of the more visually spectacular types of slides prepared in
Victorian times were the preparations of crystallized chemical
compounds for viewing between crossed polarizing filters (Polariscope).
A large selection of these slides were produced by all the
commercial preparers, and they still amaze and entertain with their instant
transformation when viewing, from drab and colourless, into a kaleidoscope of
colour and form with the rotation of the polar filter. The addition
of a thin Selenite crystal in the light path between the polarizing
filters, enhances the available colour variations.