N ray
The so-called N rays (or N-rays) were a phenomenon described by French scientist Rene-Prosper Blondlot but subsequently found to be illusory.
In 1903, Blondlot, a distinguished physicist working at the University of Nancy, perceived changes in the brightness of an electric spark in a spark gap placed in an X-ray beam which he actually photographed and he later attributed to a novel form of radiation, naming it the N-ray, for the University of Nancy. However, no other researchers seemed able to reproduce his photographs. Blondlot, Augustin Charpentier, Arsene d'Arsonval and many others claimed to be able to detect rays emanating from most substances, including the human body. Most researchers of the subject at the time used the perceived light of a dim phosphorescent surface as "detectors", although work in the period clearly showed the change in brightness to be a physiological phenomenon rather than some actual change in the level of illumination. Physicists Gustave le Bon and P. Audollet and spiritualist (Phrenologist?) Carl Huter even claimed the discovery as their own, leading to a commission of the Academie des sciences to decide priority.
The "discovery" excited international interest and many physicists worked to replicate the effects. However, the notable physicists Lord Kelvin, William Crookes, Otto Lummer and Heinrich Rubens failed to do so. Following his own failure, self-described as "wasting a whole morning", US physicist Robert W. Wood, who had a reputation as a popular "debunker" in the period, was prevailed upon to travel to Blondlot's laboratory in France to investigate further. Wood suggested that Rubens go since he had been the most embarrassed when the Kaiser asked him to repeat the French experiments and then after two weeks he had to report his failure to do so. Rubens, however, felt it would look better if Wood went since Blondlot had been most polite in answering his many questions.
In the darkened room, Professor Wood secretly removed an essential prism from the experimental apparatus, yet the experimenters still said that they observed N rays.  He also secretly replaced a large file that was supposed to be giving off N rays with an inert piece of wood, yet the N rays were still "observed".  His report on these investigations, published in Nature, suggested that N rays were a purely subjective phenomenon, with the scientists involved having recorded data that matched their expectations.  By 1905 no one outside Nancy believed in N rays.
The incident is used as a cautionary tale among scientists on the dangers of error introduced by experimenter bias. More precisely, patriotism was at the heart of this self-deception. France had been defeated by the Germans in the Franco-Prussian War in 1870, and after the major discovery by Wilhelm Rontgen of the X Ray the race was on for new discoveries.
N rays were cited as an example of pathological science by Irving Langmuir. However, the case is far more interesting than a single event, because nearly identical properties of an equally unknown radiation were recorded some 50 years before in another country by the Baron von Reichenbach in his treatise "Researches on Magnetism, Electricity, Heat, Light, Crystallization, and Chemical Attraction in their relations to the Vital Force", London, 1850, and before that in Vienna by Franz Mesmer in his "Memoire on the discovery of Animal-magnetism", 1779. It is clear that Reichenbach was aware of Mesmer's work and that researchers in Paris working with Blondlot were aware of Reichenbach's work (Revue Scientifique, Ser 5, Vol II, No.22) although there is no proof that Professor Blondlot was personally aware of it. However, this spread of nearly identical pathological science in history shows the phenomena to have greater breadth than the usually assumed patriotic self-deception.
Blondlot still has a street named after him in downtown Nancy as the belief that he had made a major discovery persisted.