POLARISATION OF LIGHT. 



2i 



No change whatever will take place in 

 this system of rings, by turning the spar 

 about its axis ; but if we turn the ana- 

 lysing plate G,fig. 21, round, as formerly 

 described, then in the azimuths C, 90, 

 180 and 270, the same system of rings 

 will be seen, while at the azimuths 45, 

 135, 225, and 3 1 5, another system will 

 be seen like that shown in Jig. 26. 

 Fig. 26. 



This system differs from the former in no 

 other respect than this, that all the co- 

 lours in the one are exactly comple- 

 mentary to those in the other, so that 

 the superposition of the two, if it could be 

 effected, would completely obliterate both 

 systems. 



If the rhomboid of calcareous spar is 

 now cut into two plates by any line MX, 

 and if the rings produced by each plate be 

 examined separately, it will be found, 



1. That the rings given by each plate 

 are larger in diameter than those produced 

 by the whole rhomboid previous to its 

 division. 



2. That the rings in the thickest of the 

 two plates are less in diameter than those 

 produced by the thinnest ; or in general, 



3. The squares of the diameters of the 

 rings produced by the same plate are 

 proportional to the number which repre- 

 sents the corresponding tint in Newton's 

 Table ; and in plates of different thick- 

 nesses, the squares of the diameters of 



similar rings are reciprocally propor- 

 tional to the square roots of their thick- 

 nesses. 



The phenomena now described may be 

 seen, with equal advantage, by cutting 

 off the solid angles of the rhomb by sur- 

 faces FN, HM perpendicular to the axis 

 B b ; but as the mineral does not cleave 

 parallel to those planes, and is difficult to 

 polish, we cannot, in this way, make 

 the comparative experiments mentioned 

 above with the same facility. 



In examining, in the same manner, 

 other crystallised bodies that have one 

 axis of double refraction, we discover in 

 all of them a system of rings similar to 

 that in calcareous spar, and the axis of 

 this system invariably coincides with the 

 axis of double refraction. In those crys- 

 tals, however, which have positive double 

 refraction, the system of rings, though 

 the same in appearance, has a very differ- 

 ent property. If we take a system of 

 rings, for example, formed by ice or 

 zircon, and combine it with a system 

 of the same diameter formed by Iceland 

 spar, we shall find that the two systems 

 destroy one another ; and hence we con- 

 clude that the system of rings produced 

 by these crystals are jooszYu^, or opposite 

 in character to the negative system of 

 rings in calcareous spar. In the follow- 

 ing Table will be found all the different 

 crystals which give a negative and a 

 positive system of rings. 



Crystals that give a Negative System. 



Carbonate of Lime. 



Carbonate of Lime and Magnesia. 



Carbonate of Lime and Iron. 



Carbonate of Zinc. 



Corundum. 



Sapphire. 



Ruby. 



Emerald. 



Beryl. 



Phosphate of Lime. 



Idocrase. 



"We me rite. 



Paranthine. 



Tourm aline. 



Rubellite. 



Mica from Kariat. 



Molybdate of Lead. 



Phosphate of Lead. 



Phosphato-Arseniate of Lead. 



Hyposulphate of Lime. 



Hydrate of Strontites. 



Arseniate of Potash. 



Muriate of Lime. 



Muriate of Strontian. 



Nitrate of Soda, 



