It's sort of hard to believe that this set up had that strong an effect. It's also hard to believe that it has much relevance to visual ghosts, although it is probably more relevant to the "third man" factor that is commonly reported by people who are in isolation, especially at times of crisis.
As for visual ghosts, there was an article in the New York Times about the suggestion that ghosts have changed from solid to semi transparent over history. Here are some extracts:
That's interesting, except that I thought that when the Society for Psychical Research did an extensive survey of ghost experiences back in the 1880s, one of the surprise findings was that ghosts often did appear solid. (Especially, if I recall correctly, the "crisis apparition" style of incident, where someone who had died miles away makes an unexpected appearance before loved ones or others at about the time of death.)We think of ghosts as wispy and translucent — a vaporous woman, perhaps, who floats down the stairs, her dress trailing in the languid air behind her. But in early modern Europe, ghosts were often perceived as solid persons. The viewer discovered that they weren’t when they did something that ordinary humans could not, like bypassing a locked door to enter a room.By the 19th century, people had begun to think of ghosts predominantly as spectral forms — ephemeral, elusive, evanescent. When the ghost of Marley appeared to Scrooge in Dickens’s “A Christmas Carol” (1843), and Scrooge looked his transparent body “through and through,” he illustrated a shift in the ways ghosts became real to people, how ghosts were seen and remembered.In “Spectres of the Self,” the cultural historian Shane McCorristine points to two reasons for this transmutation. The first was skepticism about the supernatural, generated by the new developments in science. The concept of hallucination emerged to explain experiences like seeing an apparition. As the seeing of ghosts became a psychological phenomenon, it also became a pathological one. In 1848, the British skeptic Charles Ollier spoke for many when he wrote that “anyone who thinks he has seen a ghost, may take the vision as a symptom that his bodily health is deranged.” As a result, Dr. McCorristine writes, the ghost was gradually relocated “from the external, objective and theological structured world to the internal, subjective and psychological haunted world of personal experience.”