22 



SCIENTIFIC THOUGHT. 



light in the direction which was in a straight line from 

 the origin or centre of light ; that the lateral or secondary 

 waves destroyed each other almost entirely by interference 

 or overlapping ; and that the so-called inflection, bending, 

 or lateral spreading of light, was occasioned by an incom- 

 plete coincidence or overlapping of these lateral undula- 

 tions. It appears that about the year 1815 Fresnel had, 

 through a study of the phenomena of diffraction, arrived 

 at a conviction, entertained by Young fifteen years earlier, 

 that the projectile theory of light could not explain 

 them. He had also, by a more rigorous and minute 

 study of Young's principle of interference, explained the 

 reason of the rectilinear propagation of light. Yet these 

 presented results did not materially affect the adherents of the 



by the ' 



f u r hf tion projectile theory, who had been during late years very 

 active in studying another class of optical phenomena, 

 those of polarisation the power which light possesses 

 of acquiring, either by refraction or reflexion, a differ- 

 ence not discernible merely by the eye. This differ- 

 ence consists in the fact that a ray of light very fre- 

 quently as Newton had already expressed it possesses 

 " sides," just as a flat strip or narrow tape has sides if 

 compared with an ordinary thread or wire, which has no 

 sides ; or as a wire drawn through a specially shaped die 

 acquires sides or edges. This property was later termed 

 polarity, 1 a term which implies that the particles of light 



17. 

 Difficulties 



1 The word "polarity" was in- 

 troduced by Malus in 1810. It is 

 unfortunate, as it suggests the cor- 

 puscular nature of light. Newton's 

 conception of "sidedness" ("later- 

 ality," formed by analogy on Lord 

 Kelvin's term "chirality" to 'de- 

 scribe right- or left - handedness, 



see vol. i. p. 432) is a better de- 

 scription of the phenomenon. It is 

 contained in the 26th query to the 

 second edition of the ' Opticks ' 

 (1717). Huygens had long before, 

 in his ' Traite" de la Lumiere ' 

 (written in 1678, published in 1690), 

 after having given a correct rule for 



