350 FOJJRTII RKPORT — 1834. 



theory, on the other haiul, not only tlie individual laws, but the 

 classes of phenomena are related ; and to calculate, nutnericallj/, 

 the laws of refraction, the varied phenomena of diffraction, and 

 those of thin plates, we only need to borrow o)ie result from 

 experience, — the length of a wave of light in each medium. 

 There is thus established that connexion and harmony in its 

 parts which is the never-failing attribute of truth. But power- 

 ful as is the weight of this intrinsic evidence in favour of the 

 wave-theory, it has yet stronger claims to our assent. These 

 claims are grounded on the vast body of new phenomena which 

 it explains, — and explains, (it is to be remembered,) not in a 

 vague and general manner, but in the precise language of 

 analysis, and with an accuracy which the refinements of modern 

 observation have not been able to impugn. It may be confidently 

 said that it possesses characters which no false theory ever 

 possessed before. 



Part II. — Polarized Light. 



(1) Polarization. — Transversal fl'brations. 



In the various phenomena which have been hitherto de- 

 scribed as taking place when a ray of light encounters the sur- 

 face of a new medium, it has been assumed that the direction 

 and the intensity of the several portions into which it is sub- 

 divided are wholly independent of the manner in which the i*ay 

 is presented to the bounding surface, the direction of the ray 

 remaining unchanged. In other words, it was taken for granted 

 that a ray of light had no relation to space, with the exception 

 of that dependent on its direction ; — that around that direction 

 its properties were on all sides alike ; — and that if the ray were 

 supposed to revolve round that line as an axis, the residting 

 phenomena would be unaltered. 



Huygens was the first to observe that this was not always 

 the case. In the course of his researches on the law of double 

 refraction, he found that when a ray of solar light is received upon 

 a rhomb of Iceland crystal in any but one direction, it is always 

 subdivided into two of equal intensity. But on transmitting 

 these rays through a second rhomb, he was surprised to observe 

 that the two portions into which each of them was subdivided 

 were no longer equally intense; — that their relative brightness 

 depended on the position of the second rhomb with regard to 

 the first ; — and that there were two such positions in which 

 one of the ravs vanished altogether. 



