1869.] 



and on the Polarization of Light. 



229 



some way by the vaporous fumes diffused in its air, I had a battery arid an 

 electric lamp carried 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 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 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. 



I had varied and confirmed in many ways those experiments on neutral 

 points, operating upon the fumes of chloride of ammonium, the smoke of 

 brown paper, and tobacco smoke, when my attention was drawn by Sir 

 Charles Wheatstone to an important observation communicated to the 

 Paris Academy in 1860 by Professor Govi, of Turinf . His observations 

 on the light of comets had led M. Govi to examine a beam of light sent 

 through a room in which was diffused the smoke of incense. He also ope- 

 rated on tobacco smoke. His first brief communication stated the fact of 

 polarization by such smoke, but in his second communication he announced 

 the discovery of a neutral point in the beam, at the opposite sides of which 

 the light was polarized in planes at right angles to each other. 



But unlike my observations on the laboratory air, and unlike the action 

 of the sky, the direction of maximum polarization in M. Govi's experi- 

 ment enclosed a very small angle with the axis of the illuminating beam. 

 The question was left in this condition, and I am not aware that M. Govi 

 or any other investigator has pursued it further. 



I had noticed, as before stated, that as the clouds formed in the experi- 

 mental tube became denser, the polarization of the light discharged at 

 right angles to the beam became weaker, the direction of maximum pola- 

 rization becoming oblique to the beam. Experiments on the fumes of 

 chloride of ammonium gave me also reason to suspect that the position of 

 the neutral point was not constant, but that it varied with the density of 

 the illuminated fumes. 



The examination of these questions led to the following new and re- 

 markable results : — the laboratory being well filled with the fumes of in- 

 cense, and sufficient time being allowed for their uniform diffusion, the 

 electric beam was sent through the smoke. From the track of the beam 

 polarized light was discharged, but the direction of maximum polarization, 

 instead of being along the normal, now enclosed an angle of 12° or 13° with 

 the axis of the beam. 



A neutral point, with complementary effects at opposite sides of it, was 

 also exhibited by the beam. The angle enclosed by the axis of the beam, 

 and a line drawn from the neutral point to the observer's eye, measured in 

 the first instance 66°. 



* I hope to try Alpine air next summer, 

 t Coinptes Bendus, tome li. pp. 360 & 669. 



