150 REFRACTION OF tCEIAND CRYSTAL, 



£rp. 3. In a direction at right angles with either fide, it is 

 found ftill higher, being 1,537, or its reciprocal 0,6506. 



Exp. 4. And in the plane bifecting an obtufe angle, the re-, 

 fractive power of the natural furface appears greater, and h 

 expreffed by the fine 1,571, or its reciprocal 0,6365. 

 and furfaces, Exp. 5. When either of the two greateft folid angles of the 



fpar contained under three obtufe angles, is cut off by a po- 

 lifhed furface making equal angles with each of its fides, the 

 fame refractive power 1 ,483 is found in all directions. By the 

 theory alfo, the fection of the fpheroid is in this cafe a circle, 

 and every femidiameter (FC) the fame, fince the plane is at 

 right angles to the minor axis. 



Exp. 6. If a plane furface be formed bifecting an obtufe 



angle of the fpar, and applied to a prifm, the fame minimum 



of refraction 1,488, is found in a direction that coincides with 



the preceding plane^ and therefore with the major axis of the 



generating ellipfe ; but, as the direction is varied, it increafes 



fo rapidly as foon to exceed the power of glafs, and to be no 



longer afcertainable by the angle of incipient reflection. 



Regular rcfrac- Exp. 7. The regular refraction of this fpar is alfo too great 



tion. f or examination by means of any prifm, for want of a medium 



of union of fufficient denfity ; but, by trial in the ufual method, 



itmeafured, on an average of feverai experiments, 1,657, or 



its reciprocal 0,6035. ' 



Spheroid which By affuming, as Huygens has done, the equality of this 



relacTons ' P ower w * tn tne maximum of the oblique refraction, we have 



fufficient data for conftruction of the fpheroid by which the 



refractions are regulated; for we have 0,67204 (Exp. 1.) as 



major axis of the generating ellipfe, and 0,6035 (Exp. 7) will 



be the minor axis, parallel in pofition to the thort axis of the 



fpar. 



The angle of inclination of this axis to the furfaces of the 

 fpar, if fuppofed to be equilateral, may be computed by 

 fpherical trigonometry, from any other angle that has been 

 afcertained by meafurement. 



The meafures that I have taken are not exactly thofe of 



Huygens ; but I neverthelefs hold them in equal eftimation, 



from the conformity which I find they bear to each other, by 



affiftance of his theory. 



.Deduction of Exp. 8. I meafured with care an angle at which two furfaces 



and pofition. P*" tli e fpar are inclined to each other, and found it to be 105° 5'. 



Hence 





