436 Geological Society. 



would be if the terminal planes were inclined at angles the 

 same as those of a Nicol prism, or if more oblique. And ; as a 

 matter of fact, I have had prisms constructed with faces both 

 more oblique and less oblique than those of the ordinary Nicol, 

 and some also with end-faces normal to the axis as in the flat- 

 ended Nicol prisms. My suggestion, of which the main point 

 was the orientation of the reflecting film in a principal plane 

 of section of the crystal, included the particular case which 

 has claimed Mr. Glazebrook's attention. And in the conclu- 

 ding paragraph of my paper of 1881 I claimed, for the con- 

 struction which I suggested, one of the two further advantages 

 which Mr. Glazebrook has so skilfully worked out, viz. " the 

 advantage of producing a field in which the rectilinear polari- 

 zation approximates more uniformly and symmetrically to a 

 polarization in one plane than is the case in the ordinary 

 Mcol." As this was a point to which Mr. Glazebrook took 

 exception on the occasion when my paper was read to the 

 British Association, I am the more gratified to find his ele- 

 gant analytical demonstration of this very feature. 



I have only to add that, thanks to a small grant from the 

 Wollaston fund of the Royal Society, I have been able to pro- 

 secute some further investigations on polarizing prisms, which 

 will, I trust, be shortly ready for publication. 

 I am, Gentlemen, 



University College, Bristol, Your obedient servant, 



May 19, 1883. SlLVANUS P. THOMPSON. 



LXVII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 368.] 



April 25, 1883. J. W. Hulke, Esq., F.R.S., President, in the Chair. 



THE following communications were read : — 

 1. " On the Skull of Meqalosaurus." By Prof. B. Owen, C.B., 

 F.R.S., F.G.S. 



2. "Notes on the Bagshot Sands." ByH.W.Monekton,Esq.,F.G.S. 



The author explained that his paper related to the series of 

 Bagshot Sands on and around Bagshot Heath, which forms what is 

 termed " the main mass " of the Bagshot beds in the memoirs of the 

 Geological Survey. 



The railway-cutting at Golds worthy Hill, described in 1847 by 

 Prof. Prestwich, is still the best type section of the Middle Bagshot 

 Sands ; and the succession of strata seen there was illustrated by 

 reference to newer sections near Ascot and Wellington College. It 

 was pointed out that the most marked feature in this part of the 

 series is very pure greensand, containing casts of shells of Brackles- 

 ham species, that a pebble-bed is found at nearly the same relative 

 level over a large area, and that this pebble-bed forms the most 



