1^8 REFRACTION OP ICELAND CRYSTAL, 



IV. 



On the oblique Refraclivn of Iceland Cryftal. By William 

 Hyde Wollaston, M. D. F. R. $. From the Phil. 

 Trauf for 1802. 



IN the preceding communication*, I have inferted two 



different meafures of refractive powers, diftinctly obfervable 



1:1 the Iceland cryftal, as well as an eftimate of its difperfive 



power ; but have referved for a feparate treatife, fome renarks 



which the fame mode of investigation has enabled me to mak« 



on its oblique refraction. 



Iceland cryftal The optical properties of this body have been fo amply de- 



by Huygens, bribed by Huygens in his Traite de la Lumihre, that it could 



anfwer little purpofe to attempt to make any addition to thoie 



Which he has enumerated. But, as the law to which he has 



reduced the oblique refractions occasioned by it, could not be 



verified by former methods of meafurement, without con- 



fiderable difficulty, it may be worth while to offer a new and 



eafy proof of the juftnefs of his conclufions. For, fince the 



Hli theory, theory by which he was guided in his inquiries, affords (as 



has lately been mown by Dr. Young f) a fimple explanation 



of feveral phenomena not yet accounted for by any other 



hypothecs, it mutt be admitted that it is entitled to a higher 



degree of consideration than it has in general received. 



that light is pro- According to that hypothefis, light proceeding from any 



pagated 1 by vibra- l um { nous C entre> is propagated by vibrations of a medium 



medium j highly elaftic, that pervades all fpace. In ordinary cafes, 



™ du ! ati . ng . the incipient undulations are of a fpherical form ; but, in the 



moft cafes ; butl ce l an d cryftal, light appeared to Huygens to proceed as if the 



in an oblate undulations were portions of an oblate fpheroid, of which the 



£nl mihH : CC " ax ^ s ^ s parallel to the (hort diagonal of an equilateral piece of 



the cryftal and its centre the paint of incidence of the ray. 

 Whence the law From this fpheroidical form of the undulations, he deduces 



of this refrac- ^e obliquity of refraction; and lays down a law, obfervabk? 



tion : viz. that *l J » / 



the fine of inci- in all refractions, at any furface ot the lpar, whether natural 



dence has a con- or artificial, which bears the clofeft analogy to that which ob- 



ordinate of re- tains univerfally at other refracting furfaces -, for as, in other 



fraction in the 



/pheroid, * See our Journal IV. 89. 



f Bakerian Lefture. Phil. Trans, for 1S01. 



Cafes, 

 2 



