﻿] 48 Prof. R. W. Wood : Abnormal Polarization and 



flame. It is a good plan to support the bulb in a clamp- 

 stand in a dark room and focus sun- or arc-light at its centre. 

 The red fog persists for some time, once it is formed. When 

 at its best its scattering power is so great that a reddish colour 

 is seen within the bulb at a distance of a metre from the arc 

 without any concentration. Examined by transmitted light 

 in a well-lighted room, no trace of colour is to be seen, which 

 proves that the red light is selectively scattered, and not 

 produced by absorption. With the concentrated beam from 

 the arc the scattered light is blood-red and of great intensity. 

 A nicol placed with its long diagonal horizontal, nearly, but 

 not quite extinguishes it. 



It seemed probable that the polarization might be produced 

 by refraction of the light within the small particles. The 

 light which is given off at right angles to the direction of the 

 incident beam is in part reflected of course. This portion is 

 uncoloured, and polarized with its vibration perpendicular to 

 the direction of the incident rays. Another portion of light 

 may reach the eye after two refractions and one internal 

 reflexion, and this portion will be coloured, and oppositely 

 polarized. - We are of course considering the particles as 

 partially transparent, and of such size as to permit them 

 to act as the water-drops in the theory of the rainbow. A 

 small glass bulb filled with a red liquid can be used to show 

 the phenomenon on a large scale ; if it is placed in a beam 

 of sunlight and viewed at an angle slightly greater than 90° 

 with the incident rays, two spots of light will be seen — one un- 

 coloured due to reflexion, the other coloured due to refraction; 

 and examination with a nicol shows that they are more or 

 less completely polarized in opposite directions. Attempts 

 were made to obtain fogs of other strongly-absorbing media. 

 Precipitation of the aniline dyes was tried, without much 

 success however. Nitroso- dimethyl aniline, a substance which 

 has proved of interest in connexion with many optical problems, 

 was found to give excellent results. A few grains are placed 

 in the bottom of a large test-tube, and heated with a small 

 flame. A yellow fog at once forms in the tube, which exhibits 

 the same phenomena shown by the iodine fog. Examined 

 with a nicol with its short diagonal horizontal, the scattered 

 light is yellowish green and very bright ; if the nicol is 

 turned through a right angle the scattered light is white 

 and much fainter. The nitroso absorbs the blue end of the 

 spectrum. The fog is best examined through the open 

 mouth of the tube, the arc-light being focussed at the centre. 

 Under certain conditions, these nitroso fogs showed brilliant 



