m tlie Cavities of Minerals. 9 



the topaz, in consequence of the duplication and overlapping of 

 the images formed by double refraction. 



This inconvenience, however, may be nearly removed, by ma- 

 king the plate of topaz very thin ; or it may be entirely reme- 

 died, in plates of any size, by causing the incident rays RS RS, 

 Fig. 2., to pass along one of the resultant axes of the topaz, while 

 the reflected rays SL SL pass along the other resultant axis. 



In order to compare the angle at which. total reflexion took 

 place at the upper surfaces of the fluid and the cavity, with that 

 which would have taken place had the fluid been water, I placed 

 a drop of water on part of the lower surface of the plate AB, 

 and I found that the light reflected at the same angle of inci- 

 dence, was much more brilliant from the separating surface of 

 the new fluid and the cavity, than from the separating surface of 

 the topaz and the water, a result which indicated, in the most 

 unequivocal manner, that the new fluid had a refractive power 

 inferior to water, and that it differed in this respect from every 

 other known fluid. 



Although, in this estimate, I attended carefully to the cir- 

 cumstance, that, in the one case, the light reflected from the 

 bottom of the cavity was combined with that reflected from its 

 surface, and therefore used deep cavities, where the two re- 

 flexions could to a certain degree be separated ; yet, in order to 

 remove any doubt that might remain on the subject, I took a 

 plate of topaz that contained water, or, to speak more correctly, 

 a fluid which did not expand by heat, and upon comparing the 

 reflexions from the cavities, the difference was most palpable. 



In one specimen I measured the difference between the 

 angles of incidence at which total reflexion took place, at the 

 separating surface of the new fluid and topaz, and at the separa- 

 ting surface of water and topaz, and I estimated that the re- 

 fractive power of the new fluid was below 1.300, that of water 

 being 1.336. 



VOL. X. P. I. B 



