THEORETICAL CONSIDERATIONS.
-------
Before attempting to discuss the facts now known in regard to the
Roentgen phenomena, it is well to review briefly the known ways in which
radiant energy may be transmitted.
By radiant energy is, of course, meant energy proceeding outward from a
source and producing effects at some distant point. There are two well
understood ways in which energy may be transmitted,—first, by an actual
transfer to the distant point of matter to which the energy has been
imparted from the source, as in the flight of a common ball, a bullet,
or a charge of shot. In this mode of transmission, it is evident that
the flying particles, assuming that they are subject to no forces on the
way, will move in straight lines from the source to the distant point.
They constitute real rays, diverging from the source; an obstacle in
their path, would, if the radiations proceeded from a point, cast a
shadow with sharply defined edges.
Second,—by a transfer of the energy from part to part of an intervening
medium, each part as it receives the energy, transmitting it at once to
the parts around it, no part undergoing more than a slight displacement
from its normal position. This mode of transmission constitutes wave
motion. The source imparts its energy to the particles of the medium
near it. Each of those particles transfers its energy to the particles
all around it. Each of these particles in turn transfers its energy to
the particles around it, and so on through the medium. It is plain that
there are here no such things as genuine rays. As the energy is
transferred from particle to particle, each in turn becomes a centre of
disturbance transmitting its motion in all directions. It is only
because the movements transmitted from different points annul one
another except along certain lines, that we have apparent straight lines
of transmission, and, therefore, fairly sharp shadows. But shadows
produced by wave transmissions are never absolutely sharp. The wave
movement is always propagated to some extent within the boundary of the
geometrical shadow, less as the wave lengths are shorter. With sound
waves whose lengths are measured in inches or feet, the penetration into
the shadow is considerable. With light waves 1/37000 to 1/70000 of an
inch in length, the penetration into the shadow is very small and
requires specially arranged apparatus to show that it exists.
This penetration into the geometrical shadow is characteristic of energy
propagated by wave motion, and if the fact of such penetration can be
demonstrated, it is conclusive proof of propagation by waves.
Another characteristic of wave motion is found in the phenomena of
_interference_. This is the mutual effect of two wave systems, which,
when meeting at a given point, may strengthen or annul each other
according to the conditions under which they meet. Either of those
characteristics should enable us to distinguish between propagation by
wave motion and by projected particles. But when wave lengths are very
short and radiations feeble, the tests are not easy to apply.
Again, a wave is in general propagated with different velocities in
different media. This causes a deflection or deformation of the wave as
it passes from one medium into another, and results in _refraction_, as
in the cases of light and sound. Absence of refraction would be strong
though not conclusive evidence against a wave theory of propagation.
In wave propagation, each particle of the medium suffers a small
displacement from its equilibrium position and performs a periodic
motion about that position. This displacement may be in the line of
propagation—longitudinal vibration—or it may be in a plane at right
angles to that line—transverse vibration. All the phenomena mentioned
above, diffraction, interference, refraction, and also reflection,
belong equally to either mode of wave propagation. Other phenomena must
be made use of to distinguish between these.
When the vibrations are transverse they may all be brought into one
plane through the line of propagation. They may be _polarized_, when the
ray will present different phenomena upon different sides. When the
vibrations are longitudinal, no such phenomena can be produced.
Polarization, then, serves to distinguish between longitudinal and
transverse vibrations.
Now let us consider briefly the Roentgen ray phenomena that bear upon
the question of the nature of the propagation.
[Illustration:
FROM SCIAGRAPH OF NORMAL ELBOW-JOINT; STRAIGHT, IN POSITION OF
SUPINATION.
By A. W. Goodspeed. _Phot. Times_, July, ’96.
Copyright, 1896, by William Beverley Harrison, Publisher of “X-ray”
Pictures, New York.
]
It seems to be settled beyond question that the origin of the Roentgen
rays is the fluorescent spot in the discharge tube. § § 107, 108, 111.
