474 Mr. E. Talbot Paris on the Polarization of 



infinitely small particles to those of dimensions comparable 

 with the wave-length, the direction of maximum polarization 

 first moves back towards the incident beam, the movement 

 being less marked in the case of conducting particles than 

 of dielectric particles. 



When j] = l°25 the maxim am has moved back through the 

 90° position to about = 108°, so that there must be two 

 cases when the scattered light would have its maximum 

 polarization at 90° to the incident beam, one when 9;->0 and 

 again when r\ is between 1 and 1*25. 



The curves for 77 = 1*5 and rj = 2 show the same general 

 feature as those for the perfect conductor, though in the 

 latter case the effects are much less marked. 



If the sphere is supposed large compared with the wave- 

 length the light would be regularly reflected and P would 

 show a low maximum at about = 146°. 



4. Experimental. 



1. Preparation of the Silver Suspensions. — The suspensions 

 of silver were prepared by mixing a solution of silver nitrate 

 and pyrogallol in pyridine with water. The pyridine solution 

 was prepared by dissolving 1*436 gms. of silver nitrate in 

 20 cm. 3 of pyridine, and 0*228 gm. of pyrogallol in 10 cm. 3 

 of pyridine; these two solutions were mixed*. For the pre- 

 paration of small particles it is convenient to dilute this 

 solution down to ^ - of its original strength. Fairly uniform 

 suspensions of silver can be prepared by mixing this solution 

 with water; the greater the proportion of water the smaller 

 are the resulting particles. 



2. Preparation of Uniform Suspensions. — In order to obtain 

 more uniform suspensions use was made of the process of 

 fractional centrifuging. The process, as described by Pro- 

 fessor Perrin J, consists in centrifuging a suspension con- 

 taining particles of various sizes, for a certain time, say tj. 

 Let a 1 represent the radius of a particle which, under the 

 influence of the centrifugal force, would travel from the 

 surface of the liquid in one of the test-tubes carried by 

 the centrifuge to the bottom of the tube in the time tj. 

 Then the sediment which collects at the bottom of the tube 

 contains most of the particles of radius greater than cti 

 together with smaller particles which started from positions 

 nearer the bottom of the tube. The sediment is mixed with 

 water and centrifuged again, and the process is repeated 



* Pieroni, Gazetta, 43, vol. i. pp. 197-200 (1913). 



t Perrin, Ann. d. Chim. et de Phys. 8me series, Sept. 1909 ; or Perrin, 

 'Brownian Movement and Molecular Ideality' (Enolish translation, Taylor 

 & Francis). 



