106 HISTORY OF OPTICS. 



thin crystalline plates, the plane of polarization undergoes an oscilhi 

 tiou which carries it backwards and forwards through a certain angle, 

 namely, twice the angle contained between the original plane of pola- 

 rization and the principal section of the crystal. The intervals which 

 this oscillation occupies are lengths of the path of the ray, very minute, 

 and different for different colors, like Newton's fits of easy transmis- 

 sion ; on which model, indeed, the new theory was evidently framed." 

 The colors produced in the phenomena of dipolarization really do 

 depend, in a periodical manner, on the length of the path of the light 

 through the crystal, and a theory such as M. Biot's was capable of 

 being modified, and was modified, so as to include the leading features 

 of the facts as then known ; but many of its conditions being founded 

 on special circumstances in the experiments, and not on the real condi- 

 tions of nature, there were in it several incongruities, as well as the 

 general defect of its being an arbitrary and unconnected hypothesis. 



Young's mode of accounting for the brilliant phenomena of dipo- 

 larization appeared in the Quarterly Review for 1814. After noticing 

 the discoveries of MM. Arago, Brewster, and Biot, he adds, " We have 

 no doubt that the surprise of these gentlemen will be as great as our 

 own satisfaction in finding that they are perfectly reducible, like other 

 causes of recurrent colors, to the general laws of the interference of 

 light which have been established in this country ;" giving a reference 

 to his former statements. The results are then explained by the inter- 

 ference of the ordinary and extraordinary ray. But, as M. Arago 

 properly observes, in his account of this matter, 18 " It must, however, 

 be added that Dr. Young had not explained either in what circum- 

 stances the interference of the rays can take place, nor why we see no 

 colors unless the crystallized plates are exposed to light previously 

 polarized." The explanation of these circumstances depends on the 

 laws of interference of polarized light which MM. Arago and Fresnel 

 established in 1816. They then proved, by direct experiment, that 

 when polarized light was treated so as to bring into view the most 

 marked phenomena of interference, namely, the bands of shadows ; 

 pencils of light which have a common origin, and which are polarized 

 in the parallel planes, interfere completely, while those which are 



17 See MM. Arago and Biot's Memoirs, Mem. Inst. for 1811; the whole 

 volume for 1812 is a Memoir of M. Biot's (published 1814); also Mtin. Inst. foi 

 iS17 ; M. Biot's Mem. read in ISIS, published in 1819 and for ISIS. 



1(1 Enc.. Brit. Supp. art. Polarization. 



