REFRACTION OF ICELAND CRYSTAL.' J5| 



Hence, the greater angle of the furfaces themfelves may be 

 computed to be 101° 55' ; and the angle which the (hort axis 

 makes with each plane furface is 45° 23' 25". 



If GSMP (Fig. 2.) be a plane bifecting an obtufe angle of 

 the fpar, the fection of the fpheroid in that plane paiTes through 

 the axis CS, and therefore is the generating ellipfe. By cabcu-. 

 lating from the known dimenfions of its major axis CP 0,67204, 

 its minor axis CS 0,6035, and the angle GCS=45° 23' 25'', 

 CG will be found * to be 0,6365, of which the reciprocal is 

 1,5736, differing but little from 1,571, as it appeared by mea- 

 surement. (Exp. 4.) 



Again, if ABDE (Fig. 4.) be one of the natural furfaces, 

 and PG/> the ellipfe formed by that feclionof the fpheroid, PC 

 being as before 0,67204, and CG 0,6365, the reciprocal of 

 1,571 found by meafurement, (Exp. 4) then the femidiameter 

 CT, parallel to the fide AE, which makes an angle TCP 

 39° 2f, will be found to be 0,6573, inftead of 0,6537, and 

 its reciprocal 1,5215, inftead of 1,518. (Exp. 2.) 



The femidiameter alio, in the direction of CL, perpendicular 

 to the fide, at an angle LCP 50° 51- , is found by calculation 

 0,650, and its reciprocal 1,539, inftead of 0,6506 and 1,537. 

 (Exp. 3.) 



From the foregoing data, the courfe of a ray perpendicular Oowft of a per- 

 to the furface of the fpar may likewife be computed; for, ^ t " r ^ re ^ 

 fince the fine of incidence is then nothing, the ordinate of fra&ion. 

 refraction muft be alfo nothing, and the ray will be refracted 

 along the femiconjugate diameter CM. (Fig. 2.) 



By calculation f, the angle which this conjugate makes 

 with the perpendicular is 6° 1\'. But, by the following mea- 

 furement, it appears to be 6 Q 16'. 



Exp. 9. A piece of fpar that meafured 1,145 inch in thick- Experiment to 

 nefs, was laid upon a line, and mowed two images that were co ^ f ^ he fame 

 removed from each other -j^j of an inch. Then, as 1,145 : 

 0,126 :: radius : tang, of 6 W 16'. 



The different remits deduced from theory and from obferva- Comparativt 

 tion, will be feen at one view in the following ftatement : refuitTfrom 



. obfervation and 



* (Fig. 3.) CS : CP :: tang. PCG : tang. VCp fee ?Cp : fee. from theory. 



PCG :: CP : CG* 



+ (Fig. 5.) CS : CP :: tang. PCG : tang." /CO or co-tang. 



PCC^i then CP: cs t: tan S« PC Qj Wn S- PCM j and LCP— 

 PCM=MCL. 



In 



