"Great discoveries are made accidentally less often than the populace likes to think. Nevertheless it was an accident, bound indeed sometime to occur, yet none the less an accident, that put Röntgen on the track of X-rays. Photographic plates, though protected from light, were found to become fogged and spoilt when kept in the neighbourhood of highly exhausted glass tubes or bulbs through which electric discharges were passing. It appeared that rays of some kind which could penetrate the coverings of the plates were emitted by the discharge tubes.Röntgen found that a screen covered with a phosphorescent substance, such as potassium-platino-cyanide, became luminous near such a tube, and that a thick slab of metal cast a heavy shadow when interposed between the tube and the phosphorescent screen, while light substances such as thin aluminium or wood, though opaque to light, with these new rays cast shadows hardly visible. The absorption of the rays seemed roughly proportional to the thickness and density of the absorbent, and the rays became more penetrating the higher the exhaustion of the gas in the vacuum tube. Rays of an appropriate "hardness" were found to throw a shadow of the bones within the living flesh on a phosphorescent or photographic plate, and, when the right technique had been elaborated, this fact proved invaluable in surgery."