132 FRAGMENTS OF SCIENCE 



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 par- 

 ticles reach the light the liquid sends forth laterally al« 

 most perfectly polarized light; and if the selenite be em- 

 ployed, vivid colors flash into existence. 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 

 invisible 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 pro- 

 duce any sensible effect with most other liquids, suffices 

 to bring out vividly the selenite colors. 



§ 8. The Sky op 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 illu- 

 mination, 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 



