LAWS OF DOUBLE REFRACTION. 401 



menta Crystalli Islandici, Hafniae, 1669.) He ana- 

 lyzed 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 latter refraction Bar- 

 tholin 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 extra- 

 ordinary refraction; that Newton altogether mis- 

 took it, and that it was not verified till the experi- 

 ments of Haiiy and Wollaston in our own time ; 

 we might expect that it would not be soon or 

 easily detected. But Huyghens possessed a key to 

 the secret, in the theory, which he had devised, of 

 the propagation of light by undulations, and which 

 he conceived with perfect distinctness and correct- 

 ness, 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 dis- 

 covery which we have here to consider,) with a pre- 

 cision and success which excited deserved admira- 

 tion, when the subject, at a much later period, 

 regained its due share of attention. His Treatise 

 was written 2 in 1678, but not published till 1690. 



3 See his Traite de la Lumiere. Preface. 

 VOL. II. D D 



