1821.] Crystallized Bodies on Homogeneous Light, 123 



In this plate, the virtual poles correspond very nearly to the 

 second minimum of the extraordinary pencil beyond the poles 

 P P'. The same plate was now reduced by grinding to the 

 thickness 0*08816 inch. In this operation, care was taken to 

 grind away the side of the plate most distant from the eye only, 

 leaving the other perfectly untouched and unimpaired in its 

 polish. The plate being reduced to exact parallelism by the 

 sphserometer was again examined, the same side still remaining 

 next the eye. By this arrangement, the same angles of emer- 

 gence from the posterior surface correspond rigorously to the 

 same directions of the ray in the interior of the crystal, witk 

 respect to the axes of its molecules ; and thus we avoid com- 

 pletely any errors which might arise from using plates cut at 

 different angles, it being ahnost impossible to cut two plates 

 precisely alike in this respect. 



Table II. — Sulphate of Baryta, Thickness = 0*08816 in. 



Corresponding 

 inclinations. 



58° 0' 

 57 30 



55 57 



54 5 

 52 57 



SI 30 



50 



48 40 



47 35 

 47 10 



46 7 

 45 3 

 44 33 

 43 35 

 42 23 

 40 55 

 39 30 



1200 0' 

 120 47 



122 15 



124 



125 33 



127 



128 30 

 130 



Ordinary pencil. 



Pink, verging to orange 



yellow 



Yellow 



Pale yellow. . 



Greenish white 



Light blue 



Dark blue 



Sontibre purple. 



Very indifferent sombre 



pink 



P^e yellow 



White 



Very pale violet white . . 

 Very sombre violet, almost 



black 



Sombre and dirty olive 



green 



Very pale blue 



White 



White 



Orange white 



Bright orange 



Bright scarlet 



Narrow crimson 



Purple 



Blue 



Bluish white 



Yellowish or greenish white 



Orange yellow 



Rich crimson 



Purple 



Bright blue 



Good green 



Rich pink 



Greenish purple 



Good green, but pale 



Pink red 



Pale bluish green 



Pale pink 



Extraordinary. 



Blue, somewhat greenish . 



Dark blue 



Purple 



Pink 



Yellow, verging to orange 

 Bright yellow 

 Yellow white 



Bluish white 

 Dark indigo blue 

 Sombre violet 

 Dusky greenish yellow 



Poles for the 

 mean red rays 



Tints beyond 

 the poles. 



Pure brilliant white. 



White, rather ruddy 



Orange white 



Sombre orange or brick red 



Narrow purple 



Blue 



Pale blue 



Bluish white 



White 



White 



Yellow 



Rich crimson 



Purple 



Bright blue 



Green 



Good yellow 



Pink yellow 



Rich pink or crimson 



Splendid green 



Pinkish white 



Fine pink 



Greenish blue 



Pale pink 



Very pale greenish blue 



Remavks, &c. 



Virtual poles 



