448 Notices of Memoirs — British Association — 



the ' crossed nicols ' condition the analyser is held in place against 

 S by the small stop M. The o' and e' rays from the crystal on 

 entering the analyser are resolved into two components, and of these 

 only one is transmitted. The light leaving the analyser is therefore 

 made up of the resolved portions of o' and e' , and is indicated by the 

 arrows Ko and Ke, both lying in the same plane, but one behind 

 the other. 



The way for the more rigid explanation of the birefringence colours 

 is often made easy by some such phrase as the following : " The two 

 resolved portions of the polarized ray enter the crystal plate perfectly 

 'in step ', but as one is retarded more than the other while passing 

 through the crystal, it falls behind, and wlien brought by the analyser 

 into the same plane the two rays are found to be no longer ' in step ', 

 and consequently interfere with each other." The interference can 

 be illustrated by soldering to each arrow a portion of a sine curve 

 wire (as at N). Bj' varying the distance between the arrows the 

 quartz wedge and kindred phenomena can be well shown. 



If a large cork be used for L, and the amplitudes of Ko and Ke 

 be varied as with o' and e' , one can illustrate the impossibility of 

 obtaining complete extinction with uncrossed nicols. 



Pleochroism is exemplified by removing the analyser and clipping 

 differently coloured glass plates (e.g. brown and green) over the 

 and e slots in the crystal plate i^. The o' and (?' arrows maybe 

 coloured accordingly, and the variation of the tint as the crystal is 

 rotated is brought out well by varying the lengths of the o' and e' 

 arrows as before, to give the two colour intensities in all positions. 

 A little mechanical contrivance maybe added to do this automatically, 

 but would require nice workmanship. 



The model may be used to illustrate several other points, but its 

 application to these ends will occur to every teacher, and need not be 

 mentioned here. 



I. — British Association foe thk Advancement of Science, Eightt- 

 THiKD Annual Meeting, held at Birmingham, September 10-17, 

 1913. List of Titles of Papers read in Section C (Geology) 

 and in other Sections bearing upon Geology. 



Presidential Address by Professor E. J. Garwood, IT. A., V.P.G.S. 



(see ante, p. 440). 

 Professor Charles Lapwurth, F.R.S. — On the Geology of the Country 



round Birmingham. 

 Professor W. W. Watts, F.R.S. — Notes on the Igneous Rocks of the 



Birmingham District. 

 Mr. George Barroiv. — The Spirorhis Limestones of North Warwickshire 



(see p. 463). 

 Mr. Henry Kay. — On the Stream-courses of the Black Country 



Plateau (see p. 457). 

 Professor W. J. Sol/as, F.P.S. — On the Formation of Rostro-Carinate 



iTints. 



