of Nitrate of Potash. 125 



By takiiig a mean between the two extreme observations, we 

 olitain 0-058 for the approximate dispersive power, — a result 

 which could scarcely have been anticipated from the substances 

 which enter into the composition of nitre. The following table 

 will show the relation which this measure bears to the dispersive 

 powers of other bodies : 



Sulphate of lead 0-OGO 



Nitrate of potash, 2d refraction 0*058 



Flint glass 0-048 



Water 0;035 



In order to examine the character of the rays which form the 

 two images, 1 polarized the light of a candle by reflection from 

 glass, and viewed it through two of the parallel faces of a hex- 

 aedral prism of nitre. When the edges or common sections of 

 its faces were parallel to the plane of reflection, a bright image 

 of the candle was seen in the middle of a mass of nebulous light, 

 exactly similar to what happens in the agate when its veins- are 

 parallel to the plane of reflection. But upon turning round the 

 crystal of nitre, the bright image gradually vanished^ while the 

 nebulous light increased; and when the edges of the crystal 

 were perpendicular to the plane of reflection, the bright image 

 was extinguished, and the nebulous light a maximum. When 

 the reflected image of the candle is viewed through two inclined 

 faces of the nitre, the two images vanish alternately, like those 

 formed by all doubly refracting crystals. 



A prism of nitrate of potash, having its refracting surfaces 

 equally inclined to the axis of the hexaedral crystal, possesses 

 the faculty of depolarizing light ; and hence it follows, from the 

 theory of depolarization, that the prism mu ;t, in this case, form 

 two distinct images. 



The two neutral axes of this salt are parallel and perpendi- 

 cular to the sides of the hexaedral prism ; and the depolarizing 

 axes are parallel to the diagonals of the square base common to 

 the two pyramids which compose its primitive rectangular oc- 

 taedron. ' The least refracted image is that which is produced 

 bv the extraordinary law of refraction. 



' The lieautiful coloured rings which I exhibited to the Society, 

 as produced by the action of topaz upon polarized light, and 

 which I have also discovered in the agate, and in a great variety 

 of other bodies *, exist also, but in a very singailar manner, in 

 the nitrate of potash. 



By comparing in a rude manner the coloured rings formed by 

 different bodies, with the thickness of the plates by which they 

 were produced, I concluded that the conjugate diameters of the 



* Sec Fl. 11. Trans. Loiiil. 1814, part i. p. '218. 



rin,;r- 



