REFRACTION OF LIGHT, REFR ACTOIMFTRY AND INTERFEROMETRY 



Table 1 



mens in a few minutes, and for these reasons 

 are widely used industrially. 



Selection of a Refracto metric Tech- 

 nique. The last significant figure of relative 

 refractive index obtained with the available 

 instruments serves as a guide in selecting the 

 proper refractometric technique. Routine fig- 

 ures currently accepted for some commer- 

 cially available instruments are given in the 

 Table 1. 



REFERENCES 



See next article. 



R. JONNARD 



HISTORY OF LIGHT REFRACTION (See also p. 

 454) 



The term "refraction" refers to the change 

 of direction, or "breaking" of the rectilinear 

 path of light at the surface of separation of 

 two transparent media. 



This paradoxical phenomenon has at- 

 tracted the attention of philosophers since 

 early Greco-Latin anticiuity, and it was cor- 



rectly opposed to the rectilinear propagation 

 observed when light travels through unob- 

 structed space. It is worthy of note that the 

 word Optikos ("I see") existed only in the 

 Greek language. Thus, the study of optics 

 appears to be an intellectual phenomenon 

 properly Greco-Latin in origin, arising 

 sometime toward the end of the Hellenic 

 period of history (1). 



The historical development of knowledge 

 relative to the refraction of light encompasses 

 the whole of geometric optics as well as the 

 nature of light and the mechanism of vision. 

 Reflection phenomena, linked with recti- 

 linear propagation were well described in 

 Plato's "Times" (427-347 B.C.), in Euclid's 

 "Optica", Vol. VII (2), (circa 325 B.C.), in 

 Appollodorus's "Philosophy" (3), and in 

 Claudius Ptolemy's "Physics" and "Catop- 

 tries" (2nd century A.D.). All these authors 

 knew the elementary laws of optical reflec- 

 tion. 



Simultaneously with the development of 

 knowledge about the reflection of light, the 

 apparentl}^ contradictory notion of refraction 

 was neatly distinguished by the peripateti- 

 cian of Chalcis, Aristotle (322-284 B.C.) in 

 his "Rhetoric" and "Meteorology." His 

 views were extended by the Stoic Posi- 

 donius, of Apamee (135-50 B.C.). Lucius 

 Annaeus Seneca's "Natural Questions" (circa 

 25 A.D.) contains abundant allusions to the 

 consequences of the refraction of light 

 through flasks, bottles, glass, menisci and 

 the hke. In fact, Mediterranean antiquity 

 knew how to utilize these effects practically. 



The full significance of these discoveries 

 appeared after Aristotle, referring to the 

 fifth century (B.C.) philosopher Empedocles, 

 postulated that light is a vibrational phe- 

 nomenon traveling with a finite velocity 

 through fluids of definite densities. Such 

 views were subsequently expoimded by Aris- 

 toxene of Tarente (circa 4th century B.C.), 

 to be finally developed by the Arab Averrhoes 

 in the 12th century A.D. 



As early as the 10th century A.D. one 



494 



