sin i/sin r = n = Cte {!) 



HISTORY OF LIGHT REFRACTION 



finds a most explicit mathematical theory of was able, in 1644, to interpret correctly Willi- 



refraction in the Ibn-Al-Haitham's "Treatise brod Snell's experiment (Leyden, 1621) and 



of Optical Geometry" after this author, rec- to formulate the basic law of refraction which 



ognizing the limitations of Ptolemy's ele- summarizes so well most of geometric op- 



mentary law, discovered the principle of the tics: 

 inverse return of spherical waves, which is 

 the basis of modern prismatic refractome- 



ters. During the Middle Ages the laws of where n is the refractive index characteristic 



light refraction were widely applied. For in- of the medium traversed. This law made 



stance, Roger Bacon in his "Opius Mains" modern optics possible. 



(1270) describes glass lenses without stating Thus, the early history of the refraction of 



their basic properties. Indeed, one finds in light can be traced from early Greco-Latin 



Al-Hazem's Treatise all necessary funda- antiquity, along two main lines of thought, 



mentals which might have been desirable at One is concerned with the studies derived 



the time. Thus, contrary to a common asser- from the reflection of light, running from 



tion and to the opinion of Joseph Bertrand, Empedocles (5th century B.C.) through Aris- 



the discovery of refraction considerably pre- totle, Plato, Euclid, Apollodorus, Ptolemy, 



ceded Tycho-Brahe, Vitellio (circa 1270), Bacon, Magnus, Kircher, Snell and Descar- 



and Kepler. tes. The other is concerned with the phe- 



Johannes Kepler (6) must be credited with nomena related to the non-linear propagation 



what may be considered the first practical of light, from Aristotle again, to Aristoxene, 



refractometer. Its principle is well described Posidonios, Cleomede, Lucretius, Seneca, 



in his "Dioptrice," published in 1611. The Ibn-Al-Haitham, Averrhoes, Bacon, Tycho- 



instrument permits one to determine the re- Brahe, Vitellio, Kepler, Hooke, Snell and, 



lationship between the ratio of length of the again, Descartes. 



shadows and the ratio of incidence and of For the remaining conformal picture frag- 



refraction angles, the two rays traveling ments, the historical development, the reader 



through air and an unknown medium, re- is referred to the classical works of Leonardo 



spectively. Although Kepler labored under da Vinci (10), Huygens (11), Fresnel (12), 



the wrong assumption that light has an in- Arago (13), Mach (14), and others. A very 



finite velocity, it is with such an instrument clear exposition of the phenomena of refrac- 



that he discovered the important phenome- tion from the standpoint of geometric optics 



non of the internal total reflection in glass is given by J. W. Forest in 0. Glasser's book 



for an angle of about 42°. Thus, Mees (7) (15), while the references in Reymond's book 



erred when he stated that Kepler did not (16) somewhat fill up some lacunae in Wile's 



know the law of refraction. basic work. Other important reference works 



There is some evidence that Robert Hooke include: Delambre (17), Doublet (18), 



(8), in England, also invented a refractome- Marion (19) and Tannery (20). 



ter of some kind, but no trace of Hooke's Almost simultaneously with the publica- 



instrument has been discovered. tion of Descartes' momentous synthesis, 



The 17th century continued to labor under Pierre de Fermat showed that the "sine law" 

 the assumption of an infinite velocity for could be deduced from the philosophical 

 light. To Descartes (9), light was simply a principle of "optical extremum path." The 

 "pressure transmitted to the eye," with an principle proved much broader than the ex- 

 infinite velocity which could "still be perimental data then available, and its in- 

 greater" when traveling in denser media fluence in science may be felt in the work of 

 than in lighter ones. Despite this error, he Leibnitz, Kant, Hamilton, Gauss, and even 



495 



