SCIENTIFIC USE OF THE IMAGINATION 125 



gram of clean mastic is dissolved in eighty-seven grams 

 of absolute alcohol, and the transparent 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 may be seen on exceptional days among the 

 Alps, but it is a very fair sky-blue. A trace of soap in 

 water gives a tint of blue. London, and I fear Liver- 

 pool, 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 undulatory 

 theory, was led by his experiments to regard the firma- 

 ment as an illuminated turbid medium, with the darkness 

 of space behind it. He describes glasses showing a bright 

 yellow by transmitted, and a beautiful blue by reflected, 

 light. Professor Stokes, who was probably the first to dis- 

 cern the real nature of the action of small particles on the 

 waves of ether, ^ describes a glass of a similar kind.^ Capi- 



^ This is inferred from conversation. I am not aware that Professor Stokes 

 lias published anything upon the subject. 



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

 decoction of horse-chestnut bark." Curiously enough, Goethe refers to this 

 very decoction : "Man nehme einen Streifen frischer Rinde von der Rosska- 

 stanie, man stecke denselben in ein Glas Wasser, und in der kiirzesten Zeit 

 werden wir das voUkommenste Himmelblau entstehen sehen." — Goethe's 

 "Werke," B. xxix. p. 24. 



