-------
79. ROENTGEN’S EXPERIMENTS. X-RAYS, AND A NEW ART. _Wurz. Physik. Med.
Gesell._ Jan. ’95; _Nature_, Lon., Jan. ’96; _The Elect._, Lon. April
24, 96; _Sitz. Wurz. Physik. Inst._ D. _Uni._ Mar. 9, 96.—UNINFLUENCED
BY A MAGNET IN OPEN AIR.—Although Lenard recognized several kinds of
cathode rays, which differed as to penetrating and phosphorescing power,
yet he always held, or inferred at least that they were deflected by a
magnet, outside, as well as inside, (proved § 72_a_.) of the discharge
tube. § 59. Prof. Wilhelm Konrad Roentgen subjected his newly discovered
rays to the action of very strong magnetic fields in the open air, but
no deviation was detected. This is the characteristic which more than
anything else has served to distinguish X-rays from cathode rays. This
property has been confirmed by others. He employed the principle of
magnetic attraction of internal cathode § 59, rays to shift the
phosphorescent spot, for then he noticed that the source of X-rays
fluctuated also.
80. SOURCE OF X-RAYS MAY BE AT POINTS WITHIN THE VACUUM SPACE.—In one
case, he employed a Lenard tube, and found that the X-rays were
generated from the window which was in the path of the cathode rays. §
67. Different bodies within the discharge tube were found to have
different quantitative powers of radiating X-rays when struck by the
cathode rays. He stated “If for example, we let the cathode rays fall on
a plate, one half consisting of a 0.3 mm. sheet of platinum and the
other half a 1 mm. sheet of aluminum, the pin-hole photograph of this
double plate will show that the sheet of platinum emits a far greater
number of X-rays than does the aluminum, this remark applying in every
case to the side upon which the cathode rays impinge.” On the reverse
side, however, of the platinum, no rays were emitted, but a large amount
was radiated from the reverse side of the aluminum. § 67. He admitted
that the explanation was simple; but, at the same time, he pointed out
that this, together with other experiments, showed that platinum is the
best for generating the most powerful X-rays. One form with which he
experimented is illustrated in Fig. J, in principle, being described as
a bulb in which a concave cathode was opposite a sheet of platinum,
placed at an angle of 45° to the axis of the curved cathode, and at the
focus thereof.
[Illustration:
J
]
81. REFLECTION OF X-RAYS.—He emphasized the knowledge that there is a
certain kind and a certain amount of reflection, such as that produced
upon light and, as pointed out by Lenard, upon cathode rays, by certain
turbid media. The following quotation sets forth the exact experiment to
show slight reflection at metal surfaces. “I exposed a plate, protected
by a black paper sheet 1 to the X-rays (_e.g._ from bulb J) so that the
glass side 2 lay next to the discharge tube. The sensitive film was
partly covered with star-shaped pieces (4 slightly displaced in the
Fig.) of platinum, lead, zinc and aluminum. On the developed negative
the star-shaped impressions showed dark (comparatively) under platinum,
lead and more markedly, under zinc; the aluminum gave no image. It
seems, therefore, that the former three metals can reflect the X-rays;
as, however, another explanation is possible, I repeated the experiment
with only this difference, that a film of thin aluminum foil was
interposed between the sensitive film and the metal stars. Such an
aluminum plate is opaque to the ultra-violet rays, but transparent to
X-rays. In the result the images appeared as before, this pointing still
to the existence of reflection at metal surfaces.”
82. PENETRATING POWER. The transmitted energy was tested both by a
fluorescent screen and by a sensitive photographic plate. Either one was
acted upon by the rays after transmission through what have ordinarily
been called opaque objects. § 68. for example, 1000 pages of a book. As
in Lenard’s results, so in Roentgen’s, the color of the object had no
effect, even when the material was black. § 68, near beginning. A single
thickness of tinfoil scarcely cast a shadow on the screen. § 66_a_. The
same was true with reference to a pine board 2 or 3 cm. thick. They
passed also through aluminum 15 mm. thick. 63_b_. Glass was
comparatively opaque, § 66_a_, as compared with its power of
transmitting light, but nevertheless it must be remembered that the rays
passed through considerable thickness of glass. The tissues of the body,
water § 68, near centre, and certain other liquids and gases were found
exceedingly permeable § 67. Fluorescence could be detected through
platinum 2 mm. thick and lead 1.5 mm. thick. Through air the screen was
illuminated at a maximum distance of 1 m. A rod of wood painted with
white lead cast a great deal more shadow than without the paint, and in
general, bones, salts of the metals, whether solid or in solution,
metals themselves and minerals generally were among the most resisting
materials. § 155. The experiments were performed in a dark room by
excluding the luminosity of the tube by a thick cloth or card board
entirely surrounding the tube. He performed the wonderful experiment, so
often since repeated, of holding the hand between the screen of barium
platino cyanide and the discharge tube, and beholding the shadow picture
of the bones. This was the accidental step which initiated the new
department of photography, and which gave to the whole science of
electric discharge, a new interest among scientists and electricians and
which thoroughly awakened popular interest. The whole world concedes to
him the honor of being the originator of the new art. In view of
sciagraphs of the bones of the hand upon the screen, it occurred to him
in view also of Lenard’s experiments, on the photographic plate, to
produce a permanent picture of the skeleton of the hand with the flesh
faintly outlined. § 84. The accompanying half tone illustration, page
37, was made by the _Elect. Eng_. N.Y. (June 3, ’96) by permission, and
it represents the Edison X-ray exhibit at the New York Electrical
Exposition of the Electric Light Association, 1896. Thousands of people,
through the beneficence of Dr. Edison, were permitted to see the shadows
of their bones surrounded by living flesh. The screen was made of calcic
tungstate. The hand and arm were placed behind and viewed from the
front. § 132, near beginning.
