REFRACTION OF ICELAND CRYSTAL. |£9 



cafes, the ratio is given between the fine of incidence and fine 

 of refraction, (or ordinate of the Jpherical undulation pro- 

 pagated,) fo, in the Iceland cryftal, the ratio between the 

 fine of incidence and ordinate of refraction (in any one fection 

 of the fperoidical undulation) is a given ratio. 



If ABD (Fig. I, Plate VIII.) be any furface of the fpar, Uluftration by 

 let FHOK be a fection of the fpheroid through its centre C, diagranu 

 and RC any ray of light falling on that furface; draw FO a 

 diameter of the fpheroid, in the plane of incidence RVO, and 

 CT, its femiconjugate diameter, in the plane of refraction 

 FTO. Then, if CI be the refraded ray, VR, the fine of 

 incidence, fhall be to EI, the ordinate of refraction parallel to 

 FC, in the conflant ratio of a given line N to the femidiameter 

 FC. 



In any other plane of incidence, the ratio of fine to ordi-But the refrac- 

 nate is alfo conflant ; but it is a different ratio, according to different ' 

 the magnitude of that diameter in which the plane of incidence planes of inei- 

 interfeas the ellipfe FHOK. £££$%» 



When the incidence of the ray palling from any medium of ellipfe, &c. 

 greater denfity upon the furface of this fpar, is fuch that the 

 emergent ray becomes parallel to the furface, the ordinate of 

 refraction is then a femidiameter of the fpheroid ; and, ac- 

 cordingly, the refractive power of this fpar, when-examinedas maybe 

 by means of a prifm in different directions, fhould be found feen b y the 

 to vary as that femidiameter which coincides with the plane of re fleQion. 

 incidence and refracting furface. 



The obfervations that I have made on this fubflance, accord Hypotkefis of 

 throughout with this hypothefis of Huygens ; the meafure$]? uys * ns con * 

 that I have taken correfpond more nearly than could well hap- 

 pen to a falfe theory, and are the more to be depended on, as 

 all my experiments, excepting the laft, were made prior to 

 my acquaintance with the theory, and their agreement was 

 deduced by fubfequent computation. 



Exp. 1 . The oblique refraction of this fpar is rendered vifible, Experiments on 

 by cementing a furface of it to a prifm of flint-glafs, with J^|^ oeW ' 

 little balfam of Tolu. When the line of fight befects an acute different planeg. 

 angle of a natural furface of the fpar, the refractive power is 

 feen to be lefs than in any other direction, and may be expreffed 

 by the fine 1,488, or its reciprocal 0,67204. 



Exp. 2. When the plane of incidence is parallel to one of 

 the fides, the power is 1,513, of which the reciprocal is 

 © 4 <5587. 



Exp, 



