126 Mr. Herschel on the Action of [Feb. 



In order, however, to avoid the effect of the dispersive power, 

 which, at such considerable obliquities, would rendei* the obser- 

 vations liable to some uncertainty, I cut another plate in such a 

 manner that the perpendicular to its surface, instead of coincid- 

 ing neai'ly with the optic axis, was directed very nearly to one o* 

 the virtual poles. Its thickness was then gradually reduced in 

 the manner above described for sulphate of baryta, though, 

 owing to the nature of the body, it was found impossible to 

 avoid the necessity of re-polishing the posterior surface at each 

 operation ; but as this was done with all possible care, only a 

 very sHght error can have arisen from this cause. 



Table Y.-^Mochelle Salt. Thickness = 0-11618 in. 



Ordinary penciL 



256 30 



255 10 

 254 SO 



252 30 



260 27 



248 5 

 245 45 

 243 30 



240 25 



237 30 

 237 

 234 20 

 230 10 



225 45 



220 40 



211 40 

 197 SO 



Very pale pink. 



White, tinged with orange 

 Yellowish or greenish white 

 Very pale greenish blue 



DuUblue 



Fine deep indigo 



Violet purple 



White, a little tinged with 



violet 



Yellowish white 



Pale yellow 



Pale pink yellow 



Pinkish purple , 



Rich sombre purple, some 



what fiery 



Pale green 



Extremely pale green .... 



White 



The richest damask crimson 

 Livid imperfect purple. . . 



Pale bluish green 



Pink, approaching to red. 

 Sky blue. 



Fink orange 



Sky blue, inclining to. lilac . 



PaJe bluish green 



Splendid yellow green .... 



Rich lilac 



Splendid green 



White 



Lilac blue. 



White 



Splendid green yellow .... 



White 



Pale lilac 



Fine yellow green 



Extraordinary. 



Very pale bluish green. 



Very fine intense indigo 

 Purple, rather sombre 

 Indifferent purplish pink 

 Yellowish pink white 

 White inclining to orange 

 Yellowish white 



W^hite, not very brilliant .... 



Pale violet blue 



Sombre indigo, inclining to violet, 



narrow, and well defined 

 Sombre violet white 

 Very pale greenish yellow 



White, tinged with greenish yellow 



Fine crimson 



The richest deep damask crimson 



Livid imperfect purple 



Fine pale green 



White 



Fine rich crimson 



Pale blue green 



Light pink, strongly inclining to 



orange red 

 Pale greenish blue 

 Fine yellow 



Fine pink, a little purple 

 Lilat 



Splendid green 

 Rich lilac 

 White 



Splendid yellow green 

 White 



Pale lilac blue 

 White 



Pale greenish yellow 

 Fine lilac 



Pole P for 

 mean red rays 

 Perpendicu- 

 lar incidence 



The virtual 

 pole/, 



Remarks, &c. 



