176 Mr. Her ichel on the Action of [March, 



ApophyUitt. Thickness = 0-082^ inch. 



SI 50 



55 19 

 S9 45 

 SO 50 

 S3 3 



35 50 



37 20 



38 5 

 40 10 



43 55 



44 27 

 46 45 



49 57 



50 30 

 53 40 

 66 



56 40 



59 35 



61 45 



Ordinary pencil. 



Bright white , 



White, with a trace of 

 purple 



Exceeding sombre violet 



Extraordinary. 



Black 



62 

 63 



06 



10 

 







so 



Pale greenish yellow . 



White , 



White 



White, with a strong tinge 



of violet. 



Blue, strongly inclining to 



j purple 



Sombre indigo, inclining to 



1 violet 



Sombre violet 



Tolerably good yellow green 

 White, with a trace of yellow 



A^Tiite 



Pale purple 



Sombre purple blue 



Sombre violet 



Green yellow 



Yellowish white 



Yellowish white, , , 



White 



Pale purple 



Sombre indigo 



Sombre violet 



Faint violet white 



Livid grey 



Tolerably green yellow . . 

 Yellow white 



UTiite, slightly greenish 

 Pure bright white 



Purplish white 

 Sombre violet blue 

 Extremely sombre violet. . 



Pale yellow green 



Greenish white 



White 



White , 



Purplish white 



Obscure indigo, inclining to purple 



Sombre violet 



Tolerably yellow green 

 Yellowish white 



White 



Pale purple 



Sombre indigo blue 



Sombre violet 

 Livid grey 

 Yellow green 



Pale yellowish white 



Yellowish white 



White 



White 



Purple 



Very sombre indigo 



The axis 



Ut max. of 

 illumination 



1st minimum 



3d maximum 



2d minimum 



3d maximum 

 3d minimum 



4th maximum 

 4th minimum 



In the colours of thin plates and others of the like composition, 

 the difference in the lengths of the periods of the different rays 

 is so considerable, that after seven or eight alternations of colour, 

 the rings confound one another, and are blended into a uniform 

 whiteness. Were the periods more nearly equal, a greater num- 

 ber of rings should be visible, and were they strictly so, the 

 succession of alternate whiteness and blackness should be conti- 

 nued to infinity. As the values of /, Vy &c. in the apophylHte 

 approximate pretty closely to this limit, we should expect to see 

 a much greater number of rings, and this I find to be really the 

 case. By inclosing a thick plate in balsam of copaiba in a pro- 

 per apparatus to increase the range of incidence, 1 have counted 

 as far as the 35th order, when I desisted ; not from any want of 

 alternate colours, but owing to their extreme closeness, which 

 rendered it impossible to number them distinctly. Indeed I 

 have no doubt that could a very thick and limpid specimen bQ 

 procured, hundreds might be seen without artificial aid. 



In two instances then, at least, and probably in many morej 