83. PENETRATING POWER AND DENSITY OF SUBSTANCES.—Although he found that
there was some general relation between the thickness of materials and
the penetrating power, yet he was satisfied that the variation of the
power did not bear a direct relation to the density, (referring to
solids) especially as he noticed a peculiar result when shadows were
cast by Iceland spar, glass, aluminum and quartz of equal thickness. The
Iceland spar cast the least shadow upon suitable fluorescent or
photographic plate. The increased thickness of any one substance
increased the darkness of the shadow, as exhibited by tinfoil in layers
forming steps. Other metals, namely platinum, lead, zinc and aluminum
foil were similarly arranged and a table of the results recorded. §
63_b_.
RELATIVE
THICKNESS. THICKNESS. DENSITY.
Platinum .018 mm. 1 21.5
Lead .050 mm. 3 11.3
Zinc .100 mm. 6 7.1
Aluminum 3.500 200 2.6
mm.
He concluded from these data that the permeability increased much more
rapidly than the thickness decreased.
84. FLUORESCENCE AND CHEMICAL ACTION. § 70 and 63_a_.—Among the
substances that fluoresced were barium platino cyanide, calcium
sulphide, uranium glass, Iceland spar and rock salt. In producing
sciagraphs on the photographic plates, he found it entirely unnecessary
to remove the usual ebonite cover, which, although black, and so opaque
to light, produced scarcely any resistance to the rays. The sensitive
plate, even when protected in a box, could not be kept near a discharge
tube, for he noticed that it became clouded. He was not sure whether the
effect upon the sensitive plate was directly due to the X-rays or to a
secondary action, namely, the fluorescent light which must have been
produced upon the glass plate having the film, it being well known that
light of fluorescence possesses chemical power. He called attention to
the fact that inasmuch as fluorescent light which can be reflected,
refracted, polarized, etc., was produced by the rays; therefore, all the
X-rays which fell upon a body did not leave it as such. § 67. No effect
was produced upon the retina of the eye although he temporarily
concluded that the rays must have struck the retina in view of the great
permeability of animal tissue and liquids. § 68, at end. Conclusions of
this kind not based on experiment, are never reliable, even when offered
by very high authorities. Again the rays were weak. Roentgen himself
admitted that the salts of metals in solution (§ 82, near centre)
rendered the latter rather opaque. The eye ball is continually moistened
with the solution of common salt. Further than this, Mr. Pignolet
noticed in _Comptes Rendus_, Feb. 24, ’96, an account of an experiment
of Darien and de Rochas. In anatomy it is common to experiment on fresh
pig’s eyes in order to make comparisons with human eyes. The above named
Frenchmen submitted the former to X-rays. The eyes were but slightly
permeable thereto.
[Illustration:
THE PHYSICAL INSTITUTE, UNIVERSITY OF WÜRZBURG,
WHERE PROF. ROENTGEN HAS HIS RESIDENCE, DELIVERS HIS LECTURES, AND
PERFORMS HIS EXPERIMENTS.
From photograph by G. Glock, Würzburg. (Not referred to in book.)
]
85. NON-REFRACTION AND BUT LITTLE REFLECTION OF X-RAYS.—He employed a
very powerful refracting prism made of mica and containing carbon
bi-sulphide and water. The same prism refracted light but did not
refract X-rays. No one would think of making prisms for examining light,
of ebonite or aluminum, but he made such a prism for testing X-rays. But
if there were any refraction he concluded that the refractive index
could not have been more than 1.05, which may be considered as a proof
that the rays cannot be refracted. He tried heavier metals, but the
difficulty of arriving at any satisfactory results was due to the
resistance of such metals to the transmission of the rays. Among other
tests was one consisting in passing the rays through layers of powdered
materials through which the rays were transmitted in the same quantity
as through the same substances not powdered. It is well known that light
passed into powdered transparent materials, is enormously cut off,
deviated, diffused, refracted etc., in view of the innumerable small
surfaces of the particles. Hence he concluded that there was little if
anything in the nature of refraction or reflection of X-rays. § 146. The
powdered materials employed were rock salt, and fine electrolytic and
zinc dust. The shadows, both on the screen and as recorded on the
photographic plate were of substantially the same shade as given by the
same materials of the same thickness in the coherent state. One of the
most usual ways of testing refraction of light is by means of a lens.
