SCIENTIFIC USE OF THE IMAGINATION. 143 



Water will not dissolve resin, but spirit will; and when 

 spirit which holds resin in solution is dropped into water, 

 the resin immediately separates in solid particles, which 

 render the water milky. The coarseness of this precipitate 

 depends on the quantity of the dissolved resin. You can 

 cause it to separate in thick clots or in exceedingly fine 

 particles. Professor Briicke has given us the proportions 

 which produce particles particularly suited to our present 

 purpose. One gramme of clean mastic is dissolved in 

 eighty-seven grammes of absolute alcohol, and the trans- 

 parent solution is allowed to drop into a beaker containing 

 clear water kept briskly stirred. An exceedingly fine 

 precipitate is thus formed, which declares its presence by 

 its action upon light. Placing a dark surface behind the 

 beaker, and permitting the light to fall into it from the top 

 or front, the medium is seen to be distinctly blue. It is not 

 perhaps so perfect a blue as I have seen on exceptional 

 days, this year, among the Alps, but it is a very fair sky- 

 blue. A trace of soap in water gives a tint of blue. Lon- 

 don, and I fear Liverpool milk, makes an approximation to 

 the same color through the operation of the same cause ; 

 and Helmholtz has irreverently disclosed the fact that the 

 deepest blue eye is simply a turbid medium. 



The action of turbid media upon light was illustrated 

 by Goethe, who, though unacquainted with the modula- 

 tory theory, was led by his experiments to regard the 

 firmament as an illuminated turbid medium with the dark- 

 ness of space behind it. He describes glasses showing a 

 bright yellow by transmitted, and a beautiful blue by re- 

 flected light. Professor Stokes, who was probably the first 

 to discern the real nature of the action of small' particles 

 on the waves of ether, describes a glass of a similar kind. 1 



1 This glass, by reflected light, had a color " strongly resembling that 

 of a decoction of a horse-chestnut bark." Curiously enough, Goethe 

 refers to this very decoction: "Man nelime einen Streifen frisclier Rinde 



