22 



POLARISATION OF LIGHT. 



Subphosphate of Potash. 



Sulphate of Nickel and Copper. 



Ruby Silver. 



Mellite. 



Somervillite. 



Octohedrite. 



Phosphate of Ammonia and Magnesia. 



Nepheline. 



Arseniate of Lead. 



Arseniate of Copper. 



Gmelinite. 



Oxahverite. 



Edingtonite. 



Levyne. 



Cyanuret of Mercury. 



Alunite. 



Crystals that give a Positive System. 

 Zircon. 

 Quartz. 

 Oxide of Tin. 

 Tungstate of Lime. 

 Titanite. 

 Boracite. 

 Apophyllite. 



Sulphate of Potash and Iron. 

 Superacetate of Copper and Lime. 

 Hydrate of Magnesia. 

 Ice. 



Prussiate of Potash, certain specimens. 

 Dioptase ? 



If we combine two plates of two crys- 

 tals of the positive class, such as Calca- 

 reous Spar and Beryl, the system of 

 rings will be the same as would be pro- 

 duced by two plates of calcareous spar, 

 one of which is the plate employed, and 

 the other, a plate which gives rings of the 

 same size as the plate of beryl. But when 

 a positive system of rings is combined 

 with a negative system, such as those 

 produced by zircon or ice with those pro- 

 duced by calcareous spar or beryl, the 

 resultive system of rings, in place of being 

 the siim of their separate actions, will 

 be their difference, that is, it will be 

 equal to the system produced by a thin 

 plate of calcareous spar, whose thickness 

 is equal to the difference of the thicknesses 

 of the plate of calcareous spar employed, 

 and another plate of calcareous spar that 

 would give rings of the same size as those 

 given by the zircon above. 



By comparing the numerical values of 

 the tints produced at different angles of 

 inclination to the axis, it follows from ex- 

 periment, that if the thickness of the 

 mineral is invariable, the numerical value 

 of the tints will always vary as the square 

 of the sine of the angle which the refracted 

 ray forms^with the axis of the crystal, 



For example, if at an angle of 1 with 

 the axis of double refraction, we have 

 the tint of the bright blue of the second 

 order of colours, whose value, in New- 

 ton's Table (Optics t p. 35), is 9 ; then let 

 it be required to determine what will be 

 the tint produced at an inclination of 

 20: the sine of 10 is .1736, and its 

 square .0301 ; the sine of 2G is .342, and 

 its square .117. Hence we have the ana- 

 logy, as .0301 : .1 1 7 = 9 : 35, which corre- 

 sponds to a tint a little above the red of 

 the fifth spectrum or order of colours. 



But though a tint of 35, or the red of 

 the fifth order, can only be produced at 

 an inclination of 20, the thickness of the 

 crystal being supposed the same at all 

 inclinations, yet, if we suppose the thick- 

 ness of the crystal at an inclination of 

 10, to be increased in the proportion of 

 9 to 35, we should then have at 10 the 

 same tint as we have at 20, with a 

 smaller thickness. In any given crystal, 

 any tint may be produced at any given 

 inclination. If we require to produce a 

 very low tint, such as 4, or the yellow 

 of the first order, at an inclination of 80, 

 where the polarising force is very strong, 

 we must then reduce the substance to a 

 very thin film ; and, on the other hand, 

 if we wish to develope a high tint, such 

 as 45, or the greenish blue of the seventh 

 order, at the 'inclination of 5, we must 

 then take a very great thickness of crys- 

 tal to make up for the low polarising 

 force which exists so near the axis. 



The system of polarised rings, like the 

 rings formed by thin plates, may be in- 

 creased in number by viewing them 

 through a prism ; and at inclinations to 

 the axis of a crystal at which they cease 

 to become visible, they may be readily 

 developed by the opposite action of a crys- 

 tal, but which does not exhibit them sepa- 

 rately. 



The phenomena exhibited by a single 

 system of rings undergo curious and 

 beautiful transformations, by interposing 

 thin crystallised films of sulphate of lime, 

 or mica, between two plates, each of 

 which give a system of rings. If, for ex- 

 ample, in the split rhomboid shown in 

 Jig. 24, we insert a thin and equal film 

 of mica, a very singular effect will be 

 produced upon the ring; but when the 

 two rhomboidal plates AMNB,DMNB, 

 are equal, the effect is still more beauti- 

 ful, and the character of the system 

 changes, not only by the revolution of 

 the analysing plate, but during the revo- 

 lution of the rhomboid. In order to show 

 all the varieties of this beautiful class of 



