He was a great astronomer, mathematician and physician. He wrote more than sixty books on different subjects. In all his work, the book on optics was a great revelation in the history of science. In his work he embraces all the subjects of the optics including the optical physiology.
His work includes a study on the reflection and refraction with realised experiments with the help of different mirrors (spherical, parabolics, cylindrical, concave and convex). A study on the magnifying glass, reasearch on shade, colors, rainbow and a discussion on light, that is the first philosophical scientific treaty on vision.
Optics in the western world before Alhazen
From the Vth century before J.C., a research between the eye and the seen object where the light as the principal actor was the main discussion because that light links the object to the eye. I today everyone adopts this theory without having a single thought about it and taking it as an evident reality, it is in fact a result of work and discussion of two thousand years.
The way the eye and the object communicated was one of the solutions another possible thought was a connection getting out of the eye and going to the object seen or from the object seen to the eye or a coexistence between those two things going from one to the other and vice versa.
All the ideas came into the discussion the first theory was a transmission from the eye to the object which was adopted by pythagoreans.
The atomic scientists were favorable to the transmission from the object to the eye. Empedocle (491-430 before J.C.) was the first one to conclude a two way transmission it is only a century later that a fourth theory came up (figure 1) with Aristot (384-322 before J.C.). At that time visual optic was dominated mostly by Greeks who extented their phylosophical ideas to vision.
- Fig. 1 — Rays come out of the eye and go to fill the object.
For Plato (428-348 before J.C.) what he calls the divine fire or luminous force, gets out of the eye to detect the object. When the divine fire gets out of the eye it encounters the light sent by the object. It is the union of the two lights that gives the vision for the color vision it is the quantity of the divine fire which changes from one color to another.
Epicurus (341-280 before J.C.) denies the existence of rays trandsmitted by the eye to the object.
For Zeno (485-430 before J.C.) the rays transmitted by the eye felt the object.
At the alexandrian school with Euclid (3rd century before J.C.) and Ptolemee (2nd century before J.C.) the ideas of rays transmitted by the eye remains valid. Euclid says that the rays which are transmitted by the eye are made of luminous superimposed corpuscles contained in the area. For Ptomelee the eye sends visual rays. During the 3rd century, mathematical basis necessary to optics were elaborated by Euclid, Archimedes and Apollonius.
A conclusive contribution was brought by Galien (130-201 after J.C.) which described the structure of the eye. He brought the eye together with other sensorial organs of the human being. He brings up the optical nerve, which plays a role of a cavity through which a visual fluid flows from the brain. The ideas of Galien and Plato’s theory get together, that is a fluid flows to one eye and another fluid from the eye without getting out of it which makes the organe sensitive and ables it to be impressed by the first fluid. The structure of the eye strats to be involved even if a false idea which says that the cristallin plays a role.
All these ideas are going to be conclusive in the development of theories related to the vision mechanism and therefore on light. After Galien not much progress was made. Eight centuries later from synthesis on geometric and physiologic studies by Alhazen a general theory will come up. What will remain from the antique heritage is that the rays flow from the eye and goes to feel the object and the expression to have an evil eye.
Optics by Alhazen
Alhazen establishes optics on fundamental basis and physiology. He has undertaken the first reform on optics. He has extended his study by taking into consideration other phemonenon in his work Optica Thesurus.
In the first part of his work he says that direct and bright colors hurt the eye. With this idea, he stands by those who, admitted the existence of a light independent from the human being. His argument is that when the eye is in contact with bright light, they hurt and in fact when one observes the sun, he cannot see it because the excessive light hurt his eyes. And that is the same when one looks at a mirror well polished and illuminated by the sun. If the eye is placed in the sunbeam reflected by the mirror, that light that comes from the mirror to the eye the pain will be such that it will unable him to open his eyes.
He then concludes that if the eye suffers by something dazzling than we can believe that there is something flowing from the eye to the object because there is no reason to suffer when the eye is in contact with one object and not another but it is necessary to admit that there is something that flows from the object to the eye which has the capacity to reflect in a mirror concerning light, he says that any illuminated body by any kind of light, it flows from the light in all directions (figure 2).
