SCIENTIFIC USE OF THE IMAGINATION. 117 



through the operation of the same cause ; and Helm- 

 holtz 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 Groethe, who, though unacquainted with the undu- 

 latory 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, 1 describes a 

 glass of a similar kind. 2 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 appearance 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 turbid water 

 of the Visp. When kept still for a day or so, the 

 grosser matter sank, but the finer particles remained 

 suspended, and gave a distinctly blue tinge to the 

 water. The blueness of certain Alpine lakes has 

 been shown to be in part due to this cause. Professor 



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

 Professor Stokes has published anything upon the subject. 



2 This glass, by reflected light, had a colour 'strongly resembling 

 that of a decocti-on 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 ein 

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

 menste Himmelblau entstehen sehen.' Goethe's Werke, B. xxix. 

 p. 24. 



