Mr. Herschelon the Action of [Feb. 



In this plate the virtual poles correspond to the second maxi- 

 mum of the extraordinary pencil. It is needless to detail the 

 tints between the poles. The same plate once more reduced 

 with the same precaution to leave the posterior surface un- 

 touched, developed the following series of colours beyond the 

 poles. 



Table III. — Sulphate of Baryta. Thickness = 0*05758 in. 



Ordinary pencil. 



Fine yellow ! 



Pale yellow i 



White, inclining to yellow. 



Bluish white 



Indigo 



Sombre purple 



Sombre reddish violet .... 



Dirty violet yellow 



Pale yellow 



White 



Pure brilliant white 



White 



Pale orange 



Sombre orange or brick red 

 Sombre and nariow purple 



Blue 



Pale blue 



Bluish white 



White 



Pale yellow, &c. &c 



Extraordinary. 



Indigo 



Purple 



Dull crimson red 



Dull orange 



Yellow 



White 



White 



White 



Violet white 



Sombre violet 



Black 



Sombre dirly green 



Pale dirtv bluish green 



White 



White 



Ruddy white 



Orange 



Orange red 



Narrow crimson 



Purple, &c. &c. 



Remarks, &c. 



Poles, for tlie 

 mean red rays 

 Tints beyond 

 t'ie poles 



Virtud poles 



Here the virtual poles p, /?' correspond precisely to the first 

 minimum of the extraordinary pencil. 



In a plate of Rochelle salt, cut nearly, but not quite perpendi- 

 cular to the optic axis, and whose thickness was 0*194425 in. 

 the rings beyond the poles were almost entirely obliterated, 

 while those between them exhibited the following singular suc- 

 cession of colours, which will show to what an extent the devia- 

 tion from Newton's scale is carried in this substance. 