- Fig. 2 — Light flows to the object and goes back in all directions by that way is sent to each eye that faces it.
Alhazen continues with the same logic and says that when the eye is placed in front of an illuminated object, this will provide light to the external area of the eye. But it has been established that light has an impact on the eye and it is in the nature of the eye to feel the light and to suffer from it. The conclusion that follows is that the eye can see the object from the light that flows from it.
The second part of his work deals with experimentations related to visual sensitometry. He demontrates that a mild or excessive brightness or the interposition of flames under certain conditions make an object invisible or make its particularities more or less visible under good enlightment.
The third part of his work, based on various observations he shows that the tonality of colors varies with light.
In the fourth part, he describes the structure of the eye and in following chapter he discusses the vision mechanism. Vision is the result to rays coming from the object and going the eye. Here is the point where he differs from the grecoroman theory. But one problem remains to be solved which is how that light gets into the eye, gives form and structure to the object.
Alhazen comes up with one idea. He brings an element of sensitivity without any element of direction, the eye can see light and colors mixed and confused and an organized vision links one point of the object to one impressed point of the eye from the optical diagram of the eye of Galien and considering that the several different transparent layers are concentric, the crystalline at the centre of the eyeball where the essential of the visual faculties stands; Alhazen takes into consideration Euclid’s pyramidal perspective but draws it with rays going from various part of the object and flowing at the surface of the eye and therefore places the summit of the pyramid at the centre of the eye. Colors and different points of the object are spread in a regular way along the rays to the cornea and pupil. They get printed on the first layer of the crystalline. This picture in the same order, similar to the object is therefore in the eye with all the elements to realize a vision knowing the existence and the nervous structure of the retina why Alhazen admits that printing is made at the first layer of the crystalline? The answer to that question is evidence? It is due to the difficulty to get the rays through the crystalline.
He had discovered the darkroom and knew that image of a candle is reversed. If the image had to formed on the retina, it would necessarily be pictured inversely, therefore beyond the centre of the eye. The position of the retina picture is inverse.
Alhazen has resolved a problem long of 15 centuries which was to get into the pupil any reduced form of any seen object. He says that the object is formed before it gets inversed. He could get any explanation on the image being inversed that would satisfy. He gives up the idea of the retina being the organ of vision.
He thinks that his theories are valid, when he encounters difficulty he finds solutions, temporary solutions if he is not entirely satisfied.
This false problem on the interpretation of a physical inversed image being positionned rightly has been resolved by physiologists and philosophers of 19th century.
Alhazen continues his theory by explaining the existence of rays from the object is not sufficient. The eye has to be in the direction of the object in order to receive the rays. At this stage of his study, Alhazen had to explain how two organs gave an identical vision. He discovered that by covering one eye, he provoqued diplopia. His first conclusion was that images were formed in similars ways and that a message was transmitted by the crystalline and then to the optical nerves before getting to the chiasma. At that point a message merges which was lately interpreted by psychological facts and reason (figure 3).
Alhazen says that we receive 2 images which are double but perceived as one single image he demonstrate the unification process from physiological diplopia.
Out of his seven books, an entire one deals with optic illusion. He describes a series of experiences on visual field and visual intensity. His theory is based on mechanism. He builts a theoric light that flows in a staight line and reflects in mirrors and refracts on transparent surfaces. It follows a rectilinear mouvement without any fear for intersection with other lights in a diaphanous body. The light is spread in a very fast motion. The motion in a translucide body is quicker than in deep body. In fact in any translucide body, the light that passes through depends on the resistance of its structure.
A great conclusive contribution to Alhazen’s work was brought by geometers, persian and arab mathematicians as Khawarazmy, Neiziry (Anaritius), Alkindi, Sharafdin Tusi, Ibn Sahl, Kouhie, Suzi and all those who have worked on that matter before Alhazen.
His collaborators were not known and the credit of his work was given to him.
The final outcome of Alhazen’s work is more historical. He has imagined what is today a simple evidence. Alhazen created visual optics on solid basis with valid experimentations and calculations.
Kepler, six centuries later, accomplished the first reform on optics and scientist recognize Alhazen to be the creator of what is called today physiological optics.