes to "physicists" (or natural philosophers, ashab al-tabi'a) and to "mathematicians" (ashab al-ta'alim). The first of these approaches seeks to account for visual perception in terms of "forms" received in the eye, and the second explains the visible appearance of objects by means of "visual rays" assumed to go forth from the eye. In Ibn al-Haytham's view, neither approach is self-sufficient, though each captures a certain amount of the truth. Accordingly, he became convinced that a sound and complete theory of vision must bring these two approaches together or, as he puts it, must achieve a "synthesis" (tarkib) of physical and mathematical considerations. As a first approximation, the synthesis proposed by Ibn al-Haytham in the Optics can be described as an application of the geometrical methods employed by the visual-ray theories to the "physical" doctrine of forms.46 Ibn al-Haytham writes:

Our subject is obscure and the way leading to knowledge of its nature difficult; moreover, our inquiry requires a combination of the natural and the mathematical sciences. It is dependent on the natural sciences because vision is one of the senses and these belong to natural things. It is dependent on the mathematical sciences because sight perceives shape, position, magnitude, movement and rest, in addition to its being characterized by straight lines; and since it is the mathematical sciences that investigate these things, the inquiry into our subject truly combines the natural and the mathematical sciences.47

Book II, "On the Visible Properties, Their Causes and the Manner of Their Perception," consists of four chapters: "Preface," "On Distinguishing the Lines of the Ray, "On the Manner of Perceiving Each of the Particular Visible Properties," and "On How Sight Perceives Visible Objects." Chapter 2, in particular, tackles various issues related to the psychophysics of vision or perception: perception of light, of color, of distance, of position, of solidity, of shape, of size, of separation, of continuity, of number, of motion, of rest, of roughness, of smoothness, of transparency, of opacity, of shadow, of darkness, of beauty, of ugliness, of similarity, and of dissimilarity. In Book II, Ibn al-Haytham discusses the different conditions of the radial lines and distinguishes their characteristics. In addition, he gives a detailed account of all aspects of visual perception, shows the manner in which sight perceives each of them, distinguishes the ways in which sight perceives visible objects, and shows how they differ from one another. 48

In Book II of The Book of Optics, Ibn al-Haytham maintains that the objection raised by the adherents of the visual-ray hypothesis applies only to a deficient theory, not to a properly developed one that takes into account the psychological process necessarily involved in every normal act of seeing. Accordingly, the purpose of Book II is to provide a full explanation of the psychology of visual perception - undoubtedly the most important single component of the theory of direct vision already launched in Book I. The general outline of the psychological account is as simple as it is distinctive. An image (or form) of the object seen is carried intact from the eye in which it has been received to the "common nerve" where it is perceived by a faculty called "the last sentient" (al-hass al-akhir), which resides in the front of the brain. Presumably this is where all kinds of sensation (visual, auditory, tactile) are registered after they have been delivered by the various sense organs.49
Book III, "On Errors of Direct Vision and Their Causes," consists of seven chapters: "Preface," "On What Needs to Be Advanced for Clarifying the Discussion of Errors of Sight, "On the Causes of Errors of Sight, "On Distinguishing Errors of Sight," "On the Ways in Which Sight Errs in Recognition," and "On the Ways in Which Sight Errs in Inference." Chapter 7 tackles several issues related to the psychology of visual error, in current psychological terms "visual illusions." These issues are errors of distance, position, illumination, size, opacity, transparency, duration of perception, and the condition of the eye. As far as the theory of direct or rectilinear vision is concerned, Ibn al-Haytham's fundamentally new ideas, according to Sabra, are mostly contained in the first two books of The Book of Optics. The third book is largely an exercise in the extension and application of these ideas. But the applications often seem somewhat mechanical or only a little inspired; and they frequently serve to illustrate obvious remarks, rather than explore new ground.50 Book III is an experimental proof of Ibn al-Haytham's theory of vision and perception. As he puts it: "We will now explain in detail and sum it up, and also show how these matters can be experimentally examined in such a way as to achieve certainty.51
Psychophysicists have tackled various issues including sensitivity to light and light intensity, general sensation and its variation, perception of color, the sensation of touch, perception of darkness, the moon illusion, and binocular vision. Regarding sensation, Ibn al-Haytham writes:

It has also been shown that sensation occurs only through the crystalline. Therefore, the eye's sensation of the light and color that are on the surface of the visible object occurs only through that part of the crystalline's surface which is determined by the cone formed between the object and the center of the eye. And we saw that this humor has some transparency and some density, and for this reason it is likened to ice. Thus, because it is somewhat transparent it receives the forms and these pass through it on account of the transparency that is in it; and because it is somewhat dense it resists the forms and hinders them from passing throughout it on account of that density that is in it, and the forms are fixed in its surface and its body on account of that density. Similarly, with every transparent body that is somewhat dense: when it is illuminated, the light passes through it according to the transparency that is in it, and the light is fixed on its surface according to its density; thus light appears on the surface and in the whole of the body in as much as it is fixed in it.52

