Claims of capturing a spirit with the camera lens were made as early as the 1850s, when photography was relatively new to the world. At that time, the process of photography was mysterious enough to most people that the idea of a photograph capturing the latent image of a spirit seemed quite possible.
In fact, Sir David Brewster, in his 1856 book on the stereoscope, gave step-by-step instructions for creating a spirit photo, beginning with:
Simply pose your main subjects. Then, when the exposure time is nearly up, have the ‘spirit’ figure enter the scene, holding still for only seconds before moving out of the picture. The ‘spirit’ then appeared as a semi-transparent figure…
One of the more famous–and infamous–spirit photographers was William H. Mumler of Boston. He turned his ability to make photographs with visible spirits into a lucrative business venture, starting in the 1860s. Doubts grew about his work, but even when a spiritualist named Doctor Gardner recognized some of the so-called spirits as living Bostonians, people continued to pay as much as $10 a sitting.
Mumler was charged with fraud in 1869, though not convicted, due to lack of evidence. However, his career as a photographer of the spirit world was essentially over.
Via Library of Congress.