﻿Colour of Light scattered by Small Absorbing Particles. 149 



colours which were not due to absorption. If the test-tube 

 is fastened at an angle of 45° in a clamp-stand, and the light 

 focussed about an inch below the open mouth, the fog will 

 be found to whirl about in a vortex ; and if it is examined 

 with a nicol, the stream lines will often be found to show 

 brilliant colours. One stream will be bright red and the 

 adjacent stream deep green, and the colours appear to change 

 to their complementary tints when the nicol is rotated through 

 90° ; though it is difficult to be sure of this effect, owing to 

 the rapid changes which take place in the distribution of 

 the colours. 



These changes reminded me of some curious effects which 

 I have observed with thin films of collodion deposited on silver 

 surfaces *. When a powerful beam of unpolarized white light is 

 concentrated normally upon the film, and the light scattered at 

 grazing emission is examined with a nicol, it was found that 

 one-half of the visible spectrum was polarized vertically, i. <?., 

 parallel to the surface, the other half horizontally. The com- 

 plete explanation of this very singular phenomenon has not 

 yet been found, though it was pretty definitely established 

 that it was due to the frilling of the film, the granular 

 structure being just resolvable with the highest powers of 

 the microscope. 



The diameter of the particles in the nitroso fogs averaged 

 about "003 mm., as was determined by inverting the tube 

 over a glass plate, allowing the particles to settle, and examin- 

 ing the deposit with a microscope. The particles are much 

 larger than the ones treated by Lord Hayleigh, which are too 

 small to regularly refract or reflect the incident light, and 

 which operate in a wholly different manner. 



Somewhat similar clouds can be formed by shaking up 

 some of the finely powdered aniline dyes in a test-tube, the 

 finest particles remaining suspended in the air. Soluble 

 prussian-blue makes a good cloud, the scattered light being 

 deep blue when examined with a nicol with its short diagonal 

 parallel to the incident rays, and white under the reversed 

 conditions. The polarization effects are not as marked, 

 however. 



* "On some New Cases of Interference and Diffraction/' Phil. Ma; 

 vii. p. 376 (1904). 



