ARTIFICIAL SKY. 119 



throughout the entire beam, when the line of vision 

 was perpendicular to its length. 



The horizontal column of air, thus illuminated, was 

 18 feet long, and could therefore be looked at very ob- 

 liquely. I placed myself near the end of the beam, as 

 it issued from the electric lamp, and, looking through 

 the Nicol and selenite more and more obliquely at the 

 beam, observed the colours fading until they disappeared. 

 Augmenting the obliquity the colours 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 polarised 

 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 Eoyal Institution. The track of the 

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

 length of 14 or 15 feet being attainable. This beam 

 exhibited all the effects observed with the beam in the 

 laboratory. Even the uncondensed electric light falling 

 on the floating matter showed, though faintly, the effects 

 of polarisation. 



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 polarised by particles, not by 

 molecules 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 experi- 

 ments on neutral points, when my attention was drawn 

 by Sir Charles Wheatstone to an important observation 

 communicated to the Paris Academy in 1860 by Pro- 