The evidence seems overwhelming that within the tube, the phenomena are
the result of streams of electrified particles of the residual matter,
shot off from the cathode in straight lines, perpendicular to its
surface. § 57. This was Crookes’ original theory, § 53, _near centre_,
and it seems to have stood well the test of scientific criticism. These
flying particles falling upon anything in their path, give rise to
X-rays. It is preferable, but not essential, that the bombarded surface
should be connected electrically with the anode. § § 113, and 116. The
best results are obtained by using a concave cathode, and placing at its
centre the surface which is to receive the bombardment, thereby
concentrating the effect upon a small area.
Nearly all experimenters agree in locating the origin of the X-rays at
this bombarded spot. The energy here undergoes a transformation, and the
X-rays represent one of the forms of energy developed.
What are the characteristics of this particular form of radiant energy?
It causes certain salts to fluoresce, § § 66, 84, and 132, and it
affects the photographic plate. § § 70 and 84. In these respects, it is
like the short wave length radiations from a luminous source. It is,
however, totally unlike these in its power of penetrating numerous
substances entirely opaque to light, such as wood, paper, hard rubber,
flesh, etc. In passing through hard rubber and some other opaque
insulators, X-rays are like the long wave length radiations from heated
bodies, but X-rays penetrate many substances that are opaque to these
long wave length radiations, and they are especially distinguished from
all forms of radiant energy previously recognized, in their relative
penetrating power for flesh and bones which makes it possible to obtain
the remarkable shadow pictures which have become within three or four
months, so familiar to all the world.
But these phenomena, although they serve to distinguish the X-rays from
all other forms of radiant energy, do not furnish any clew to the nature
of the X-rays themselves.
In attempting to formulate a theory of X-rays, the idea that first
naturally presents itself is that they are due to some form of wave
motion.
[Illustration:
FROM SCIAGRAPH OF KNEE-JOINT, STRAIGHT, SIDE VIEW, SHOWING PATELLA, OR
KNEE-CAP.
By Prof. Goodspeed. _Phot. Times_, July, ’96.
]
The characteristics of wave motion are diffraction and interference
phenomena. So far, no positive evidence of diffraction, § 110, nor
interference, § 89, have been recognized, although experiments, have
been tried that would have shown plainly, diffraction phenomena, had
light been used in place of the Roentgen radiations. § 170. We must,
therefore, conclude, either that the Roentgen radiations in the
experiments were too feeble to produce a record of the diffraction
effects, or, that they are not due to wave motion at all, unless of a
wave length very small even when compared with waves of light. The
absence of refraction is also opposed to any wave theory of the Roentgen
radiations, for it is difficult to believe that waves of any kind could
travel with the same velocity through all media, which they must do if
they suffer no deviation. § 86.
The next supposition naturally is, that the phenomena are due to streams
of particles. It has been suggested that the rays may be streams of
_material_ particles, but this theory cannot be maintained in view of
the fact that the rays proceed, without hindrance, through the highest
vacuum. § § 72_b_ and 133, _near end_. Neither is it consistent with the
high velocity of propagation. Molecules of gas could not be propelled
_through air_ with any such velocity or to any such distance as X-rays
are propagated. Tesla has claimed § 139, that the residual gases are
driven out through the glass of the vacuum bulb by the high potential
that he employs. This has not been confirmed by other experimenters. It
has been observed that the vacuum may be greatly improved by working the
bulb, § 121, that is, sending the discharge through it, but
experimenters generally have found that heating the bulb impairs the
vacuum and restores the original condition. The gases, were, therefore,
occluded during the electrical discharge, to be again set free by
heating the bulb. § 139_b_. The rays may be ether streams, perhaps in
the form of moving vortices, but of such streams we have no independent
knowledge, and can only determine by mathematical analysis, what their
characteristics should be. They would not suffer refraction, and would
not produce interference nor diffraction phenomena. Whether they would
_do_ what the X-rays do, go through the flesh and not through bone,
through wood and not through metal, excite fluorescence, or affect the
photographic plate, cannot be said. There is evidence that there are at
least two kinds of X-rays, § 152, differing in penetrating power, though
perhaps not differing in other respects.
