The Asphyx (aka The Horror of Death, Spirits of the Dead)

A philanthropist and amateur psychic researcher embarks on a fateful quest for immortality. Hugo Cunningham (Robert Stephens) has a morbid hobby of taking photographs of dying people, and this leads to his discovery of a nebulous spirit of the dead-known in mythology as the Asphyx-that appears (only visible on photographic plates) at the moment of death. Hugo becomes obsessed with capturing his own Asphyx and thus ensuring that he cannot die, but of course this is an ill-fated ambition that puts him on a ruinous path to destruction and death.