For example, iron-gall ink, commonly used on ancient documents, is transparent to infra-red light and most visible in the ultraviolet region of the spectrum.

ECONOMIST: Monitor

It slipped enlarged photographs of the latent prints and the prints from Mr Mitchell into transparent plastic sleeves, and marked red dots on the sleeves to suggest which of Mr Mitchell's prints matched the latent ones and where.

ECONOMIST: Fingering fingerprints

Though the carbon nanotubes absorb infra-red light, they are almost totally transparent to the visible variety.

ECONOMIST: Monitor