TRANSACTIONS OF SECTION A. 305 
TUESDAY, AUGUST 25. 
Jot Meeting with Section G (Engineering). 
Discussion on Wireless Telegraphy. Opened by 
Sir Outver Lopes, F.R.S. 
The following Papers were then read in Section A :— 
1. Some Measurements of the Wave-length in Air of Electrical Vibra- 
tions associated with a Thin Straight Terminated Rod. By 
Professor J. A. PouLock. 
2. High-Frequency Spectra. By H. G. J. Mosetey. 
3. On the Scattering of Light by Small and Large Particles of 
Conducting and Non-conducting Substances. By Professor 
ALFRED W. Porter, F’.R.S., and E. TauBot Paris, B.Sc. 
The work summarised herein is a continuation of an investigation by Porter 
and Keen’ on the diffraction of light by particles comparable with the wave- 
length. In that work the scattering was produced by a sulphur suspension, and 
observations were restricted to the transmitted light. In the present paper the 
degree of polarisation has been determined (by means of a double-image prism 
and nicol) for the light scattered in different directions; and suspensions of 
silver and copper have been investigated as well as suspensions of sulphur. The 
metallic suspensions were made by the method of Pieroni,” which we have found 
to give stable suspensions. 
The results for sulphur particles show a good general agreement with the 
theoretical values calculated by Lord Rayleigh,® but exact comparison is_ not 
possible owing to the difficulty of determining the size of the particles. In the 
case of the silver particles comparison with theory is easier, because the total 
amount of silver present can be so readily determined chemically, the number 
of particles per unit volume can be counted, and thence their size can be 
calculated. Measurements of the size were also made by Perrin’s method, i.e., 
by counting the number of particles in each of two layers a small vertical 
distance apart, and attributing the difference (as in an atmosphere of gas) to 
the total weight of particles in the intervening space. Both methods give 
practically the same results. 
Complete curves have been obtained for the polarisation for different-sized 
particles in different directions. Mention will be made here only of the direction 
of maximum polarisation for silver suspensions for light of wave-length 550up. 
This is shown in the following table :— 
. 3 Direction of max. | Relative electric 
Diameter of particles. | polarisation. conductivity. 
fu | 
80 | 90° | -- 
| 98 90° | 2-96 
108 98°-36' 3-70 
131 1099-54 4:12 
164 113°-36' | — 
310 130°-30' — 
{ 
1 Proc. Roy. Soc. A, Vol. 89, 1914. 
2 Garetta, 48 (1), 197 (1913). 
3 Proc. Roy. Soc. A, Vol. 84, 25 (1910). 
1914. x 
