AETIEICIAL SKY. 121 



A neutral point, with complementary effects at oppo- 

 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 

 turning 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 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 : 



1st observation 



2nd 



3rd 



4th 



6th 



6th 



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 polarisation had again become normal to 

 the beam. 



The laboratory was next filled with the fumes of 

 gunpowder. In five successive experiments, correspond- 

 ing to five different densities of the gunpowder-smoke, 

 the angles enclosed between the line of vision to the 



