nd that when the short diagonal of
the prism was vertical, the quantity of light reaching the eye was
greater than when the long diagonal was vertical. When a plate of
tourmaline was held between the eye and the bluish cloud, the quantity
of light reaching the eye when the axis of the prism was perpendicular
to the axis of the illuminating beam, was greater than when the axes
of the crystal and of the beam were parallel to each other.

This was the result all round the experimental tube. Causing the
crystal of tourmaline to revolve round the tube, with its axis
perpendicular to the illuminating beam, the quantity of light that
reached the eye was in all its positions a maximum. When the
crystallographic axis was parallel to the axis of the beam, the
quantity of light transmitted by the crystal was a minimum.

From the illuminated bluish cloud, therefore, polarised light was
discharged, the direction of maximum polarisation being at right
angles to the illuminating beam; the plane of vibration of the
polarised light was perpendicular to the beam. [Footnote: This is
still an undecided point; but the probabilities are so much in its
favour, and it is in my opinion so much preferable to have a physical
image on which the mind can rest, that I do not hesitate to employ the
phraseology in the text.]

Thin plates of selenite or of quartz, placed between the Nicol and the
actinic cloud, displayed the colours of polarised light, these colours
being most vivid when the line of vision was at right angles to the
experimental tube. The plate of selenite usually employed was a
circle, thinnest at the centre, and augmenting uniformly in thickness
from the centre outwards. When placed in its proper position between
the Nicol and the cloud, it exhibited a system of splendidly-coloured
rings.

The cloud here referred to was the first operated upon in the manner
described. It may, however, be greatly improved upon by the choice of
proper substances, and by the application, in proper quantities, of
the substances chosen. Benzol, bisulphide of carbon, nitrite of amyl,
nitrite of butyl, iodide of allyl, iodide of isopropyl, and many other
substances may be employed. I will take the nitrite of butyl as
illustrative of the means adopted to secure the best result, with
reference to the present question.

And here it may be mentioned that a vapour, which when alone, or mixed
with air in the experimental tube, resists the action of