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 rn;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 J 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 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, f 
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 nehtne einen Streifen 
frischer Rinde von der Rosskastanie, man stecke denselben in ein 
Glas Wasser, und in der kiirzesten Zeit werden wir das vollkom- 
menste Hiuiiuelblau entstehen sehen." Goethe's Werke, B, 
p. 24- 
