ARTIFICIAL SKY' 127 



Nicol and selenite more and more obliquely at the beam, 

 observed the colors fading until they disappeared. Aug- 

 menting the obliquity the colors appeared once more, but 

 they were now complementary to the former ones. 



Hence this beam, like the sky, exhibited a neutral 

 point, on opposite sides of which the light was polarized 

 in planes at right angles to each other. 



Thinking that the action observed in the laboratory 

 might be caused, in some way, by the vaporous fumes 

 diffused in its air, I had the light removed to a room at 

 the top of the Royal Institution. The track of the beam 

 was seen very finely in the air of this room, a length of 

 fourteen or fifteen feet being attainable. This beam ex- 

 hibited all the effects observed with the beam in the labo- 

 ratory. Even the uncondensed electric light falling on 

 the floating matter showed, though faintly, the effects 

 of polarization. 



When the air was so sifted as to entirely remove the 

 visible floating matter, it no longer exerted any sensible 

 action upon the light, but behaved like a vacuum. The 

 light is scattered and polarized by particles, not by mole- 

 cules or atoms. 



By operating upon the fumes of chloride of ammo- 

 nium, the smoke of brown paper, and tobacco-smoke, I had 

 varied and confirmed in many ways those experiments on 

 neutral points, when my attention was drawn by Sir 

 Charles Wheatstone to an important observation com- 

 municated to the Paris Academy in 1860 by Professor 

 Govi, of Turin. 1 M. Govi had been led to examine a 

 beam of light sent through a room in which were succes- 



1 "Comptes Rendus," tome li. pp. 360 and 669. 



