irammitted through crystallized Badles. 333 



which is almost exactly the ratio oi {m—iy to {m—\y. The 

 Value of m' m the tartrate being 1-5 15. 



11. Nitrate of potash. This salt, which is remarkable for 

 its optical properties*, exhibits along the axis of the hexaedral 

 prism a series of beautiful miniature rings, tivelve of which are 

 distinctly visible. In a plate of the nitrate of potash -^^ of an 

 inch thick, the fourth ring subtended an angle of 5° 45', 

 whereas, in a plate of topaz yWo ^^ ^"^ "^^"" thick, it subtended 

 an angle of S° 25'. But 



O ^O • CJ ^lP 1000 iOOO 



the thickness of topaz that would give the fourth ring a diame- 

 ter of 5* 45'. Hence the thicknesses at which the nitrate of 

 potash and topaz produce rings of equal magnitude are as -j»„y\p 

 to tWVj or as 1 to 3*97 nearly. But assuming the rings to 

 vary as (?n — 1)', those formed by the nitrate should have 

 been larger than those exhibited by the topaz in the ratio of 

 636^:515>, or nearly 1-88 to 1. Hence the rings formed by 

 nitrate of potash are 1-88 x 3-97, or /'O times smaller than they 

 should be if their conjugate diameters had varied as (w— 1)' f- 



12. Acetate of lead. This doubly refracting crystal melts at 

 a temperature not much greater than that which bees' wax re- 

 quires, and it takes a long time to cool and crystallize. When 

 it is formed by heat into a thin film between two plates, the 

 crystals shoot from different centres, and exhibit by polarized 

 light the most beautiful alternations of the prismatic colours. 

 When the eye is kept at a distance from the plate, the colours 

 radiate like the spicula of the salt, and vary at every inclination 

 of the plate. 



13. Mother of pearl. The coloured rings are extremely bril- 

 liant in this substance when the polarized light is transmitted 

 almost perpendicularly; but they do not appear when it pene- 

 trates by an oblique path. 



The other substances, which have already been mentioned as 

 affording coloured rings by polarized light, exhibit only imper- 

 fect segments of the fringes ; l)ut in all of them these segments 

 are distinctly visible, excepting in caoutchouc, where the colours 

 are extremely faint. 



It is highly probable that the coloured rings will be found in 



* I liave endeavoured to give a full account of these in the Transactions 

 of the lloyr.l Society of Edinburgh, vol. vii. part ii. 



t 'I'lie thickness of tlie plates of ice, sulphate of potash, and nitrate •» 

 potash, and the inclination of the incidmt pencil, ^ere measured in thi 

 rudest manner, as iny object was merely to ascertain in e.eneral it there 

 was any connection between the masnitude of the coloured rings and the 

 fcfruttive power of the body which pr(jdiiced thcni. 



astili 



