116 



parency, transmitting a yellowish, and reflecting a bluish light ; an- 

 other is translucent, and a third opake : the two first varieties be- 

 come transparent, and evolve air when immersed in water : the third 

 evolves air also, but remains opake. If the first varieties be only 

 slightly wetted they become quite opake. The property of acqui- 

 ring transparency by the evolution of air from, and the absorption of 

 water by its pores, belongs also to the hydrophanous opal ; but the 

 faculty of becoming opake by a small quantity, and transparent by 

 a larger, of water, shows a singularity of structure in tabasheer. 

 As the tabasheer disengages more air than hydrophane, its pores 

 must be more numerous ; and therefore the transmission of light, so 

 as to form a perfect image, indicates either a very feeble refractive 

 power or some peculiarity in the construction of its pores. To de- 

 termine this, Dr. Brewster formed a prism of tabasheer with an 

 angle of 34 15', and upon measuring its refractive power found it 

 very low, though various in different specimens, the index of refrac- 

 tion varying from Til to 1'18, that of water being 1*33, of flint- 

 glass 1'60, of sulphur 2'11, of phosphorus 2'22, and of the dia- 

 mond 2'47. So that tabasheer has a lower refractive power than any 

 other solid or liquid, and holds an intermediate place between water 

 and the gases. Dr. Brewster then gives a formula for computing the 

 absolute refractive power of bodies, and a table of results, from 

 which it appears that, in this respect, the refractive power of taba- 

 sheer is so low as to be separated by a considerable interval from all 

 other bodies. 



The author next proceeds to detail a variety of experiments upon 

 the absorbent powers of the different kinds of tabasheer, in respect 

 to several liquids, and the corresponding effects upon its optical pro- 

 perties and specific gravity, and concludes with observations on the 

 cause of the paradox exhibited by the transparent tabasheer, in be- 

 coming opake by absorbing a small quantity of water, and transpa- 

 rent when the quantity is increased. 



An Account of a Membrane in the Eye, now first described. By 

 Arthur Jacob, M.D., Member of the Royal College of Surgeons in 

 Ireland, Demonstrator of Anatomy and Lecturer on Diseases of the 

 Eye in the University of Dublin. Communicated by James Macart- 

 ney, M.D. F.R.S. Read July 1, 1819. [JPhil. Trans. 1819, 

 p. 300.] 



In this paper the author describes a delicate transparent mem- 

 brane, covering the external surface of the retina, and united to it 

 by cellular substance and vessels. Its appearance varies in the dif- 

 ferent classes of animals, and at different ages. In young animals 

 it is transparent and tender ; but in the adult firm, and stained by 

 the pigment. In fishes, it has been described by Haller and Cuvier 

 as the medullary layer of the retina ; but the author thinks incor- 

 rectly, since it presents no character of nervous structure, and the 

 retina remains perfect before it. The author concludes this com- 



