SCIENTIFIC USE OF THE IMAGINATION. H7 



is seen to be distinctly blue. It is not perhaps so per- 

 fect 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 

 Liverpool, milk makes an approximation to the same 

 colour, 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 illus- 

 trated 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 beauti- 

 ful 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,* de- 

 scribes a glass of a similar kind.f Capital specimens of 

 such glass are to be found at Salviati's, in St. James's 

 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 ap- 

 pearance of the bloom of a plum. By rubbing the 

 varnish with a silk handkerchief optical continuity is 

 established and the chill disappears. Some years ago I 

 witnessed Mr. Hirst experimenting at Zermatt on the 



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

 Professor Stokes has published anything 1 upon the subject. 



f This glass, by reflected light, had a colour 'strongly resem- 

 bling that of a decoction of horse-chestnut bark.' Curiously 

 enough, Goethe refers to this very decoction : 'Man nehme einen 

 Streifen frischer Rinde von dcr Rosskastanie, man stecke densel- 

 ben in ein Glas Wasser, und in der kUrzesten Zeit werden wir das 

 vollkommcnste Ilimmelblau entstehen sehen.' Goethe's Werke, 

 B. xxix. p. 24. 



