istory is littered with examples
of discoveries made by accident,
but as Louis Pasteur said more
than a century ago, “Chance favours only
the prepared mind.” When the German
scientist Wilhelm Röntgen accidentally
found X-rays in 1895, he knew to
pursue them. Röntgen wasn’t the first
to observe the effects of X-rays, but he
is widely credited with their discovery.
The Crookes, or cathode ray, tube he
was experimenting with was common
at the time in the labs of physicists
interested in how electric charges passed
through gases. It had been invented by
the English physicist William Crookes in
1875, and it’s likely that some Crookes
tubes had been emanating X-rays prior
to Röntgen’s discovery. Crookes himself,
for example, found that photographic
plates placed near the tubes became
cloudy – later recognised as an effect of
being exposed to X-rays.
Röntgen first noticed X-rays on 8
November 1895, having only been using
a Crookes tube for a month. No one
knows for sure what happened that day
because he requested that his notes be
burned after his death, but it’s thought
that he was investigating cathode raysexperimentation. He cast aside his other
work and locked himself away for six
weeks to investigate the new rays. He
found that they could pass through a
variety of materials, such as books and
paper, but not others, such as lead. A
few days before Christmas that year, he
made an image of his wife Bertha’s left
hand by placing it between the X-ray
source and a photographic plate. The
image, complete with visible bones and
wedding ring, was probably not the first
X-ray image, but it may well be the first
to have been made deliberately.Harnessing X-Rays
We now know that X-rays are a form of
electromagnetic radiation, on the same
spectrum as visible and ultraviolet light.
They are generated when electrons from
the cathode in the X-ray tube collide
with the anode – around 1 per cent of
the energy generated is emitted as X-rays.
While visible light is absorbed by the body,
higher frequency X-rays can pass through
us. Different materials absorb different
amounts of X-rays. Dense material like
bone absorbs more, which is why
they show up so well in X-raywith the tube (see ‘The key experiment’)
when he noticed a screen in the room
fluorescing. He realised this must have
been caused by a new phenomenon,
which he called ‘X’-rays in recognition
of their mysterious nature.
It’s not clear why Röntgen pursued
X-rays where others hadn’t, but
he was known for his meticulousTaken in 1895, this X-ray image shows a wedding ring
on the left hand of Wilhelm Röntgen’s wife BerthaHOW TO MAKE
X-R AY IMAGES
It was a relatively fast road from the discovery of X-rays to them being put
to use in hospitals; their remarkable properties were quickly harnessed by
doctors and were a catalyst for ever more advanced scanning technologiesBY KATHERINE NIGHTINGALE
HOW DO WE KNOW?
H
PHOTO: SCIENCE & SOCIETY, SCIENCE PHOTO LIBRARY