﻿vi PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 1905, 



refractometer designed by Prof. E. Bertrand in 1885 was portable, 

 but in it no attempt had been made to compensate for the curvature 

 of the hemisphere. The focal surface corresponding to the hemi- 

 sphere was — apart from the effects of chromatic and spherical 

 aberration — for rays which were parallel inside the glass, a spherical 

 envelope concentric with the hemisphere. Hence the ocular scale 

 could not possibly coincide with the focal surface for the whole 

 range required, and the difficulties owing to parallax and bad 

 definition of the shadow-edge rendered accurate results impossible. 

 He (the speaker) had, in the present instrument, overcome this 

 defect by introducing between the hemisphere and the scale a 

 corrective lens, by means of which the focal surface became almost 

 exactly a plane, and the edges separating the light and dark fields 

 were sharply defined throughout the whole range. 



The scale was graduated by means of observations on known 

 substances. Each interval corresponded to a difference in refractive 

 indices of about O'OIO. In monochromatic light the shadow-edge 

 appeared as a delicately-traced line, and an estimate of the refractive 

 index might be obtained correct to two units in the third place of 

 decimals. The instrument had been constructed to give the best 

 results between 1*45 and 1*75. The glass of which the hemisphere 

 was composed had a refractive index of 1-7938. 



The refractometer was to be used in the ordinary way. The 

 indices of minute fragments might be determined indirectly, by 

 finding with a microscope a liquid of the same refraction as them- 

 selves, and then observing with the instrument the index of the liquid. 



Mr. J. H. Steward, of London, was the maker of this refractometer. 



The following communication was read : — 



1 On the Geology of Arenig Fawr and Moel Llyfnant.' By 

 William George Fearnsides, M.A., F.G.S. 



In addition to the exhibits described on pp. v-vi, the following 

 specimens were placed on the table : — 



Fossiliferous Arenig rocks and volcanic intrusives, with micro- 

 scope-sections, exhibited by W. G. Fearnsides, M.A., F.G.S., in 

 illustration of his paper. 



Reproductions of an interesting series of new sea-urchins, of the 

 family Spatangidse, recently found in blocks of a Middle Miocene 

 rock, apparently brought up by ice-action from the floor of the 

 Schaalsee, near Zarrentin (S.E. Holstein), exhibited by Dr. F. A. 

 Bather, M.A , F.G.S. 





