transmitted through crystallized Bodies. S2d 



these planes are parallel to each other, an arrangement which is 

 represented in Plate IV., fig. 10. 



When the observer looks into the topaz in the direction r/, 

 he will perceive the second set of fringes. If the rays rr' are 

 transmitted through a plate of agate having its laminae perpen- 

 dicular to the plane of reflection, the fourth set of fringes will 

 be seen, but they are very much fainter than they appeared in 

 the former experiments. When the lamins of the agate are 

 parallel to the plane of reflection, the second set is faintly visi- 

 ble. The central spots are, however, rather larger than before, 

 so that this set has the appearance of being the reverse of the 

 Jiist rather than of the third set. 



When the light transmitted in the direction CF is seen by the 

 naked eye, it exhibits the first set of rings. If it is examined 

 through" a plate of agate having its veins perpendicular to the 

 planeof reflection, the first set is still visible ; but when the 

 agate is turned round 90^ the 'second set is perceived. 



All the preceding observations were made with a plate of topaz 

 •jVoV of an inch thick. When the plate has a greater thick- 

 ness the rings are mucli smaller, and when it has a less thickness 

 the rings are extremely large, so tiiat in very thin plates only 

 a small portion of a ring caii be perceived at once. We have 

 already seen that with a plate -,-',^V-,r of ein inch thick, the fourth 

 red ring subtends angle of IS' 30'. With another plate tWs- 

 of an inch thick, the angle subtended by the san^e ring is S'' 

 24'. But since 



_2S_7 • 10 3 IQo "(Y . QAo Oj . 



-nro-o'TiX'ij'o — '■'^ '^^ • ^^ -^ > 

 it follows that the conjugaie diameters of the rings are iiiversdij 

 as the thickness of the plaics. 



According to the Abbe Ilaiiy, the angle formed by two of the 

 primitive faces of the topaz is 124'^ 22'; and therefore the axes 

 or longest diagonals of the primitive rectangular prism will form 

 angles of 60° 31' 15"* with a line perpendicular to the lamintc, 

 a result which agrees so nearly with tiO- 38', the iuclination of 

 tlie axes of the coloured rings, that we can have no lit-sitation in 

 concluding that the ores of the coloured rings are coincident 

 with the longest diagonals of the pr/niilive rectang/tlar prism. 



The rings which have now been described, as produced hy 

 topaz, I have discovered in rock cr/fsfal., mica, the agate, the 

 oriental ruby, tlie emerald, native hydrate of magnesia, amber, 

 ice, sulphate of potash, tartrate of potash a?id soda, nitrate of 

 potash, acetate of lead, acetate of lead welted and cooled, prus- 

 siate of potash^ mother of pearl, hones of a cod, quill, the hu- 



* According to my own measurcinciils, tlic nw^c is XIZ" 58', wliich gives 

 00" *'l}' 'id" lor llic iiicliimtioii of tlic diaj^oii.ils. 



