the slide is constructed of mahogany with an image of a swan between 2 polarizing film filters, the handle rotates one of the filter to reveal the effect of changing colour, the swan image is made up of selenite crystals cut to cut to reveal the colour change under polarised light, a delicate and skilled job that only a few tradesmen were capable of producing, 18cm x 10cm Footnote: the vendor says 'Many University students were able to observe their lectures demonstrating the basic scientific principles with the use of the magic lantern. Some of these experiments were modified to be used in public entertainment lantern shows. The polarizing of light was a good example of a pure physics experiment adapted to produce a puzzling optical illusion as a slide of a white swan becomes a black swan as if by magic.'