524 TRANSACTIONS OF SECTION A. 



rays.' These travel away from the cathode in the same direction as the cathode 

 rays; these rays are of types (1) and (2). I have not succeeeded in detecting 

 rays of type (3) among these rays. The retrograde rays have negative 



constituents. 



2. A New Spectrophotometer of the Hvfner Type. 

 By E. A. Houstoun, M.A., Ph.D., D.Sc. 



This instrument consists of a spectroscope, the ordinary eyepiece of which 

 has been replaced by an eyepiece containing a glass Thompson prism and in front 

 of the slit of which is fixed a prism of special design made of glass and Iceland 

 spar. This prism performs the double function of dividing the field of view 

 into halves which touch each other sharply, and of polarising these halves at 

 right angles to each other. The halves are matched by rotating the nicol eye- 

 piece. The instrument can be arranged for use as an ordinary spectroscope in 

 one minute by removing the special prism and substituting the ordinary eyepiece. 

 It has the advantage over other Hiifner spectrophotometers that any dispersion 

 prism may be used with it. It has been used for three researches during the 

 past two years, and has proved accurate, free from systematic error, and very 

 suitable for measuring the intensity of weak lights. 



3. A New and Simple Means of producing Interference Bands. 

 By E. A. Houstoun, M.A., Ph.D., D.Sc. 



If a right-angled isosceles glass prism the right angle of which is a few 

 minutes short of 90° is placed in front of a slit, two virtual images of the slit 

 are formed behind the prism, and these produce interference bands in front of 

 the prism. The experiment is being used as a student's exercise in the physical 

 laboratory of the University of Glasgow. The method was at first thought 

 to be new, but has recently been anticipated in Berlin. 



4. A New Gyroscopic Apparatus. By Professor A. E. H. Love, F.R.S. 



FRIDAY, SEPTEMBER 2. 



Joint Meeting with Section B. 

 Discussion on Combustion. — See Eeports, p. 501. 



The following Papers and Report were then read : — 

 I. The MolecularW eight of Radium Emanation . By Sir William Eamsay, 

 K.C.B., F.R.S., and Dr. E. W. Gray. 



2. On the Number of Electrons in the Atom. By Dr. J. A. Crowther. 



The present experiments were made to determine, by means of experiments 

 in the /?-rays, the number of electrons in the atom. 



Professor Sir J. J. Thomson has recently calculated the mean deflection 4> 

 experienced by a /3-particle of known velocity in passing through a thickness t 

 of material. The summation is performed by a theorem due to Lord Rayleigh. 

 which leads to the relationship 



<p/ s/ t = c 

 where c is a constant involving only the number of electrons in the atom of 



