This Crookes X-ray tube has a three-pronged glass cone invented by William Crookes in the late nineteenth century. The air within the glass tube is almost entirely removed and then a high voltage is applied between the two electrodes. As a result, electrons (or cathode rays, as they are often called) are emitted from the cathode and accelerated toward the anode. These electrons then emit X-rays as they hit the end of the tube, which can be focussed upon the human body to create a visual image of bone structures. The tube cannot, however, produce x-rays continuously and so has been improved upon since its introduction.