Editing Cathode ray (section)

===Straight line motion===
[[Julius Plücker]] in 1869 built a tube with an anode shaped like a [[Maltese Cross]] facing the cathode. It was hinged, so it could fold down against the floor of the tube. When the tube was turned on, the cathode rays cast a sharp cross-shaped shadow on the fluorescence on the back face of the tube, showing that the rays moved in straight lines.<ref name="Pais">{{cite book | last = Pais | first = Abraham | author-link = Abraham Pais | title = Inward Bound: Of Matter and Forces in the Physical World | publisher = Oxford Univ. Press | year = 1986 | location = UK | url = https://books.google.com/books?id=mREnwpAqz-YC&pg=PA81 | isbn = 978-0-19-851997-3 }}</ref>{{rp|79}} This fluorescence was used as an argument that cathode rays were electromagnetic waves, since the only thing known to cause fluorescence at the time was [[ultraviolet]] light. After a while the fluorescence would get 'tired' and the glow would decrease.<ref name="Thomson" />{{rp|143}} If the cross was folded down out of the path of the rays, it no longer cast a shadow, and the previously shadowed area would fluoresce more strongly than the area around it.