ARTIFICIAL SKY 129 



site 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 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 83. 



The distances, roughly measured, of the neutral point 

 from the lamp, corresponding to the foregoing series of 

 observations, were these: 



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 



