X-rays were discovered in 1895 by German physicist, Wilhelm Röntgen. He also discovered the medical application of x-rays when he passed his hand in front of a barium screen and noticed a shadow of his skeleton. The ability to see inside the body has been a big boom for diagnosis in dentistry. A tiny burst of x-rays aimed between the teeth will show cavities in their early stages before they can be detected by visual examination. As the x-rays pass through the teeth, the high density of tooth structure will stop the x-rays from reaching the film. Decay is less dense because the acid from bacterial plaque leaches out the calcium. Therefore, more x-rays are able to pass through that point. The more x-rays that pass through, the darker the point on the x-ray film. The dentist will look at the film for dark spots against a white background in order to detect decay. The middle and later stages of gum disease can also be detected because the disease will attack the bone. With less density of the bone, the damaged bone will show up on an x-ray film as darker and therefore the extent of the disease can be better quantified.