of Light wit/im Solid and Fluid Bodies. 411 



tion are at once exhibited to us. The innumerable reflecting 

 surfaces, receiving the intromitted beam at all possible angles, 

 reflect the incident light in all possible directions, so that the 

 eye, wherever it is placed, sees the beam as if it were self- 

 luminous ; and while the eye is made to revolve in a circle 

 round the cylindrical beam, it receives a pencil of polarized 

 light — polarized in a plane passing through the eye and the 

 axis of the cylinder ; or, what is the same thing, a thousand 

 spectators viewing diis beam in the same azimuth, but in di- 

 rections differently inclined to the horizon, would all see 

 exactly the same phEenomena of reflexion and polarization ! 



4. On the Causes of the Internal Decomposition and Dispersion 

 of Light. 



In imperfectly crystallized minerals, such as particular spe- 

 cimens of udidaria, chrysoheryl, o])al and sapphire, the white 

 and coloured opalescence, and the asterial radiations, have been 

 shown to arise from minute vacuities, or from open spaces with 

 crystallized sides, or from narrow pipes, or linear spaces parallel 

 to the edges of the primitive or secondary forms of the mineral. 

 In tabasheer, where the vacuities contain air, which we can 

 expel and send back at pleasure, a fine blue light is dispersed, 

 depending no doubt on the size of the vacuities. In a very 

 remarkable specimen of calcareous spar, crowded with hemi- 

 trope veins, I have observed a copious internal dispersion 

 produced by the reflexion of light at the different surfaces, 

 which, though in optical contact, have different degrees of 

 extraordinary refraction. 



All these phaenomena, however, are essentially different 

 from those which form the subject of this paper, with the ex- 

 ception of the phaenomena of fluor-spar, in so far at least as 

 they are the result of imperfect crystallization. The epipolism 

 which Sir John Herschel ascribes to this mineral, or its internal 

 dispersion, according to my experiments, does not belong to 

 the species, but only to particular varieties, and not even to 

 the variety, but merely to particular parts of it ; it is therefore 

 the result of unequal or imperfect crystallization. The nucleus 

 is perfect, a coating supervenes, having a different tint by 

 transmitted light, and dispersing a fine blue light, and so on 

 through a succession of strata, dispersing differently coloured 

 lights, and separated by non-dispersing spaces. An extra- 

 neous element, therefore, depending on the state of the solu- 

 tion, has been successively intioduced into the crystal; and if 

 it liad the same refractive and dispersive power as the fluor- 

 spar, it could not reflect any portion of the intromitted beam. 

 But if there is any difference in the mean refraction, or in the 



