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 I have seen on 

 exceptional days, this year, 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 Helmholtz has irreverently dis- 

 closed the fact that a blue eye is simply a turbid medium. 



Numerous instances of the kind might be cited. The 

 action of turbid media upon light was fully and beauti- 

 fully illustrated by Goethe, who, though unacquainted 

 with the undulatory theory, was led by his experiments 

 to regard the blue of the firmament as caused by 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. 

 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, which was 

 charged with the finely divided matter ground down by 

 the glaciers. When kept still for a day or so the grosser 

 matter sank, but the finer matter remained suspended, 

 and gave a distinctly blue tinge to the water. No doubt 



