﻿of the Sky, and on the Polarization of Light. 391 



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 

 experimental tube became denser, the polarization of the light dis- 

 charged at right angles to the beam became weaker, the direction of 

 maximum polarization becoming oblique to the beam. Experiments 

 on the fumes of chloride of ammonium gave me also reason to sus- 

 pect 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 

 remarkable results : — The laboratory being well filled with the fumes 

 of incense, and sufficient time being allowed for their uniform diffu- 

 sion, the electric beam was sent through the smoke. From the track 

 of the beam polarized light was discharged, but the direction cf 

 maximum polarization, instead of being along the normal, now en- 

 closed 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°. 



The windows of the laboratory were now opened for some minutes, 

 a portion of the incense smoke being permitted to escape. On again 

 darkening the room and turning on the beam, the line of vision to 

 the neutral point was found to enclose with the axis of the beam an 

 angle of 63°. 



The windows were again opened for a few minutes, more of the 

 smoke being permitted to escape. Measured as before, the angle 

 referred to was found to be 54°. 



This process was repeated three additional times ; the neutral point 

 was found to recede lower and lower down the beam, the angle be- 

 tween a line drawn from the eye to the neutral point and the axis 

 of the beam falling successively from 54° to 49°, 43°, and 33°. 



The distances, roughly measured, of the neutral point from the 

 lamp, corresponding to the foregoing series of observations, were 

 these : — 



1 st observation, 2 feet 2 inches. 



2nd 



>> 



2 



„ 6 , 



3rd 



a 



2 



„ 10 „ 



4th 



a 



3 



„ 2 , 



5th 



)) 



3 



„ 7 „ 



6th 



>) 



4 



,y 6 „ 



At the end of this series of experiments the direction of maximum 

 polarization had again become normal to the beam. 



The laboratory was next filled with the fumes of gunpowder. In 

 five successive experiments, corresponding to five different densities 

 of the gunpowder-smoke, the angles enclosed between the line of 

 vision to the neutral point and the axis of the beam were 63°, 50°, 47°, 

 42°, and 38° respectively. 



After the clouds of gunpowder had cleared away, the laboratory was 

 filled with the fumes of common resin, rendered so dense as to be 



