W. K. Rontgen, “the x-‐rays” (1895) 1
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It seemed that nothing in the natural world could stop the West’s progress. Western
scientists, engineers, and inventors appeared able to surmount every obstacle and to
find solutions to every problem. Even the invisible realms revealed their secrets to
them. On November 8, 1895, while experimenting with electric current flow, the
German scientist Wilhelm Röntgen (1845–1923) produced and detected radiation on
the electromagnetic spectrum in a wavelength range now known as X-‐rays or—in
many countries—Röntgen rays. For this achievement he won the first Nobel Prize in
Physics in 1901. In addition to medical uses, including radiography and radiation
therapy, X-‐rays were also found to help determine the structure of crystals, to test the
soundness of diverse materials, and of course to screen passengers in airports.
In the passages below, Röntgen describes the systematic observations and
experiments by which he confirmed the existence and behavior of X-‐rays and his
hypotheses as to their nature. His careful use of the scientific method is clearly shown.
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By W. C. RÖNTGEN
I.—UPON A NEW KIND OF RAYS
1. If the discharge of a great Ruhmkorff
induction coil be passed through a
vacuum-‐tube, or a Lenard
tube, or similar apparatus
containing a sufficiently high vacuum, then, the tube being covered with a close
layer of thin black pasteboard and the room darkened, a paper screen covered on
one side with barium-‐platinum cyanide and brought near the apparatus will be seen
to glow brightly and fluoresce at each discharge whichever side of the screen is
toward the vacuum tube. The fluorescence is visible even when the screen is
removed to a distance of 2 meters from the apparatus.
The observer may easily satisfy himself that the cause of the fluorescence
to be found at the vacuum tube and at no other part of the electrical circuit.
W. C. Röntgen, “The X-‐Rays,” in Annual Report of the Board of Regents of the Smithsonian Institution
(Washington: Government Printing Office, 1898), 137–39, 141, 142, 143.
Heinrich Daniel Ruhmkorff (1803–77) devised an induction coil used to produce high-‐voltage
current from a low-‐voltage supply.
Johann Wilhelm Hittorf (1824–1914) was a German physicist who successfully calculated the
electric capacity of charged atoms and ions.
Philipp Eduard Anton von Lenard (1862–1947) was a German physicist who won a Nobel Prize for
Physics in 1905 for his research on cathode rays.
William Crookes (1832–1919) was a British chemist and physicist who developed a device that
controls electric current in a container.
The emission of light by a substance that has absorbed light or other electromagnetic radiation.
2. It is thus apparent that there is here an agency which is able to pass
through the black pasteboard impenetrable to visible or ultra violet rays from the
sun or the electric arc, and having passed through is capable of exciting a lively
fluorescence, and it is natural to inquire whether other substances can be thus
It is found that all substances transmit this agency, but in very different
degree. I will mention some examples. Paper is very transmissible.
I observed fluorescence very distinctly behind a bound book of about 1,000
pages. The ink presented no appreciable obstacle. Similarly fluorescence was seen
behind double whist
pack. A single card held between the fluorescent screen and
the apparatus produced no visible effect. A single sheet of tin foil, too, produces
hardly any obstacle, and it is only when several sheets are superposed that their
shadow appears distinctly on the screen. Thick wooden blocks are transmissible.
Slabs of pine 2 or 3 centimeters thick absorb only very little. A plate of aluminum
about 15 millimeters thick diminished the effect very considerably, but did not
cause the fluorescence to entirely disappear. Blocks of hard rubber several
centimeters thick still transmitted the rays.
Glass plates of equal thickness behave very differently according to whether
they contain lead (flint glass) or not. The first class are much less transmissible than
If the hand is held between the vacuum tube and the screen, the dark shadow
of the bones is seen upon the much lighter shadow outline of the hand. Water,
carbon, bisulphide, and various other liquids investigated proved very
transmissible. I could not find that hydrogen was more transmissible than air. The
fluorescence was visible behind plates of copper, silver, lead, gold, and platinum,
when the thickness of the plate was not too great. Platinum 0.2 millimeter thick is
still transmissible, and silver and copper plates may be still thicker. Lead 1.5
millimeters thick is practically impenetrable, and advantage was frequently taken of
this characteristic. A wooden stick of 20 millimeters square cross section, having
one side covered with white lead, behaved differently when interposed between the
vacuum tube and the screen according as the X-‐rays traversed the block parallel to
the painted side or were compelled to pass through it. In the first case there was no
effect appreciable, while in the second a dark shadow was thrown on the screen.
Salts of the metals, whether solid or in solution, are to be ranged in almost the same
order as the metals themselves for transmissibility.
3. These observations and others lead to the conclusion that the
transmissibility of equal thicknesses of different substances depends on their
density. At least no other characteristic exerts so marked an influence as this.
The following experiment shows, however, that the density is not the sole
factor. I compared the transmissibility of nearly equally thick plates of glass,
and quartz. The density of these substances is substantially
the same, and yet it was quite evident that the calcspar was considerably less
transmissible than the others, which are about alike in this respect.
A card game.
Calcspar is a mineral and polymorph of calcium carbonate.
