POLARISATION OF LIGHT. 



thirty-five rings. Under a more careful 

 examination, however, he found that 

 they had the following colours. 

 First Order. Black, greenish white, 

 bright, white, purplish white, sombre 

 violet blue. 



Second Order. Violet almost black, 

 pale yellow green, greenish white, 

 white, purplish white, obscure indi- 

 go inclining to purple. 

 Third Order. Sombre violet, tolerable 

 yellow green, yellowish white, white, 

 pale purple, sombre indigo. 

 Fourth Order. Sombre violet, livid ffrey, 

 yellow green, pale yellowish white, 

 white, purple, very sombre indigo. 

 Mr. Herschel likewise found a remark- 

 able deviation of the tints in hypo-sul- 

 phate of lime, and in Vtsuvian *. 



The explanation of such remarkable 

 deviations from the usual tints, as exhi- 

 bited in apophyllite, was deduced by Dr. 

 Brewster from his Theory of Rectangu- 

 lar Axes, and by means of it, all the 

 preceding phenomena are capable of the 

 most rigorous computation. In the Phil. 

 Trans.lSIS, p. 249, he has shown "that 

 a single positive axis (/o-.38)may be repre- 

 sented by three rectangular positive axes 

 (C, A, and B) provided two of them (A, B) 

 are equal, and the third (C) has a less 

 intensity than the other two." The same 

 author has also shown (Phil. Trans. 

 1813) that double refracting crystals have 

 also two dispersive powers ; and he con- 

 cluded that in crystals with two axes, 

 each axis has a different action upon the 

 differently coloured rays. 



In the case of apophyllite, then, the two 

 positive axes A, B, (fig. 38) will produce 

 a negative resultant axis at C ; and as 

 the real axis at C is positive, the ap- 

 parent or finally resultant axis at C will 

 be a single axis, negative if the negative 

 be the strongest. &ndposilive if the posi- 

 tive axis be the strongest. Now let us 

 suppose that in the apophyllite the two 

 axis at C have equal intensity, viz. + C 

 and-C, (-C being the resultant of + A 



* "Among crystals with one axis," says Mr. 

 Hersrhel,"Dr Brewster has enumerated the Idocrase 

 or Vesuvian, and correctly. Had he noticed, how- 

 ever, in the specimens examined by him, the very 

 striking inversion of the tints of Newton's scale ex- 

 Mbifed in the rings of that now before us, he would 

 doubtless have made mention of it." Treatise on 

 Light, \ 112.3. Dr. Brewster examined on'// one speci- 

 men of Vesuvinn, which was a large and valuable 

 crystal lent to him for the purpose, and which he 

 was not allowed to cut. It was of a nuibrown 

 colour, siiinuient to nnsk completely any peculiarity 

 in its tints ; and was in other respects quite unntted 

 for the observations made by Herschel. 



and +B,) for yellow light, and that G 

 acts more powerfully upon the red rays, 

 than + C, while + C acts more energe- 

 tically upon the violet rays. In this case 

 the two axes +C and C will com- 

 pensate one another exactly for yellow 

 light, or there will be no double refrac- 

 tion and polarisation for yellow rays, or 

 the diameter of the rings will be infinite. 

 In red light the predominance of C will 

 leave a single negative axis of double re- 

 fraction for red rays, and will conse- 

 quently produce a negative system of 

 rings. In violet light, on the contrary, the 

 predominance of the action of + C will 

 leave a single positive axis of double 

 refraction for violet rays, and will con- 

 sequently produce a positive system of 

 rings. The compensation here described is 

 exactly analogous to that of a compound 

 lens, consisting of a convex and concave 

 lens of equal curvatures, of such glass 

 that their indices of refraction for yellow 

 rays is equal, while the index of refrac- 

 tion for the violet rays is greater in the 

 convex lens, and the index for the red 

 rays greater in the concave lens. Such 

 a lens will converge the violet rays, di- 

 verge the red rays, and produce no de- 

 viation at all in the yellow ones. That 

 is, the same compound lens will be a 

 plane lens in yellow light, a convex one 

 in blue light, and a concave one in red 

 light. 



In this view of the subject each order 

 of colours in apophyllite is, as it were, a 

 secondary residual spectrum arising from 

 the opposite action of the negative and 

 positive axes, and the tints of which 

 these orders are composed will conse- 

 quently vary according to the locality 

 of the ray of compensation. 



From the circumstance of some speci- 

 mens of apophyllite exercising a negative 

 action upon light, Dr. Brewster states, 

 that he had no doubt that apophyllites 

 D 2 



