124 FRAGMENTS OF SCIENCE. 



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 dis- 

 tilled water, for example, a thick slice of light is neces- 

 sary to make the polarisation 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 effect with most 

 other liquids, suffices to bring out vividly the selenite 

 colours. 



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 

 distinguished from surrounding space by its excess or 

 defect of light in relation to that space. By altering 

 the illumination, 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 polarised, 

 may, as the reader of the foregoing pages knows, be in 

 great part quenched by a Nicol's prism, while the light 

 of a common cloud, being unpolarised, cannot be thus 

 extinguished. Hence the possibility of very remarkable 

 variations, not only in the aspect of the firmament, 

 which is really changed, but also in the aspect of the 

 clouds, which have that firmament as a background. 

 It is possible, for example, to choose clouds of such a 

 depth of shade that when the Nicol quenches the light 

 behind them, they shall vanish, being undistinguishable 



