130 FRAGMENTS OF SCIENCE 



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 very irritating to the lungs. 

 The direction of maximum polarization enclosed, in this 

 case, an angle of 12, or thereabout, with the axis of the 

 beam. Looked at, as in the former instances, from a posi- 

 tion near the electric lamp, no neutral point was observed 

 throughout the entire extent of the beam. 



When this beam was looked at normally through the 

 selenite and Nicol, the ring- system, though not brilliant, 

 was distinct. Keeping the eye upon the plate of selenite, 

 and the line of vision perpendicular, the windows were 

 opened, the blinds remaining undrawn. The resinous 

 fumes slowly diminished, and as they did so the ring- 

 system became paler. It finally disappeared. Continuing 

 to look in the same direction, the rings revived, but now 

 the colors were complementary to the former ones. The 

 neutral point had passed me in its motion down the beam, 

 consequent upon the attenuation of the fumes of resin. 



With the fumes of chloride of ammonium substantially 

 the same results were obtained. Sufficient, however, has 

 been here stated to illustrate* the variability of the position 

 of the neutral point. * 



By a puff of tobacco- smoke, or of condensed steam, 



1 Brewster has proved the variability of the position of the neutral point for 

 skylight with the sun's altitude, a result obviously connected with the foregoing 

 experiments. 



