Crystal balls (often made of glass), crystals, cut glass, mirrors, magick mirrors (black glass), swinging pendants, crystal bowls, painted eyes, icons and images painted in flashing colors (complimentary colors placed together), spinning disks, and other devices that focus the attention but tire the vision. Sometimes a small focal object is placed above the eye level forcing one to soon close the eyes. Combined with either a hetero or an internal dialogue, their used can be very effective in hypnotic induction.
Andrews: Crystal Balls & Crystal Bowls, Tools for Ancient Scrying & Modern Seership
Cunningham: Divination for Beginners, Reading the Past, Present & Future