ARTIFICIAL SKY 131 



blown into the illuminated beam, the brilliancy of the 

 selenite colors may be greatly enhanced. But with differ- 

 ent 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 cloud of 

 chloride of ammonium be thrown into the beam at the 

 point looked at ; the ring-system flashes out with aug- 

 mented brilliancy, but the character of the polarization 

 remains unchanged. This is also the case when phos- 

 phorus, or sulphur, is burned underneath the beam, so 

 as to cause the fine particles of phosphorus or of sulphur 

 to rise into the light. "With the sulphur-fumes the bril- 

 liancy 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, causing 

 the centre of the ring-system to change from black to 

 white, and the rings themselves to emit their comple- 

 mentary colors. 1 



Almost all liquids have motes in them sufficiently 

 numerous to polarize sensibly the light, and very beau- 

 tiful effects may be obtained by simple artificial devices. 

 When, for example, a cell of distilled water is placed in 

 front of the electric lamp, and a thin slice of the beam is 



1 Sir John Herschel suggested to me that this change of the polarization from 

 positive to negative may indicate a change from polarization by reflection to 

 polarization by refraction. This thought repeatedly occurred to me while look- 

 ing at the effects ; but it will require much following up before it emerges into 

 clearness. 