[Illustration:
FROM SCIAGRAPH OF NORMAL KNEE-JOINT, FLEXED.
_Phot. Times_, July, 96.
Copyright, 1896, by William Beverley Harison, Publisher of “X-ray”
Pictures, New York.
]
X-rays have their origin only in electrical discharges in high vacua.
They are absent from sun-light and from light of the electric arc, and
other sources of artificial illumination, § 136. Proceeding from the
bombarded spot, they are not deflected by a magnet, except in an
evacuated observing tube, as proved by Lenard, § 72_a_, and show no
evidence of carrying an electric charge like cathode rays, § 61_b_, p.
47. On the contrary, they will discharge either a negatively or
positively charged body in their path. The evidence seems conclusive
(Chap. VIII.) that the ultra-violet rays from an illuminating source
also discharge charged conductors. In this respect, therefore, there is
a similarity between the X-rays and ultra-violet light.
The action of the waves of light upon a cell formed of selenium lowers
the resistance of the latter and herein is circumstantial evidence at
least, concerning the similarity of the properties of X-rays and light,
because the former are also found to increase the conducting power of
selenium. § 171.
The experiments of Roentgen, § 90, seem to show that the discharging
effect of X-rays is due to the air through which the rays have passed.
It is certain that the discharge of electrified bodies by light occurs
more generally for negatively than for positively charged bodies, § §
99_B_, 99_I_, and 99_S_, that it depends upon the nature, § 97_b_, and
density, § 97_a_, of the gas surrounding the body, and also upon the
material of the charged body itself. § 98. The discharge would,
therefore, seem to be connected with a chemical action, § 153, _near
end_, which is promoted by the rays. This seems all the more probable,
since it was found, § 98, that the more electro-positive the metal, the
longer the wave length that would influence the discharge. In this
connection, it is well to note that Tesla found, § 146_a_, that in their
power of reflecting (or diffusing X-rays), the different metals stand in
the same order as in the electric contact series in air, the most
electro-positive being the best reflectors. It would be interesting to
know whether connecting the reflecting plate to earth, would, in any
way, vary its reflecting power.
The X-rays seem to discharge some bodies, when positively charged, and
other bodies when negatively charged. They will also give to some bodies
a positive, and to others a negative charge (§ 90_c_). Is the order here
also that of the electrical contact series in air? Are not all the
phenomena of electrical charge and discharge, of reflection or
diffusion, and of X-rays, connected with chemical action, as the
apparent difference of potential, due to contact, undoubtedly is? § 153.
An experiment by La Fay (§ 139_a_) seems to show that X-rays, in air,
after passing through a charged silver leaf, acquire the property of
being deflected by a magnet, as are the cathode rays inside the
generating or exhausted observing tube, § 72_a_. If this is confirmed,
it would go far to support the theory that these rays are streams of
_something_.
[Illustration:
FROM SCIAGRAPH OF HEAD BY PROF. GOODSPEED. NASAL BONES APPEAR LIKE
EYELASHES.
_Inter. Med. Mag._, June, ’96.
The cervical vertebræ are distinguishable in the original, but barely
so in the half-tone. Fillings are located.
]
The burden of proof, up to the present, seems to be against any wave
theory of the X-rays, for, although they are like the ultra-violet rays
in producing fluorescence and in affecting the photographic plate, and
have some points of similarity to these rays in their effect upon
charged bodies, the X-rays are totally unlike the ultra-violet, in
respect to diffraction and interference phenomena. In fact, the absence
of such phenomena, if they are really absent, is conclusive proof that
the X-rays cannot be wave motions, unless of a wave length extremely
short even as compared to waves of light.
Since writing the above, I have seen an account of experiments in
relation to diffraction of X-rays, presented to the French Academy by
MM. L. Calmette and G. T. Huillier, in which the authors claim to have
obtained evidence that diffraction occurs. The following translation of
MM. Calmette and Huillier’s paper is taken from the _Electrical
Engineer_, N.Y., for July 22, 1896.
