Light scattered by Spherical Metal Particles. 479 



the iris diaphragm I. The scattered light was examined 

 through the telescope tube of the spectrometer, which was 

 adapted to carry the rectangular opening A (1 cm. x 0*5 cm.), 

 the double-image prism P, and the nicol N. Two images of 

 the rectangular openings are seen side by side through the 

 nicol and double-image prism, and the nicol is set in two 

 positions successively for which the intensities of the two 

 halves of the field appear equal. If the readings on the 

 divided circle are &>! and co 2 , then 



P = ^—^ 100= -100 cos (©, -co 2 ). 



The angle between the direction of observation and the beam 

 passing through the beaker is read off from the scale on the 

 spectrometer table, a preliminary setting being made to find 

 the reading for = tt. Readings were taken every 10°, and 

 intermediately in special cases. Reflexions from the side of 

 the beaker obscured the readings for # = 0° to 30° and 150° 

 to 180°. 



To obtain light of the required wave-length the incident 

 light from the arc passed through a Wratten mercury -green- 

 line filter. This filter transmits a bright beam at about 

 X550 and a much less intense beam at the extreme red end 

 of the spectrum. 



The following sources of error may be noted : — 



(a) The presence of a small quantity of red light trans- 

 mitted by the filter. This, though small, would tend to 

 diminish the polarization of the scattered light, for the 

 polarization phenomena were observed to be less marked 

 with red than green light, for a given size of the particles. 



(b) The scattering of light by dust and other particles 

 which were always found to be present in the distilled water 

 used for preparing the suspensions. Observations made on 

 distilled water alone showed the intensity of this light to be 

 extremely feeble, but exhibiting a direction of maximum 

 polarization at 90° to the incident beam. 



(c) The reflexion of light from the walls of the beaker. 

 This was very small except in the direction noted above. 



(d) The polarization of the light by its passage through 

 the walls of the beaker. 



It is doubtful if any of these sources of error would make 

 any perceptible difference in the polarization of the scattered 

 light. 



The results of the observations are shown in fig. 6, where 

 the experimentally determined values of P for particles of 



