92 FEA GMENTS OF SCIENCE. 



position near the electric lamp, no neutral point was ob- 

 served throughout the entire extent of the beam. 



When this beam was looked at normally through the 

 selenite and Niool, 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. Tlie 

 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, blown 

 into the illuminated beam, the brilliancy of the selenite 

 colors may be greatly enhanced. But with different clouds 

 two different effects are produced. Let the ring-system ob- 

 served in the common air be brought to its maximum 

 strength, and then let an attenuated cloud of chloride of 

 ammonium be thrown into the beam at the point looked at; 

 the ring system flashes out with augmented brilliancy, but 

 the character of the polarization remains unchanged. 

 This is also the case when phosphorus or sulphur is burned 

 underneath the beam, so as to cause the fine particles of phos- 

 phorus or of sulphur to rise into the light. With the 

 sulphur-fumes the brilliancy of the colors is exceedingly 

 intensified; but in none of these cases is there any change 

 in the character of the polarization. 



But when a puff of the fumes of hydrochloric acid, 

 hydriodic acid, or nitric acid is thrown into the beam, 

 there is a complete reversal of the selenite tints. Each of 

 these clouds twists the plane of polarization 90 degrees, 

 causing the center of the ring-system to change from black 



* 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. 



