William Crookes (1832-1919) was an English chemist and physicist who worked on spectroscopy. He was a pioneer of vacuum tubes for the study of physical phenomena, and invented the Crookes tube. He discovered that the rays (which were made of electrons) made the sides of the glass tube fluorescent. He also showed that they travelled in straight lines and could be deflected by a magnetic field. Crookes invented the spinthariscope, which measured alpha radiation using scintillation (light flashes) caused by these charged particles. Crookes also discovered the element thallium.