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.