A pioneer in the field of astrophysics, Johann Zöllner (1834–1882) was a professor of physics and astronomy at the University of Leipzig and an associate of the Royal Astronomical Society. Zöllner was best known for his work on astronomical photometry and spectrum analysis, on which he published widely. He invented the astronomical photometer used for measuring stellar magnitudes. He was also interested in optical illusions: the 'Zöllner illusion' consists of straight parallel lines which appear to be unparallel. This book, published in German in 1878–1879 and translated into English by Charles C. Massey in London in 1880, exemplifies the shift in Zöllner's interests in later life: he became involved in the public debate surrounding the scientific veracity of spiritualism. Here Zöllner describes his observations of experiments conducted by the medium Henry Slade in his own home.