CHEMICAL REACTIONS. 93 



to white, and the rings themselves to emit their comple- 

 mentary colors.* 



Almost all liquids have motes in them sufficiently 

 numerous to polarize sensibly the light, and very beautiful 

 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 polarized light is discharged, 

 and scarcely any color 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 polarized light; 

 and if the selenite be employed, vivid colors flash into ex- 

 istence. A still more brilliant result is obtained with 

 mastic dissolved in a great excess of alcohol. 



The selenite rings, in fact, constitute an extremely 

 delicate test as to the collective quantity of individually in- 

 visible particles in a liquid. Commencing with distilled 

 water, for example, a thick slice of light is necessary to 

 make the polarization of its suspended particles sensible. 

 A much thinner slice suffices for common water; while, 

 with Briicke's precipitated mastic, a slice too thin to 

 produce any sensible eifect with most other liquids, suffices 

 to bring out vividly the selenite colors. 



3. THE SKY OF THE ALPS. 



The vision of an object always implies a differential 

 action on the retina of the observer. The object is dis- 

 tinguished from surrounding space by its excess or defect 

 of light in relation to that space. By altering the illumi- 

 nation, either of the object itself or of its environment, we 

 alter the appearance of the object. Take the case of clouds 

 floating in the atmosphere with patches of blue between 

 them. Anything that changes the illumination of either 

 alters the appearance of both, that appearance depending, 

 as stated, upon differential action. Now the light of the 

 sky, being polarized, may, as the reader of the foregoing 



* Sir John Herscliel suggested to me that this change of the polar- 

 ization from positive to negative may indicate a change from polar- 

 ization by reflection to polarization by refraction. This thought re- 

 peatedly occurred to me while looking at the effects; but it will 

 require much following up before it emerges into clearness. 



