Today, of course, we think of spiritualism--the notion that the dead reside in a spirit world parallel to ours and can communicate with us via a medium--as completely guffaw-worthy, an proper fraud.
But in its heyday--roughly 1850-1920--Spiritualism was considered very modern, the scientific answer to religion. It was a religion for reformers! Believers supported charity, the abolition of slavery and the vote for women but had none of the baggage, none of the rules and none of the props of standard religion. Tissot was just one of many highly respectable people who took part in seances. Others included Abraham Lincoln, William Gladstone, chemist William Crookes, socialist Robert Owen, Alfred Russel Wallace, several Nobel-laueates (Pierre Curie, for example) and even Arthur Conan Doyle and Queen Victoria.
Spiritualism got its start in 1848 in that hotbed of weird religious ideas, upstate New York. (Joseph Smith lived in upstate New York in 1823 when he "saw" the angel that inspired the church of the Latter Day Saints.) The three Fox sisters began to "communicate" via taps with the spirit of, they said, a peddlar who had been murdered and buried in their basement. The rumour spread from there.
It helped, of course, that hundreds of thousands of sons, brothers and husbands had been killed senselessly in the devastating wars of the late 19th century. This was an era when traditional religious beliefs and reassurances were collapsing (as Matthew Arnold wrote in the 1867 poem, Dover Beach: "The Sea of Faith was once, too, at the full, and round earth's shore lay like the folds of a bright girdle furl'd.But now I only hear its melancholy, long, withdrawing roar.") Those who were bereaved sought comfort. They wanted to believe that they could still communicate with their children, husbands, or brothers.
Traditional religion, its authority undermined by Science, could no longer comfort with the "sure and certain hope" of the resurrection. Unlike faith, science actually could cure the sick, heal the wounded and provide food for the hungry. But what about Life after Death? What comfort could science provide? Spiritualism, which proposed that the dead are among us still, turned to photography to convince the doubtful. Photography was science, and photographs didn't lie! An industry grew up to provide spirit photos such as this one of Abraham Lincoln (owned by Mary Todd Lincoln.) Tissot's mezzotint of his vision of Kathleen Newton, though not a photograph, was regarded another piece of "evidence."
So one way to think about Spiritualism is as an intellectual and emotional stopgap. As the certainties of traditional religion crumbled before scientific truth, people sought ways to replace that which had comforted them in the old way of knowing. Maybe religion was bogus, but life eternal...they could still believe in that.