70 HISTORY OF OPTICS. 



hcdral crystals are usually very smooth and transparent, and often of 

 considerable size ; and it was observed, on looking through them, that 

 all objects appeared dcuble. The phenomena, even as early as 1G69, 

 had been considered so curious, that Erasmus Bartholin published a 

 work upon them at Copenhagen, 1 (Experimental Crystalli Islandici, 

 Hafnise, 1669.) He analysed the phenomena into their laws, so far 

 as to discover that one of the two images was produced by refraction 

 after the usual rule, and the other by an unusual refraction. This lat- 

 ter refraction Bartholin found to vary in different positions ; to be 

 regulated by a line parallel to the sides of the rhombohedron ; and to 

 be greatest in the direction of a line bisecting two of the angles of the 

 rhombic face of the crystal. 



These rules were exact as far as they went ; and when we consider 

 how geometrically complex the law is, which really regulates the 

 unusual or extraordinary refraction ; that Newton altogether mistook 

 it, and that it was not verified till the experiments of Hauy and "YVol- 

 laston in our own time ; we might expect that it would not be soon 

 or easily detected. But Huyghens possessed a key to the secret, ii> 

 the theory, which he had devised, of the propagation of light by un- 

 dulations, and which he conceived with perfect distinctness and cor- 

 rectness, so far as its application to these phenomena is concerned. 

 Hence he was enabled to lay down the law of the phenomena (the 

 only part of his discovery which we have here to consider), with a 

 precision and success which excited deserved admiration, when the 

 subject, at a much later period, regained its due share of attention. 

 His Treatise was written 2 in 1078, but not published till 1C90. 



The laws of the ordinary and the extraordinary refraction in Ice- 

 land spar are related to each other ; they are, in fact, similar construc- 

 tions, made, in the one case, by means of an imaginary sphere, in the 

 other, by means of a spheroid ; the spheroid being of such oblateness 

 as to suit the rhombohedral form of the crystal, and the axis of the 

 spheroid being the axis of symmetry of the crystal. Huyghens fol- 

 lowed this general conception into particular positions and conditions ; 

 and thus obtained rules, w ? hich he compared with observation, for cut- 

 ting the crystal and transmitting the rays in various manners. " I 

 have examined in detail," says he, 3 " the properties of the extraordi- 



1 Priestley's Optics, p. 550. ' See his Traite de la Zumiere. Preface. 



3 See Maseres's Tracts on Optics, p. 250; or Huyghens, Tr. sur la Linn, ch. v 

 .rt. 43. 



