USE OF THE IMAGINATION. 429 



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 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 m;iy 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 Liverpool, milk makes 

 an approximation to the same color, through the operation 

 of the same cause; and Helm hoi tz 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 firmament 

 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 

 discern the real nature of the action of small particles on 

 the waves of ether, * describes a glass of a similar kind. \ 

 Capital specimens of such glass are to be found at Sal- 

 viati's, in St. James' Street. What artists call " chill " 

 is no doubt an effect of this description. Through the 

 action of minute particles, the browns of a picture often 

 present the appearance of the bloom of a plum. By rub- 

 bing the varnish with a silk handkerchief optical continuity 



* This is inferred from conversation. I am not aware that Pro- 

 fessor Stokes has published anything upon the subject. 



f 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 Rosskastanie, man stecke denselben in cin 

 Glas Wasser, und in der kiirzesten Zeit werden wir das vollkom- 

 menste Hinimelblau entstehen sehen." Goethe's Werke, B. xxix. 

 p. 24. 



