ON THE CONSTRUCTION OF KALEIDOSCOPES WHICH COMBINE THE
COLOURS AND FORMS PRODUCED BY POLARIZED LIGHT.
In the preceding chapters we have supposed that the objects are
illuminated by common light, and that the forms which compose the
symmetrical figure are those of material bodies. If we employ polarized
light we may introduce into the Kaleidoscopic figures, the splendid
colours produced by crystallized bodies, and also the forms which
these colours assume, round the optical axes of crystals, or through
different thicknesses of the doubly-refracting substance. The part of
the polarizing apparatus which polarizes the light, may be a large
Nicol’s prism, or a bundle of thin glass plates, or a single plate of
black glass, fixed at the object end of the Kaleidoscope. The analysing
part of the apparatus may be a Nicol’s prism, or plates of the sulphate
of iodo-quinine, discovered by Dr. William Herapath, of Bristol.
Owing to the thickness of a Nicol’s prism, it is not well fitted for
the analyser, as it prevents the eye of the observer from getting
sufficiently near the small eye-hole of the Kaleidoscope. The plates
of the sulphate of the iodo-quinine, are therefore peculiarly adapted
for analysers; and when they can be obtained of the same size as the
angular aperture of the Kaleidoscope, with fixed reflectors, or of the
whole circular aperture when the reflectors are movable, they will also
form the best polarizers.
The crystals which are to give the colours and forms produced by
polarized light and its subsequent analysis, may be either _uniaxal_
crystals, such as calcareous spar, or quartz, or beryl, or _biaxal_
crystals, such as selenite, topaz, mica, arragonite, nitre, etc.[9]
These crystals must be placed at the end of the reflectors, and when
they transmit polarized light, their brilliant colours and forms will
vary by turning the cell which contains them, or by giving a motion
of rotation to the analyser. Thin films, or laminæ of selenite of
different thicknesses, and generally of such a thickness as gives the
bright rings of the _second_ order of colours in Newton’s scale, may
be placed in a narrow cell or object-box, and may have their outlines
of various curvatures, so as to combine both form and colour in the
Kaleidoscopic figure. Different forms may also be obtained by using
pieces of colourless glass of different shapes, or pieces of thin wire
bent into a variety of curves. If the outlines of the pattern are to be
obtained from pieces of glass or wire, the films of selenite might be
cemented to one of the glass plates of the object-box, so as to have
their axes lying in different directions.
[9] See my _Treatise on Optics_, Edit. 1853, Chaps. XXVIII, XXIX.
The coloured figures produced by glass quickly cooled, might also
be advantageously employed, and, likewise, the remarkable forms of
circular crystals and crystalline groups, when they are sufficiently
large to be seen by the naked eye.
Very beautiful objects may be made by cementing a plate of sulphate of
lime (_selenite_), 0.01818 of an inch thick, to a plate of glass, and
cutting out, upon a turning-lathe, grooves or bands of such different
depths, as to give different colours by polarized light. Beautiful
patterns may be thus executed in bands or lines of different colours;
but for the purposes of the Kaleidoscope, it will be sufficient to have
curves or portions of curves of such different forms and curvatures, as
will produce agreeable figures by their combination. These curves and
irregular forms of any kind, may be scratched or excavated in the plate
of selenite by the point of a sharp knife, and afterwards polished, or
even deepened, by the action of water, which has the property of slowly
dissolving the selenite.
In order to observe the effects produced by polarized light, it is
not necessary to have the polarizing and analysing apparatus attached
to the Kaleidoscope. The polarizer, in the form of a plate of black
glass, or a bundle of plates of common window-glass, may be laid on
the table so as to reflect the light into the Kaleidoscope at an angle
of 56°, and the analyser may be held in one hand, and the Kaleidoscope
in the other. In this case, however, it would be better to have the
Kaleidoscope fixed, in order that the observer may have the use of his
left hand, to turn the object-box, which contains the doubly-refracting
crystal.
Without the use of polarized light, very fine forms, and these
splendidly coloured, may be obtained by means of the coloured rings
exhibited in the _Iriscope_.[10] When two such systems of rings are
formed by breathing through a tube upon black glass, over which soap
has been rubbed and subsequently wiped off, by a piece of chamois
leather, they form a double system, similar to the biaxial system in
mica and topaz, with curves of various shapes, which exhibit beautiful
combinations in the Kaleidoscope.
[10] _Treatise on Optics_, p. 120.