the reflectors, therefore, we increase also the quantity of light; but, unfortunately, this increase of length is unfavourable to the other properties of the instrument; for we must now use a lens L L of a great focal length, which will render it necessary to receive the image at a great distance from the instrument. In order to render the effect as brilliant as possible, the inclination of the reflectors should, in the present case, never exceed 30°, and might be even 36° or 45°. The objects should be selected as thin as possible, and none with dark tints ought to be admitted into the objectboxes. CHAPTER XVI. 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 doublyrefracting 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. 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 objectbox, 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. 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. 1 See my Treatise on Optics, Edit. 1853, Chaps. xxviii. xxix. 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. When two such systems of rings are formed by breathing through a 1 Treatise on Optics, p. 120. 2 |