274 Dr BREWSTER on a remarkable peculiarity 



pendicular to one of the faces P of the primitive form, as given 

 by HAUY, and the plane of the axes at right angles to a line bi- 

 secting the acute angle of tho same face. 



When Mr HERSCHEL discovered the very remarkable pro- 

 perty in Apophyllite, in virtue of which it exercised a negative 

 influence over the red rays, a positive influence over the blue 

 rays, and no influence at all over the yellow ones, I shewed in 

 a paper read before this Society, and printed in their Transac- 

 tions *, that these apparently irreconcileable actions, related, as 

 they seemed to be, to a single axis of double refraction, could be 

 calculated in the most rigorous manner, by supposing the crystal 

 to have three positive axes at right angles to each other, each of 

 which exercises a different dispersive action upon the differently 

 coloured rays. This result, which is of considerable importance 

 in the theory of double refraction, is strikingly confirmed by the 

 phenomena of Glauberite, while these at the same time present 

 us with a new and still less equivocal case of the composition of 

 axes. 



In the case of Glauberite, observation exhibits to us one ne- 

 gative axis A, which is the single axis for the violet light, and 

 the principal axis for the red and the other less refrangible rays ; 

 and, at the same time, it presents to us a second axis B, which 

 may be either negative or positive, but which must be 90 distant 

 from A. If it is negative, it must be in a plane perpendicular to 

 the plane passing through the two resultant axes for red light ; 

 and it must bear to A the ratio of the square of the sine of 2<^ 

 (half the inclination of the resultant axes) to unity. If it is po- 

 sitive, it must lie in the plane passing through the resultant 

 axes, and it must bear to A the ratio of the square of the sine, 

 to the square of the cosine of 2^. But whether it be positive 

 or negative, it exercises no action whatever upon violet light, a 



* Vol. IX. p. 317. 



