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 aether,
[Footnote: This is inferred from conversation. I am not aware that
Professor Stokes has published anything upon the subject.] describes
a glass of a similar kind. [Footnote: This glass, by reflected light,
had a colour 'strongly resembling that of a decoction of
horse-chestnut bark.' Curiously enough, Goethe refers to this very
decoction: 'Man nehme einen Streifen frischer Rinds von der
Rosskastanie, man stecke denselben in ein Glas Wasser, und in der
kuerzesten Zeit werden wir das vollkommenste Himmelblau entstehen
sehen.'--Goethe's Werke, B. xxix. p. 24.]

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 Roscoe has noticed several striking
cases of a similar kind. In a very remarkable paper the late
Principal Forbes showed that steam issuing from the safety-valve of a
locomotive, when favourably observed, exhibits at a certain stage of
its condensation the colours of the sky. It is blue by reflected
light, and orange or red by transmitted light. The same effect, as
pointed out by Goethe, is to some extent exhibited by peat-smoke. More
than ten years ago, I amused myself by observing, on a calm day at
Killarney, the straight smoke-columns rising from the cabin-chimneys.
It was easy to project the lower portion of a column against a dark
pine, and its upper portion against a bright cloud. The smoke in the
former case was blue, being seen mainly by reflected light; in the
lat