392 NATURAL SCIENCE. 



JULY. 



no higher mathematical knowledge than appertains to the study of 

 the ellipsoid. 



In an interesting piece of historical criticism, Mr. Fletcher 

 shows that Fresnel did not really deduce the wave-surface from 

 his theory of the elastic ether, as is commonly supposed, but by a 

 generalisation from the empirical results of Huggens ; and he makes 

 it clear that any theory which supposes light to travel in straight 

 lines (rays) and to be due to a periodic change, whether a wave 

 motion or other disturbance, at each point of the ray, must with 

 the aid of certain natural assumptions lead to the same wave- 

 surface. 



Discarding, therefore, entirely any assumption regarding the 

 physical constitution of the ether, he deduces the optical characters of 

 crystals by an entirely new and extremely simple geometrical method 

 from the Optical Indication, which is really nothing more than the 

 elhpsoid (spheroid or sphere) famihar to physicists as the ElHpsoid 

 of Polarisation of Cauchy. The simplicity of the method is greatly 

 enhanced by the exclusive use of rays and not waves throughout the 

 process. 



We venture to predict that those students who are sufficiently 

 courageous to master the first four chapters of Mr. Fletcher's treatise 

 will not only find a lucid and logical explanation of the nature of 

 crystalline double refraction, but will obtain an insight into the 

 historical development of the subject which will be of vast assistance 

 in the study of the writings of other authors. 



Anniversary Addresses to the Geological Society of London, 1891 and 1892. 

 By Sir A. Geikie, F.R.S. (Quarterly Journal of the Gcol.Soc., Proceedings, vol. xlvii., 

 pp. 48-162, and vol. xlviii., pp. 38-179). 



In his presidential address to the Geological Society of London, in 

 February, 1891, Sir Archibald Geikie set before himself the task of 

 presenting "a general outline of the whole volcanic history" of the 

 British Isles ; and this outline has just been completed by the 

 publication of his second address in May of the present year. As the 

 author remarked, "placed on the edge of a continent and the margin 

 of a great ocean-basin, the site of Britain has lain along that critic a 

 border-zone where volcanic energy is most active and continuous ; " 

 and the records which he unfolds show a recurrence of volcanic 

 phenomena in the west and north-west of the area, even after very 

 long intervals of time. Thus, the Exeter eruptions being referred 

 back to the Permian, the whole Mesozoic era passed without external 

 manifestations ; yet in early Eocene times the outbreaks again began 

 on the continental border, giving us the plateaus and necks of Antrim 

 and the Inner Hebrides. 



Beginning in his first address with the Archaean masses, the 

 author well states his position when he accepts "the term 'Archaean ' 

 as a general designation for the oldest gneisses and their associated 

 rocks," and observes that he is unaware " of any reason why rocks 

 undistinguishablc in composition and structure from Archaean masses 

 may not be found in much younger formations." The Lewisian 

 gneiss is regarded as " originally a mass of various eruptive rocks," 

 which may or may not have given rise to volcanic products at the 

 surface. Certain pebbles of true lavas occurring in the Torridon 

 Sandstone may actually be relics from volcanoes of Archaean age. 



In discussing the " younger schists " a new term is introduced. 