4. All bodies became less transmissible with increasing thickness. For the
purpose of finding a relation between transmissibility and thickness I have made
photographic exposures, in which the photographic plate was partly covered with a
layer of tin foil consisting of a progressively increasing number of sheets. I shall
make a photometric measurement when I am in possession of a suitable
5. Sheets were rolled from platinum, lead, zinc, and aluminum of such
thickness that all appeared to be equally transmissible. The following table gives the
measured thickness in millimeters, the relative thickness compared with platinum,
and the specific gravity:
Thickness Relative Specific
Platinum . . . . . . . . . . . . . . . . 0.018 1 21.5
Lead . . . . . . . . . . . . . . . . . . . . 0.05 3 11.3
Zinc . . . . . . . . . . . . . . . . . . . . . 0.20 6 7.1
Aluminum . . . . . . . . . . . . . . . 3.5 200 2.6
From these values it may be seen that the transmissibility of plates of
different metals so chosen that the product of the thickness and density is constant
would not be equal. The transmissibility increases much faster than this product
6. The fluorescence of barium-‐platinum-‐cyanide is not the only action by
which X-‐rays may be recognized. It should be remarked that they cause other
substances to fluoresce, as for example the photophorescent calcium compounds,
uranium glass, common glass, calcspar, rock salt, etc.
It is of particular importance from many points of view that photographic dry
plates are sensitive to X-‐rays. It thus becomes possible to fix many phenomena so
that deceptions are more easily avoided; and I have where practicable checked all
important observations made with a fluorescent screen by photographic exposures.
It appears questionable whether the chemical action upon the silver salts of
the photographic plate is produced directly by the X-‐rays. It is possible that this
action depends upon the fluorescent light which, as is mentioned above, may be
excited in the glass plate, or perhaps in the gelatine film. “Films” may indeed be
made use of as well as glass plates.
I have not as yet obtained experimental evidence that the X-‐rays are capable
of giving heat. This characteristic might, however, be assumed as present, since in
the excitation of fluorescent phenomena the capacity of the energy of the X-‐rays for
transformation is proved, and since it is certain that of the X-‐rays falling upon a
body not all are given up.
The retina of the eye is not sensitive to these rays. Nothing is to be noticed by
bringing the eye near the vacuum tube, although according to the preceding
observations the media of the eye must be sufficiently transmissible to the rays in
. . .
Taking this result together with the observation that powder is as
transmissible as coherent substance, and further, that bodies with rough surfaces
behave in the transmission of X-‐rays and also in the experiments just described
exactly like polished bodies, the conclusion is reached that there is, as before
remarked, no regular reflection, but that the bodies behave toward X-‐rays in the
same manner as a turbid
medium with reference to light.
As I have not been able to discover any refraction in the passage from one
medium to another, it appears as if the X-‐rays travel with equal velocity in all bodies,
and hence in a medium which is everywhere present and in which the particles of
the bodies are embedded. These latter act as a hindrance to the propagation of the
X-‐rays, which is in general greater the greater the density of the body in question.
9. In accordance with this supposition it might be possible that the
arrangement of the molecules of the body would exert an influence on its
transmissibility, and that, for example, a piece of calcspar would be unequally
transmissible for equal thicknesses when the rays passed along or at right angles to
the axis. Experiments with calcspar and quartz gave, however, a negative result.
10. It will be recalled that Lenard, in his beautiful experiments on the
transmission of the Hittorf cathode rays through thin aluminum foil, obtained the
result that these rays are disturbances in the ether, and that they diffuse themselves
in all bodies. We may make a similar statement with regard to our rays.
. . .
Most other substances are, like the air, more transmissible for X-‐rays than for
the cathode rays.
. . .
12. According to the results of experiments particularly directed to discover
the source of the X-‐rays, it is certain that the part of the wall of the discharge tube
which most strongly fluoresces is the principal starting point. The X-‐rays therefore
radiate from the place where, according to various observers, the cathode rays meet
the glass wall. If one diverts the cathode rays within the tube by a magnet, the
source of the X-‐ray is also seen to change its position so that these radiations still
proceed from the end points of the cathode rays. The X-‐rays being undeviated by
magnets cannot, however, be simply cathode rays passing unchanged through the
glass wall. The greater density of the gas outside of the discharge tube cannot,
according to Lenard, be made answerable for the great difference of the deviation.
I come therefore to the results that the X-‐rays are not identical with the
cathode rays, but that they are excited by the cathode rays in the glass wall of the
. . .
17. If the question is asked what the X-‐rays—which certainly are not cathode
rays—really are, one might at first, on account of their lively fluorescent and
chemical action, compare them to ultra-‐violet light. But here one falls upon serious
difficulties. Thus, if the X-‐rays were ultra-‐violet light, then this light must possess
the following characteristics:
(a) That in passing from air into water, carbon bisulphide, aluminum, rock
salt, glass, zinc, etc., it experiences no notable refraction.
(b) That it is not regularly reflected by these substances.
(c)That it cannot be polarized by the usual materials.
(d) That its absorption by substances is influenced by nothing so much as by
In other words, one must assume that these ultra-‐violet radiations comport
themselves quite differently from all previously known infra-‐red, visible, and ultra-‐
I have not been able to admit this, and have sought some other explanations.
A kind of relation seems to subsist between the new radiation and light
radiation, or at least the shadow formation, the fluorescence, and the chemical
action, which are common phenomena of these two kinds of radiation, point in this
direction. It has been long known that longitudinal as well as transverse vibrations
are possible in the ether, and according to various physicists must exist. To be sure,
their existence has not, up to the present time, been proved, and hence their
characteristics have not thus far been experimentally investigated.
Should not the new radiations be ascribed to longitudinal vibrations in the
ether? I may say that in the course of the investigation this hypothesis has
impressed itself more and more favorably with me, and I venture to propose it,
although well aware that it requires much further examination.
WÜRZBURG, PHYSIK. INSTITUT D. UNIV., December, 1895.
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