N 
96 Mr. J. F. W. Herschel on the action oj 
the other 6th January, 1817, by Sir G. Mack enzie. President of the Physical Class 
of the Royal Society.” 
No comments on the above extracts are necessary. They establish at once the 
priority of Dr. BRtwsTtR’s observations, and the independence of mine. With 
regard to the division of crystals into two classes, which observation has alike sug- 
gested to both of us, it is unnecessary, if we regard either of the two classes r.s having 
the angle between the resultant axes greater than a right angle. In Dr. Brewster’s 
table, Phil Trans. 1818, p.230, succinic acid and sulphate of iron are stared as 
having this angle 90°. If this determination corresponds, as in all probability it 
does, to the yellow rays, they belong at once to both classes, and are, in fact, instances 
of the limit where one class passes into the other. Bi-carbonate of ammonia, in which 
I can perceive no separation of the axes of different colours, nor of course, any 
virtual poles, belongs in like manner to both classes, or to neither. 
JOHN F. W. HERSCHEL. 
APPENDIX. 
Description oj an instrument employed in the foregoing experi- 
ments on the polarised rings. 
The singular property possessed by the tourmaline, by 
which a plate of it of any moderate thickness cut in a direc- 
tion parallel to its axis of double refraction, is enabled to 
absorb the whole, or nearly the whole, of an incident pencil 
polarised in a plane parallel to that axis,* was pointed out by 
* The same property is observable in the epidote, the axinite, and all other na- 
tural and artificial crystals which exhibit any degree of dichroism when examined 
by unpolarised light. Muriate of palladium and potash possesses it in the highest 
perfection. This remarkable effect is easily explained by a reference to the general 
principles laid down by Dr. Brewster in his paper on absorption, Phil. Trans. 
1819, p. 11. The incident pencil is separated by the doubly refractive force into 
two, oppositely polarised, one of which is partly absorbed, the other emerges 
(polarised in its proper plane) of nearly its original intensity. 
