Double focus X-ray tube, 1896.
© Science Museum / Science & Society Picture Library
Wilhelm Roentgen (1845-1923) discovered X-rays when he encountered emisions from a Crookes' discharge tube. Experiments revealed that these rays penetrated some substances more easily than others, and also fogged photographic plates. The fact that X-rays could produce images differentiating between the densities of body tisues, was a discovery which the medical profesion was keen to exploit. The tube features two cup-shaped cathodes which serve to focus the cathode rays onto the target (a platinum anode). When the rays hit the anode, their energy changed into invisible X-rays, which pased out through the glas.