190 Proceedings of the Royal Society of Edinburgh. 



composing which, however, are still less developed than those of that 

 part of the membrane which lies over the tapetum, in the eyes of the 

 horse, ox, &c. They are in fact not hexagonal, but circular, a struc- 

 ture similar to which the author has found in the eye of a very 

 young human foetus. 



Behind and around the ciliary processes, and on the posterior sur- 

 face of the iris, the membrane of the pigment ceases to present the 

 hexagonal structure, although still composed of small irregularly 

 rounded masses of about the same size as the hexagonal plates, to 

 which they are evidently analogous. 



This change in the structure of the membrane of the pigment, 

 which is only partial in the eyes of the Mammiferse, the author has 

 found to obtain in its whole extent, in the eyes of those animals 

 lower in the zoological scale which he has examined, except in the 

 eye of the Cuttlefish, in which there is an approach to the hexagonal 

 structure in that part of the pigment which lies on the posterior sur- 

 face of the part in which the crystalline lens is fixed. 



May 6. — SiE Thomas Makdougall Brisbane, President, 

 in the Chair. The following communication was read : — 



On the Composition of some Iron Slags. By J. F. W. 

 Johnston, Esq. F. R. S. E. 



This paper described the composition of some crystallized slags 

 from Birtley Iron- Works, in the County of Durham. These slags, 

 like those from Wales described by Professor Miller in the Cam- 

 bridge Transactions, had the form of the olivine, and in composition 

 were nearly pure silicate of iron, containing about 0.4 per cent, of 

 foreign matter, chiefly magnesia. The author also gave a short ac- 

 count of a method by which the magnet might be made available in 

 giving immediately very near approximations to the quantity of iron 

 contained in the basic silicates of iron. 



At the Birtley Iron- Works, the author stated that beautiful crys- 

 tals of titanium had likewise been found. 

 The Society then adjourned to the general meeting in November. 



Dec. 16.^ — Sir Thomas M akdougal Brisbane, Bart. Presi- 

 dent, in the Chair. — 



Communication relative to the Fresh- Water Limestone of 

 Burdiehouse, near Edinburgh, belonging to the Carboni- 

 ferous group of Rocks. By Dr Hibbert. 



In his paper, the author states that the limestone in question, 