Ibn al-Haytham does not identify physiology and psychology, nor does he take the seeing of an object to be merely a matter of holding up an image to the mind's eye, so to speak. To present his theory simply as an explanation of how an optical image flows through the visual apparatus would be to concentrate on the results of Book I of his Optics, and to disregard those of the two following books. He was fully aware that we see things, not images, and he insisted on the distinction between sensation and perception, although he applied the same word, idrak (comprehension/perception) to both, calling the former "perception by pure sensation" (idrak bi-mujarrad al-hiss or comprehensio solo sensu). Book II, in fact, expounds an elaborate theory of what we should call "visual perception." The theory involves the notion of form as the total visible appearance of a thing or the totality of the thing's visible characteristics. The "form of an object," in this sense, is made up of all the object's "visible properties" (al-ma'ani al-musara), as he calls them.53 Regarding touch, he writes:

Now the sensation of touch and of pain extends from the organs only through the filaments of the nerve and through the spirit extending within those filaments. So when the forms of visible objects occur in the body of the vitreous humor and are sensed by this organ, the sensation extends from it into the sentient body that fills the cavity of the nerves that joins the eye and the front of the brain. The form extends, along with the extension of the sensation, while preserving the arrangement of its structure and the relative positions of its parts. For it is in the nature of the sentient body to preserve the arrangement of these forms. And this arrangement is preserved in the sentient body because the parts of this body that receive the parts of the forms, and the distribution of the receptive power that exists in the parts of the sentient body, are similarly arranged in the vitreous body and throughout the subtle body that fills the nerve's cavity. That being so, when a form arrives at any point on the surface of the vitreous, it runs along a continuous line the position of which remains untouched in the nerve's cavity through which the sentient body extends.54

Ibn al-Haytham's description of the unconscious processes that underline our perception of the physical world are not confined to considerations of features internal to the optical image, but include other experiences, not all of which are connected to the organ of sight. Tactile and muscular sensations that do relate to the organ of sight are, for example, those experiences of opening and closing our eyes, which, he says, are at the basis of our judgement that objects of vision lie before us in external space and not inside our eyes or our heads. Another example is the muscular sensation associated with turning the head or orienting the eyeball, which is involved in judgements about direction. Other examples involve other types of experience. Some of the most remarkable explanations in this connection are concerned with estimating the distance of an object, itself an essential factor in estimating the object's size.55
Fechner, according to Schultz, developed the method of average error in the nineteenth century.56 Ibn al-Haytham asked why objects very close to the eye appear to be larger than they are. Vision estimates the size of a visible object by comparing the angle subtended by the object at the center of the eye with the estimated distance of the object. In all cases, the distance that sight is capable of estimating is that between the surface of the eye and the object, and this falls short of the "real" distance by an amount equal to the radius of the eyeball. For moderately remote objects, the difference between the real and the estimated distance is negligible. The difference becomes critical when the distance of the object from the surface of the eye is less than, equal to, or not much larger than the radius of the eye. In this case the comparison is made between a large angle and an estimated distance appreciably smaller than the real one.57
Saturation is a psychophysical quality of light that might be described as the degree to which a color appears to be free of whiteness or blackness or the extent to which it is colored as opposed to achromatic. Although many variables can affect saturation (for example, intensity, retinal locus of stimulation, stimulus size, temporal factors), the two most important determinants are colorimetric purity and wavelength.58 Regarding saturation of colors, Ibn al-Haytham writes:

Now to distinguish between two greens is not the same as the sensation of green, for the latter is due to the eye's becoming green by the action of the green, and the eye has become green by the action of the two greens; and as a result of becoming green by the action of both greens the sense perceives them to be of the same kind. Thus its perception that one of the greens is stronger than the other, and that they are of the same kind, is a discernment of the coloration that has taken place in the eye, and not a sensation of the coloration itself. Similarly, when two colors are of similar strength and of the same kind, the sense will perceive them and perceive that they are of the same kind and of similar strength. And it is similarly the case with lights in regard to the sense of sight. For the sense of sight perceives the lights, differentiates between strong and weak lights and perceives their similarity in strength and weakness. Therefore, the sense of sight's perception of the similarity and dissimilarity of colors and lights, and its perception of the similarity and dissimilarity of the outlines and structures of the forms of visible objects, is not due to mere sensation but to their being discerned and compared with one another.59

According to Fechner, a lengthy stay in the dark gives one the capacity to see in the dark; by staying in the light for a time one loses this ability. But what does it mean, to see in the dark? It means that one can still distinguish from darkness a light, which photometrically differs very little from the dark night. Indeed, we have to consider here not so much an absolute sensation as a difference, since the nocturnal darkness still has its own photometric intensity. Thus it appears as if the tiring of the eye by the stimulation of light also blunts its sensitivity for differences.60 Ibn al-Haytham notes this clearly in Book II of The Book of Optics, in his view of the perception of darkness:

As for darkness, sight perceives it by inference from the absence of light. For darkness is the total absence of light. Where, therefore, sight perceives a certain place without perceiving any light in it, it will sense darkness. Darkness is perceived by the sentient from lack of sensation of light.61

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