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The liquid here employed is the iodide of allyl, but I
might choose any one of a dozen substances here before
me to produce the effect. You have seen what may be
done with the nitrite of butyl. With nitrite of amyl, bisul—
phide of carbon, benzol, benzoic ether, etc., the same blue
color may be produced; In all cases, where matter slowly
passes from the molecular to the massive state the transi‘
tion is marked by the production of the blue. More than
this : you have seen me looking at the blue color (I hardly
like to call it a blue “ cloud,” its texture and properties are
so different from ordinary clouds) through this bit of spar.
This is a Nicol’s prism, and it is to be wished that one of
them could be placed in the hands of each of you. Now,
this blue that I have been regarding turns out to be, if the
expression be allowed, a bit of more perfect sky than the
sky itself. On looking across the illuminating beam as we
look across the solar rays in the atmosphere, we obtain not
only partial polarization, but perfect polarization. In one
position of the Nicol the blue light passes freely to the eye;
in the other it is absolutely cut off, the experimental tube
being reduced to optical emptiness. It is well to place a
black surface behind the experimental tube, so as to prevent
foreign light from troubling the eye. In one position of
the prism this black surface is seen without softening or
qualification; for the particles within the tube are them-
selves invisible, and the light which they scatter is quenched.
If the light of the sky were polarized with the same per-
fection, on looking properly toward it through a Nicol we
should meet, not the mild radiance of the firmament, but
the unillumined blackness of space.

The construction of a N icol’s prism is such that it
allows the passage of Vibrations Which are executed in a
certain determinate direction, and these only. All vibra-
tions executed at right angles to this direction are com-
pletely stopped: while components only of those executed

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