122 FEAGMENTS OF SCIENCE. 



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 the line of vision perpendicular, the windows 

 were opened, the blinds remaining undrawn. The resi- 

 nous 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. The 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. 1 



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 

 the ring-system observed in the common air be brought 

 to its maximum strength, and then let an attenuated 



1 Brewster has proved the variability of the position of tho 

 neutral point for skylight with the sun's altitude, a result obviously 

 connected with the foregoing experiments. 



