ARTIFICIAL SKY 129 



site sides of it, was also exhibited by tte 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 per- 

 mitted to escape. On again darkening the room and turn- 

 ing on the light, 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 between a line drawn from the eye 

 to the neutral point and the axis of the beam falling suc- 

 cessively 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: 



1st obse 



2d 



3d 



4th 



5th 



6th 



vation . • • , . 2 feet 2 inches 



* 2 " 6 " 



• 2 " 10 " 



3 *• 2 " 



3 " 7 " 



4 " 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 gun- 

 powder. In five successive experiments, corresponding to 



