The man in this glass slide is Sir William Crookes (1832-1919), a prominent scientist known for his discovery of thallium and his research into cathode rays. Crookes was also a spiritualist, who wanted to investigate spiritualism using scientific methods.
Originally Crookes had been sceptical about the phenomenon and believed it to be mere trickery. His observations and photographs of the young medium Florence Cook (c1856–1904) brought his investigations to critical attention. Crookes published a report in 1874 asserting that Florence Cook, as well as other mediums, were producing genuine spirits. It caused an uproar.
The image on the left is the celebrated spirit Katie King, who Crookes was able to photograph several times. King was said to have appeared first between 1871 and 1874 in seances conducted by Florence Cook in London, and later in 1874-1875 in New York. Cook would materialise King, who walked and talked to participants in the seance. She even allowed herself to be weighed and measured.
Though Katie King resembled Florence Cook, others claimed King was taller than Cook, with a larger face and different hair and skin. They also claimed King and Cook were both visible at the same moment, so Cook could not have been playing the role of the spirit.