Minutes of Proceedings. xi 



in the course of his mapping last week, and as a result of diligent 

 searching, Mr. A. W. Eogers, assistant geologist to the Geological 

 Commission, discovered outcrops in the clay-slate near Houw-Hoek, 

 Caledon, which have yielded fossils belonging to the invertebrate 

 groups Brachiopoda and Gasteropoda, as well as more indefinite 

 remains, probably of Trilobites. The importance of the discovery 

 was twofold. They were now likely to be in a position to assign 

 the clay-slate series to some definite geological age, and they also 

 had evidence, apart from structural conditions, that ought to enable 

 them to settle whether the Bokkeveldt fossiliferous shales and these 

 clay-slates were the same as Von Hochstette and E. N. Eubidge 

 held, or whether, with Bain and Wyley and most succeeding 

 geologists, they were to consider this one as lying below and the 

 other above the Table Mountain sandstone. 



Professor Holm said that from 1865 Clerk-Maxwell, from theo- 

 retical considerations, propounded his electro-magnetic theory of 

 light which suffered electrical phenomena to be due to disturbance 

 of the same medium and of the same kind as those which excite the 

 sense of sight. The experimental verification came thirty years 

 later from Dr. Hertz, of Bonn, who not only verified Maxwell's 

 suppositions, but performed all the ordinary optical experiments, 

 such as reflection, refraction, interference, polarisation, &c, with 

 electrical waves. Hertz used as his generator of electrical oscillation 

 two large sheets of brass, separated by a small spark-gap. The 

 sheets were connected to the terminals of a Euhmkorff induction coil, 

 and when the coil was in action a rapid succession of sparks passed 

 across the spark-gap, these giving rise to electrical disturbances, 

 which radiated out into space. These disturbances in the neigh- 

 bourhood of the oscillator were detected by means of a simple circle 

 of stiff wire having two metal knobs on its ends which could be 

 separated by a small distance, giving a second spark-gap. It was 

 found that when the length of the latter was suitably adapted with 

 reference to the former, the disturbances caused minute sparks to 

 pass across this second spark-gap. The methods by which Hertz 

 showed reflection, refraction, and polarisation with these electrical 

 waves were then explained. Since Hertz's time other observers 

 had substituted for the spark-gap a Geissler vacuum tube which 

 glowed at each discharge, thus rendering the effects of the Hertzian 

 waves visible to an audience. The appearances presented when the 

 discharge takes place in a vacuum tube were- explained. When the 

 exhaustion is fairly high, bluish bands of violet light pass from the 

 negative cathode, a reddish light passing from the anode. The two 

 were separated by a dark space known as the Faraday space. As 



