in the Structure of Glauberite. 275 



supposition so absurd, that it cannot for a moment be received. 

 Since the combination of axes, therefore, indicated by experi- 

 ment for the single system of rings in violet light, and for the 

 double system in the other rays, involves a physical absurdity, 

 we must seek for a new combination, not liable to such an ob- 

 jection. 



If we suppose that the axis A for violet light is the result- 

 ant of other axes, and that these other axes are two posi- 

 tive axes B and C at right angles to each other, and also to 

 the apparent axis A, we shall obtain an explanation of all the 

 phenomena. If the axes B, C, exercise the same action on 

 the violet rays, they will produce a single negative axis at A for 

 violet light, as given by observation ; and if the relative intensi- 

 ties of their action upon red light are in the ratio of the square 

 of the cosine of 2^ to unity, the intensity of the weakest gra- 

 dually diminishing to zero for the rays between the red and the 

 violet, then we can calculate, with mathematical precision, all 

 the phenomena of double refraction and polarisation exhibited 

 by Glauberite. 



The structure of Apophyllite and Glauberite, therefore, fur- 

 nishes us with two unequivocal examples of minerals where the 

 real axes of double refraction are not pointed out by observation. 

 Their crystallographic structure does not indicate, with any cer- 

 tainty, the locali ty of the axes which we have now inferred from 

 the laws of double refraction ; but we have no doubt that the 

 results of crystallography and optical structure will ultimately 

 coincide, when our knowledge of the primitive and secondary 

 forms of minerals shall have attained a higher degree of perfec- 

 tion *. 



While repeating these experiments on Glauberite during a 

 low state of temperature, I was surprised to observe, that the 



* The following paragraphs have been added since the paper was read. 

 VOL. XI. PART II. *M m 



