The developments leading up to this stage are recent and familiar enough to be briefly summarized. The first step was once more an accident, when Röntgen (or rather one of his assistants) noticed that a bunch of keys, laid down by chance on top of an unopened box of photographic plates near a cathode tube, had produced an inexplicable shadow-image of itself on one of the plates. The cathode tube was apparently giving off some hitherto unknown type of radiation, capable of penetrating opaque substances. Röntgen was an experimentalist, not a theorist; his pupils used to say privately that in publishing this discovery of X-rays he attempted a theoretical explanation for the first and only time in his life - and got it wrong!