The Hesperus-Additor is an article written by Florizel von Reuter published in Psychic Science in january 1927. Arthur Conan Doyle wrote an introduction added at the beginning of the article, see below.
Herr Florizel von Reuter is a musician of European fame, and one of the greatest German virtuosos upon the violin. He is a man of many accomplishments, which seem to include both the psychic temperament and the power of writing a clear and arresting statement. His story speaks for itself. Whether the very remarkable results obtained were, as would appear, independent messages, or whether they were the products of a dramatisation of his own subconscious powers, presents a problem to the researcher. I may add that Herr von Reuter has kindly sent me a specimen of his "Hesperus," and that it is at the disposal of any psychic whom the College may recommend as proficient at planchette work. — A. Conan Doyle.
The full article by von Reuter can be read here.