18 



fixplanation was that when the polarised beam 

 ■cime to the selenite it was divided into two, the 

 vibrations of which were at right angles to each 

 other. These two travelled through the selenite 

 with different velocities owing to the difference of 

 structure in different directions, and on emergiuer 

 one had lost a half wave length of red or of some 

 other colour Then when these came to the 

 analyser, each at an anele of 45 degrees 

 neither could get through, bat both were 

 resolved into vertical and horizontal com- 

 ponents, and then either vertical or horizontal 

 would get through according to the position of the 

 analyser. But the emerging light must of 

 necessity be coloured as some colour had been 

 destroyed, and when the analyser was turned 

 through a right angle the other component would 

 pass with a colour the complementary of the first 

 one. This experiment was really the explanation 

 of all the wonderful illustrations that were given, 

 different substances such as crystals, horn, 

 granite, etc.. showing exactly similar effects — all 

 proving that these substances were not of uniform 

 structure. 



Aftf-r these experiments quartz plates were sub- 

 jected to analysis, but these always showed colours, 

 as the vibrations of the polarised beam, on passing 

 through them were no longer plane but circular. 

 Next some specimens of unannealed glass were 

 tested. These, by the ordinary light, showed no 

 difference in structure; but in polarised light 



colour was immediately visible, and this colour 

 indicated unevenness of structure, and therefore 

 strain. To prove this the lecturer took pieces of 

 thick glass, which in either ordinary or polarised 

 light showed no trace of colour, but when he sub- 

 jected them to bending or pressure vivid colours 

 appeared, indicating the strain in the interior. 

 He explained that by exactly similar methods, 

 many false gems can be detected, for the true 

 ones have uneven structure, and therefore show 

 colours in polarised light whereas the false do 

 not. To illustrate " refraction," and the theory 

 of colours, a beam of white light from an electric 

 arc was passed thiough a triangular bottle con- 

 taining bisulphide of carbon. The beam was de- 

 flected from its course, or refracted, but it was 

 also spread out into a beautifully coloured band 

 two feet in length showing brilliant tints from 

 red to violet. In this way it is seen that white 

 light is really composed of these various colours — 

 and if one colour be destroyed and the others 

 combined — the resultant is not white, but the 

 complementary of the colour destroyed. The 

 illustrations and experiments showed brilliantly 

 on the screen. 



The President proposed a vote of thanks to the 

 lecturer, who.in acknowledging it, paid a graceful 

 compliment to his colleague, Mr. Leeming who 

 rendered him such efficient help in the preparation 

 and carrying out of the experiments. 



TWELFTH WINTER MEETING.— APRIL 25th, 1905. 



"THE EVOLUTION OF OUR PLANET.'— By Mr. T. DeVERE. 



The last meeting of the winter session was held 

 in the Reference Library of the Beaney Institute 

 on Tuesday, April 25, when there was a fair 

 attendance. < >n the proposition of Mr. A. Lander, 

 Mr. A. W. Good and Miss Burch were elected 

 members of the Society. Previous to the lecture, 

 Mr. Sidney Harvey, who presided, showed and 

 explained metallic calcium— a metal which, he 

 said, was now produced by electrolysis at a low 

 price, and would, he thought, prove of immense 

 bc^nefit when more was kuown of its great 

 properties. Mr. Harvey also gave an interesting 

 demonstration of the Blondlot's or N-rays. 



Mr. T. De Vere then read an interesting paper, 

 entitled " The evolution of our planet." His 

 remarks were illustrated with lantern views, and 

 traced the subject from the existence of the earth 

 as a nebulous body. In the course of his remarks 

 Mr. De Vere said : 



Recent observations have disclosed to us the 

 wonderful fact that not only the west coast of 

 South America, but also other large areas, some 

 of them several thousands of miles in cir- 

 cumference, such as Scandinavia, and certain 

 archipelagos in the Pacific, are slowly and in- 

 sensibly rising ; while other regions, such as 

 Greenland, and parts of the Pacific and Indian 



Oceans, in which atolls or circular coral islands 

 abound, are as gradually sinking. That all the 

 existing continents and submarine abysses may 

 have originated in movements of this kind, 

 continued throughout incalculable periods of time, 

 is undeniable, for marine remains are found in 

 rocks at almost all elevations above the sea, and 

 the denudation which the dry land appears to 

 have suffered, favours the idea that it was raised 

 from the deep by a succession of upward move- 

 ments, prolonged throughout indefinite periods. 

 Rain and rivers, aided sometimes by slow and 

 sometimes by sudden and violent movements of 

 the «arth'a crust, have undoubtedly excavated 

 some of the principal valleys, but there are also 

 wide spaces which have been denuded in such a 

 manner as can only be explained by reference to 

 the action of waves and currents on land slowly 

 emerging or sinking again to the deep. The 

 Andes have probably been rising several feet 

 century after century, while the Pampas have 

 only risen a few inches at the time. Darwin found. 

 in crossing from the Atlantic to the Pacific, that 

 the elope was at first very gentle, then greater as 

 he neared Mendoza, which is 4,000 feet high. The 

 mountAiQor.s district then begins suddenly, and in 

 the next 120 n.iles to the Pacific toe average 



