122 FRAGMENTS OF SCIENCE. 



spending 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 very irritating to the 

 lungs. The direction of maximum polarisation enclosed, 

 in this case, an angle of 12, or thereabouts, with the axis 

 of the beam. Looked at, as in the former instances, 

 from a position 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 sele- 

 nite, and tke 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 re- 

 vived, but now the colours were complementary to the 

 former ones. TJie neutral point had passed me in its 

 motion down the beam, consequent upon the attenua- 

 tion of the fumes of resin. 



With the fumes of chloride of ammonium substan- 

 tially the same results were obtained. Sufficient, how- 

 ever, 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, 

 blown into the illuminated beam, the brilliancy of the 

 selenite colours may be greatly enhanced. But with 

 different clouds two different effects are produced. Let 



* Brewster has proved the variability of the position of the 

 neutral point for sky light with the sun's altitude, a result obvi- 

 ously connected with the foregoing experiments. 



