POLARISATION OF LIGHT. 



will produce at F the same tint of red 

 light that A does at F. In this case, the 

 tints will destroy one another, and the 

 black spot at F will be the pole of one of 

 the systems of rings of red light. If C 

 and A had the same proportional action 

 on the violet and other rays, as on the red 

 rays, which is the case in bi-carbonate 

 of ammonia, then F would also be the 

 point of compensation for violet or other 

 light, and the pole of the violet or other 

 rings. In this case, there would be no 

 virtual poles, no displacement of the 

 rings in homogeneous light, as in Ro- 

 chelle salt ; and the tints would be 

 exactly those of Newton's scale. But, if 

 the axis C has a greater proportional 

 action upon the violet and other rays 

 than A, it will produce a higher tint at 

 F than that produced by A; and the 

 point of compensation will now be at f, 

 which will become the centre of the violet 

 system of rings. The centres of the other 

 systems of rings for yellow and green light 

 will occupy intermediate points between 

 F and /; and FF' will be the inclination 

 of the resultant axes for red light, and 

 ff for violet light. This is the case 

 with all the crystals in Class I. of the 

 preceding Table. On the other hand, if 

 the axis C had a less proportional action 

 upon the violet rays than A, the points of 

 compensation would be at c and c', and cc' 

 would be the inclination of the axes for 

 blue light, which is the case with all the 

 crystals in Class II. Here, then, we have 

 a complete explanation of all the pheno- 

 mena observed by Mr. Herschel, and are 

 able to calculate them in the most rigo- 

 rous manner, by supposing the real axes 

 to be at A and C, and to have an inva- 

 riable position coincident wilh fixed lines 

 in the primitive form of the mineral. 



The most remarkable example of de- 

 viation from the tints of Newton's scale 

 occurs in apovhyllite, which has gene- 



rally one axis of double refraction. In 

 the Tyrol apophyllite, according to Dr. 

 Brewster, the system of coloured rimrs 

 with which its axis is surrounded, 

 is composed of unusual tints, the only 

 colours being bluish violet and greenish 

 yellow, separated by a ring of white light. 

 By examining the apophyllite, however, 

 in homogeneous light, Mr. Herschel suc- 

 ceeded in determining that some spe- 

 cimens exercise a negative or repulsive 

 action upon the rays at one end of the 

 spectrum, a positive or attractive action 

 upon the rays at the other end of the 

 spectrum, and no action at all upon the 

 mean refrangible rays. In one case the 

 doubly refracting action ceased in the 

 yellow rays, and in another in the indigo 

 rays. The following were the tints ob- 

 served in these two cases. 

 FIRST SPECIMEN. First Order. Black, 

 sombre red, orange yellow, green, 

 greenish blue, sombre and dirty 

 blue. 



Second Order. Dull purple, pink, 

 ruddy pink, pink yellow, pale yel- 

 low, (almost white) bluish green, 

 dull pale blue. 

 Third Order. Very dilute purple, 



pale pink, white, very pale blue. 

 SECOND SPECIMEN. First, and only 

 Order. Black, sombre indigo, in- 

 digo, indigo inclining to purple, pale 

 blue purple, very pale reddish purple, 

 pale rose red, white, white with a 

 hardly perceptible tinge of green. 

 In these two specimens the rings in- 

 crease in diameter with great rapidity 

 from the red end of the spectrum ; they 

 become infinite in diameter in the yel- 

 low rays in Specimen first, and in the 

 indigo rays in Specimen second ; after 

 which they again become finite and con- 

 tinue to contract up to the violet end of 

 the spectrum, where they have still a 

 considerably larger diameter than in the 

 red rays. 



In 'other specimens of apophyllite, 

 which Mr. Herschel calls leucocyclite, 

 from the rings being white and black, the 

 action of the doubly refracting force was 

 so equal upon all the rays of the spec- 

 trum, that the diameter of the rings was 

 nearly alike for all colours. If this were 

 accurately the case, the system of rings 

 formed in white light by the super-po- 

 sition of all these rings would be simple 

 alternations of perfrct black and white. 

 This equality was so nearly the case in 

 one specimen, that Mr. Herschel counted 



