ARTIFICIAL SKY. ]23 



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 augmented brilliancy, but the character of the 

 polarisation remains unchanged. This is also the case 

 when phosphorus, 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 brilliancy of the colours is exceedingly inten- 

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

 the character of the polarisation. 



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 

 complementary colours.* 



Almost all liquids have motes in them sufficiently 

 numerous to polarise 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 permitted to pass through it, scarcely any 

 polarised light is discharged, and scarcely any colour 

 produced with a plate of selenite. But if a bit of soap 

 be agitated in the water above the beam, the moment 

 the infinitesimal particles reach the light the liquid 

 sends forth laterally almost perfectly polarised light; 



* Sir John Herschel suggested to me that this change of the 

 polarisation from positive to negative may indicate a change 

 from polarisation by reflection to polarisation by refraction. 

 This thought repeatedly occurred to me while looking at the 

 effects ; but it will require much following up before it emerges 

 into clearness. 



