By the end of the 19th century numerous major discoveries were made concerning the structure of the microcosm and laws which govern many important discoveries in the field of microstructure took place. One of the most important was the discovery of X-rays.

In 1895 Wilhelm Conrad Roentgen was carrying out researches of the phenomenon of cathode rays. As you probably remember in1896 Joseph John Thomson ascertained that radiation was a beam of moving electrons. By accident Roentgen put a piece of cardboard covered with fluorescent mineral near the experimental set. He noticed that the substance started to glow in the dark when the source of cathode rays was turned on. Roentgen immediately started doing experiments aimed at the investigation of the phenomenon.

He discovered that if vacuum tube, used for experiments with cathode rays, was covered tightly with thin, black cardboard and placed in a darkened room, bright glow was observed during each discharge on a screen covered with fluorescent barium platinum cyanide (placed near the device). It did not matter which side (the one covered with the mineral or the other one) of the screen faced the device. Fluorescence occurred even when the screen was placed two meters from the vacuum tube.

He assumed that the fluorescence must have been caused by an agent which could infiltrate from within the vacuum tube through dark cardboard (impermeable to visible or ultraviolet radiation) to the outside of the set. In subsequent experiments the scientist showed that this agent (which he called X rays) could penetrate through various bodies to a different extend. (The level of transparency of a given body was described by Roentgen in terms of the ratio of the brightness of the fluorescing screen placed behind the body to the brightness of the uncovered screen). For example paper and zinc foil are highly transparent. Slightly less is wood. Even less is aluminium. Thin plates of copper, silver, gold or platinum are transparent (but not thick plates). Compounds containing additives of lead (for example flint glass) are much less transparent than those. And layers of lead are virtually non-transparent. Human body is quite transparent for X-rays but bones are almost not at all. An eye can't see X-rays. But when the radiation fall on a photographic plate, the plate becomes exposed. Now, when for example placing somebody's hand between a source of the rays and a photographic plate one would get a picture of hand bones. The fact was first noticed by Roentgen in 1895; and in January 1896 in Dartmouth New Hempshire X rays were used for the first time to help set a broken arm of Eddie McCarthy. Physicians could take advantage of the new, powerful tool against injuries and diseases.

The scientist also showed that the main centre emitting X-rays in all directions was the fluorescent spot on the wall of the discharge tube (the spot emerged where the beam of cathode rays fell). If the beam of cathode rays is deflected in a magnetic field and causes fluorescence in a different place, X-rays appear in a different spot. Hence it would seem that they are simply cathode rays filtered through a glass casing. However, as opposed to cathode rays, X-rays do not get deflected in a magnetic field.

The scientist made numerous observations and photographs using X-rays he had discovered. For example, he took a photograph of a shadow of the profile of the door separating the room containing the experimental devices and the room where he exposed the photographic plate. He took a photo of a shadow of the bone of an arm, a shadow of a covered wooden spool with wire wound on, a galvanometer with a magnetic needle covered with metal. He also took a photo of a piece of metal, inside of which he managed to notice certain heterogeneity invisible from the outside.

What are the X-rays, anyway? For a long time scientists couldn't have agreed what the nature of those rays was. Finally it emerged that (just like visible light) it was an electromagnetic wave. The frequency of that wave is however many times higher than of the visible light. The nature of X-rays is dual, of course. They can be also described as photons. X-ray photons have their energy much higher than visible light photons.

But how do the X-rays form? Well when very fast moving electrons fall on a metal plate they are rapidly stopped. So they loose much energy, emitting a highly energetic X-radiation photon. The place of incidence becomes a source of X-radiation.