1 96 Mr. J. Bryce's Addendum to a Descriptive Catalogue 



power of the eye itself. As soon as sufficient attention has 

 been excited towards this curious phenomenon, I shall give 

 the world a new discovery in this branch of optics, one so very 

 curious that it deserves to be ushered into notice when the 

 mind has contemplated all the preparatory steps which your 

 knowledge shall make familiar. 



It is not mere speculation when I assert that the office of 

 the retina is to regulate the admission of light. I go on safe 

 grounds : experiment has convinced me that the lines which 

 compose the meshes of the retina are thicker or thinner as 

 the opening of the pupil varies. In some future communica- 

 tion I will show you one of these meshes magnified. Mr. 

 Horner is correct in saying that the eye sustains no injury 

 in making these experiments. 



The brilliancy of colouring and the kaleidoscope patterns 

 of which Sir David Brewster speaks, belong exclusively to 

 the peculiar shape and structure of the retina. With all the 

 multiplied and various experiments that I have made, I have 

 observed that all the brilliant as well as opake figures are 

 square or angular. I have never seen anything approaching 

 to circularity but the stars which I have described in my first 

 paper. That star, or those stars, for there is one in the centre 

 of each square or mesh, may be in reality composed of points 

 or angles, but they are too minute to ascertain this fact. 

 They scintillate. 



XXIX. Addendum to a Descriptive Catalogue of the Minerals 

 of the North of Ireland. By James Bryce, Jun. 9 M.A., 

 KG.S.,$c. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 

 T N a note appended to a paper of mine on the minerals of 

 -*- the North of Ireland, published last August, you inquire, 

 in relation to a new mineral, which Dr. Thomson has named 

 hydrocarbonate of lime and magnesia, " Whether this mi- 

 neral is allied to that which the late Mr. W. Phillips, at the 

 suggestion of Mr. Brayley, described under the appellation of 

 Hydrocarbonate of Lime, in the last edition (1823) of his 

 Elementary Introduction to Mineralogy, p. 161." It is then 

 added, from a communication by Mr. Brayley, that the latter 

 mineral " is the result of the action of the trap dykes of the 

 Giants' Causeway upon the chalk which they have intersected, 

 and, according to the analysis of the late Dr. Da Costa, would 

 appear to be composed of four atoms carbonate of lime and 

 there atoms water," &c. In reply I may observe, that I be- 

 lieve the minerals to be distinct, though obviously closely al- 



