TRANSACTIONS OF SECTION A. 537 



nature of the positive and negative ions in the air, the vertical earth-air current, 

 and the phenomena of radioactivity. The potential gradient and its diurnal 

 and annual variation have been the subject of study for a good many years, 

 and we know that the phenomena vary largely with the season of the year at any 

 given station, and that there are notable differences between different stations 

 at ground-level. As yet but little is known of the diurnal and annual changes 

 at different heights in the free atmosphere. Observations made near the top 

 of the Eiffel Tower suggest that the phenomena alter rapidly as the height 

 above the ground increases. Thus observations from balloons and kites, if these 

 could be maintained at a fixed level, would be of great importance. The influence 

 of electricity on the growth of plants, first seriously studied by Professor 

 Lemstrom, seems not unlikely to prove in the future of economic importance. 

 The phenomena of thunderstorms have received considerable study from 

 meteorologists, but many lines of investigation present themselves. The loss 

 of charge of insulated bodies and the ionic charges in the atmosphere have been 

 most studied in Austria and Germany, Elster and Geitel in particular having 

 done much pioneer work. In this country Mr. C. T. R. Wilson has investigated 

 the electric charge brought down by rain and has invented an instrument for 

 measuring the earth-air current. While a great number of theories have been 

 advanced to account for the several phenomena, there are few, if any, of them 

 which command anything like universal acceptance. 



Department of Mathematics and Physics. 



The following Papers and Report were read : — ■ 



1. An Auto-collimating and Focussing Prism, and a Simple Form of 

 Spectrograph. By Professor C. Fery, D.Sc. 



The value of the spectroscope, and especially of the quartz spectrograph, 

 which includes the ultra-violet rays, as a means of scientific research, has led 

 the author to attempt the simplification of the apparatus to render it suitable 

 for industrial investigations concerning metals, alloys, &c. The employment 

 of the principle of auto-collimation, with a 30° prism, simultaneously shortens 

 the apparatus, simplifies the lens system, and avoids trouble due to the rotatory 

 properties of the quartz, since the prism is traversed twice in opposite directions. 

 But the difficulty still remains with the ordinary lens system, that the non- 

 achromatism of the lens leads to a somewhat impure spectrum, and to a very 

 great inclination (about 64°) of the plate. Hence a means was sought of 

 eliminating this lens. The general condition for producing a pure spectrum 

 is that all the incident or refracted rays should make the same angle with the 

 refracting surface. By giving suitable spherical curvatures to the front and 

 back surfaces of the prism this condition is very closely realised, and a sharp 

 spectrum is obtained on a cylindrical surface, exactly as with a curved reflecting 

 grating, and with a much smaller inclination of the plate. 



The spectrograph therefore consists simply of a heavy base, upon which 

 is fixed the new prism, a carefully made slit, and a slide for holding the plates 

 and bending them to the required cylindrical form, and also permitting of a 

 vertical displacement of the plate, so as to get several successive spectra in 

 exact juxtaposition for easy comparison. The adjustments can be made and 

 clamped, and a light-tight cover slipped into grooves, leaving only the slit and 

 the back of the slide exposed. A quartz cylindrical lens is fixed on a sliding 

 rod, so that its optic axis is always coincident with that of the slit and prism, 

 and serves to concentrate the light from the arc or other source on the slit. 

 The exposure for an arc spectrum under normal conditions is about thirty 

 seconds. 



The author described the theory of the prism and the details of the 

 apparatus, and also gave examples of the photographs obtained. 1 



1 See Engineering, October 7, 1910. 



