He was an early investigator into X-ray imaging, but his claim to have made the first X-ray image in the United States is incorrect. He learned of Röntgen's discovery of unknown rays passing through wood, paper, insulators, and thin metals leaving traces on a photographic plate, and attempted this himself. Using a vacuum tube, which he had previously used to study the passage of electricity through rarefied gases, he made successful images on 2 January 1896. Edison provided Pupin with a calcium tungstate fluoroscopic screen which, when placed in front of the film, shortened the exposure time by twenty times, from one hour to a few minutes. Based on the results of experiments, Pupin concluded that the impact of primary X-rays generated secondary X-rays. With his work in the field of X-rays, Pupin gave a lecture at the New York Academy of Sciences. He was the first person to use a fluorescent screen to enhance X-rays for medical purposes. A New York surgeon, Dr. Bull, sent Pupin a patient to obtain an X-ray image of his left hand prior to an operation to remove lead shot from a shotgun injury. The first attempt at imaging failed because the patient, a well-known Lawyer, was "too weak and nervous to be stood still nearly an hour" which is the time it took to get an X-ray photo at the time. In another attempt, the Edison fluorescent screen was placed on a photographic plate and the patient's hand on the screen. X-rays passed through the patients hand and caused the screen to fluoresce, which then exposed the photographic plate. A fairly good image was obtained with an exposure of only a few seconds and showed the shot as if "drawn with pen and ink." Dr. Bull was able to take out all of the lead balls in a very short time.