“We have the honor of submitting to the Academy some photographic proofs
obtained with the Röntgen rays by means of the following arrangement.”
“Very near the Crookes tube there is a screen “E” (_diagram omitted_),
of brass, perforated by a slit, the width of which has rarely reached a
half mm. A second metal screen, E´, is formed of a plate provided with
two slits or pierced with a window in which is fixed a metal rod of 1
mm. in diameter. This screen is placed at the distance, _a_, behind the
former. Lastly, a photographic plate, enfolded in two leaves of black
paper, is placed at the distance, _b_, behind the second screen, E´.”
“The following table indicates, for each proof, what is the screen E´
used, and the value of _a_ and _b_ + _a_:
E´.
No. _a_ _b_ +
Cm. _a_
Cm.
1. Rod of 1 mm. in 5 19.5
diameter
3. Rod of 1 mm. in 5.5 20
diameter
5. Rod of 1 mm. in 8.9 30
diameter
7. Two narrow slits, ? ?
separated by a
cylindrical rod of
1 mm. in diameter
“On the proofs 1, 3, 5 the shadow thrown by the metallic rod is bordered
on each side by a light band which shows a maximum of intensity. Within
this shade we observe a zone less dark, which seems to indicate that the
Röntgen rays penetrate into the geometrical shadow. Lastly, in proofs 3
and 5 we see, in like manner, a maximum of intensity along the margins
of the window in which the rod is placed.”
“In the proof No. 7 we perceive, in the middle of the two white bands, a
fine dark ray, while in the shadow of the rod which separates the two
slits there is seen a light ray.”
“If we compare these results with those obtained with light in the same
conditions, the slit being relatively wide and the intensity weak, it
seems difficult not to ascribe them to the diffraction of the Röntgen
rays.”
“The proofs obtained in these experiments—which we propose to
continue—are not yet so distinct that we can measure the wave length
with any precision. But we are still led to believe that this wave
length is greater than that of the luminous rays.—_Comptes Rendus._” Of
course, if diffraction phenomena can be demonstrated, the question as to
the radiations being wave propagations, is settled, though the question
whether the vibrations are longitudinal or transverse, is still open.
Before accepting any stream or vortex motion theory, we need to know
more about the X-ray phenomena, and more about stream and vortex motion.
------------------------------------------------------------------------
● Transcriber’s Notes:
○ Since this is a collection of articles by different scientists,
there may be differences in spelling and usage.
○ There is no experiment 64. There last experiment on page 51 is
number 63_b_, and the first experiment page 52 is 65. (A note
attached to the 63_b_ table of contents entry says that there is
no experiment 64.)
○ The missing entries in the Table of Contents for experiments
128_a_ and 149_a_ were added.
○ There are two experiments labeled 61_b_, page 46 (Thomson’s
Experiment) and 47 (Perrin’s Experiment). The instance on page 47
was relabeled 61_c_.
○ There are two experiments numbered 159 in the Table of Contents.
The first follows entry 110_a_ and the other is in order, after
experiment 158. The first instance was changed to 159a.
○ Missing or obscured punctuation was silently corrected.
○ Typographical errors were silently corrected.
○ Inconsistent spelling and hyphenation were made consistent only
when a predominant form was found in this book.
○ Text that was in italics is enclosed by underscores (_italics_);
text that was bold by “equal” signs (=bold=).
○ The use of a caret (^) before a letter, or letters, shows that the
following letter or letters was intended to be a superscript, as
in S^t Bartholomew or 10^{th} Century.
○ Superscripts are used to indicate numbers raised to a power. In
this plain text document, they are represented by characters like
this: “P^3” or “10^{18}”, _i.e._ P cubed or 10 to the 18th power.
○ Variables in formulæ sometimes use subscripts, which look like
this: “A_{0}”. This would be read “A sub 0”.