They do it with mirrors

From Arnaud Maillet, The Claude Glass: Use and Meaning of the Black Mirror in Western Art:

This lack of knowledge [of the Claude mirror] is sometimes compounded by prohibitions that prevent these objects from being exhibited. For example, the Musee National des Arts et Traditions Populaires in Paris possesses a mirror of about 28 centimeters in diameter, with a slight outward curvature and a black backing, designed to be suspended from hooks. Its last owner, a Parisian sculptor, had hung it in his dining room. A complicated series of events came about: The artist got rid of this magic mirror by donating it to the museum in the early 1980s. A magnetizer who had come to examine it inserted some bits of paper inscribed with signs (for example, Solomon's seal) between the backing and the glass and recommended that it be kept in charcoal, which is reputed to absorb evil forces. This mirror is therefore not exhibited, since someone who knows how to cast spells would be able to use it, even through a glass case. In accordance with these magical recommendations, it is stored in a large case designed for black magic. The guards are afraid of it, and anyone who looks inside feels a great unease. Despite its ethnographic value, and for the reasons just mentioned, this mirror cannot be lent out, since it has no inventory number: thus it "doesn't exist"!