X-rays could not be brought to a focus with the lens of what ever
material it was made. Among the substances tried were ebonite and glass.
As expected, therefore, the sciagraph of a round rod was darker in the
middle than at the edges; and a hollow cylinder filled with a more
transparent liquid showed the centre portion brighter than its edges. If
one considers this observation in connection with others, namely the
transparency of powders, and the state of the surface not being
effective in altering the passage of the X-rays through a body, it leads
to the probable conclusion that regular reflection does not exist, but
that bodies behave to the X-rays as turbid media to light, § 69.
86. VELOCITY OF X-RAYS IN DIFFERENT BODIES. p. 46.—Although he performed
no direct experiment in this direction yet he inferred in view of the
absence of refraction at the surfaces of different media, that the rays
travel with equal velocities in all bodies.
87. DOUBLE REFRACTION AND POLARIZATION.—Neither could he detect any
action upon the rays by way of refraction by Iceland spar at whatever
angle the crystal was placed. As to this property of light see Huygen’s
Works of 1690 and Malus’ Works of 1810. quartz also gave negative
results. Prof. Mayer of Stevens Institute submitted to _Sci._, Mar. 27,
’96, the report of a crucial test for showing the non-polarization of
X-rays. On six discs of glass, 0.15 mm. thick and 25 mm. in diameter,
were placed very thin plates of Herapath’s iodo-sulphate of quinine. The
axes of these crystals crossed one another at various angles. When the
axes of two plates were crossed at right angles no light was
transmitted; the overlapping surfaces of the plates appearing black. If
the Roentgen rays be polarizable, the Herapath crystals, crossed at
right angles, should act as lead and not allow any of the Roentgen rays
to be transmitted. Prof. Mayer is well known as exceedingly expert in
connection with minute measurements and in the manipulation of
scientific experiments. Dr. Morton, Pres. Stevens Inst., attested the
results as an absolute demonstration that X-rays are incapable of
polarization. _Stevens Indicator_, Jan., ’96.
88. THE PROPAGATION OF X-RAYS RECTILINEAR.—There would be no difficulty
in producing photographs of the bones of the hand with the rays of
light, if it were not for the tremendous amount of reflection and
refraction causing so much diffusion that no sharply defined shadow of
the bones would be produced. By means of a powerful lens and a funnel
pointed into a dark room, the author noticed that the condensed light
thereby obtained when passed through the hand, and when the incident
rays were parallel, came out so diffused that one would think that the
light went through bones as easily as any part of the hand. An
experiment of this kind serves to emphasize that the success of
sciagraphy by X-rays is due not only to the great penetrating power, but
to practically no refraction nor reflection. In view of the sharp
shadows cast of objects even when located in vegetable or animal media,
Roentgen was justified in giving the name of ray to the energy. He
tested the sharpness of the shadow by making sciagraphs and fluorescent
pictures not only of the bones of the hand, but of a wire wound upon a
bobbin, of a set of weights in a box, of a compass, card and needle,
conveniently closed in a metal case, and of the elements of a
non-homogeneous metal. To prove the rectilinear propagation further, he
received the image of the discharge tube upon a photographic plate by
means of a pinhole camera. The picture was faint but unmistakable.
89. INTERFERENCE. The rays of light may be caused to interfere with each
other. See _Newton’s Principia_, Vol. III.; _Young’s Works_, Vol.
I.—Theory points out that waves of ether of two pencils of light, when
caused to be propagated at certain relative phases partially or wholly
neutralize or strengthen each other. Roentgen could obtain no
interference effects of the X-rays, but did not conclude that the
interference property was absent. He was not satisfied with the
intensity of the rays and therefore could not test the matter severely.
[Illustration:
FIG. L.
]
90. ELECTRIFIED BODIES DISCHARGED BY X-RAYS. p. 47.—After Roentgen’s
first announcement, others, and probably J. J. Thomson as the first,
found that the X-rays would discharge both negatively and positively
electrified bodies. Roentgen, in his second announcement, stated that he
had already made such a discovery, but had not carried the investigation
far enough to report satisfactorily on the details. At last he put forth
an account of the whole phenomena and stated that the discharge varied
somewhat with the intensity of the rays, which was tested in each
instance by the relative luminosity of the fluorescent screen, and by
the relative darkness produced upon the photographic plate in several
instances. Electrified bodies, whether conductors or insulators, were
discharged when placed in the path of the rays. All bodies whatsoever
behaved in the same manner when charged. They were all discharged
equally by the X-rays. He noticed that “If an electrical conductor is
surrounded by a solid insulator such as paraffin instead of by air, the
radiation acts as if the insulating envelope were swept by a flame
connected to earth.” Upon surrounding said paraffin by a conductor
connected to earth, the radiation no longer acted on the inner
electrified conductor. The above observations led him to believe that
the action was indirect and had something to do with the air through
which the X-rays passed. In order to prove this, it was necessary for
him to show that air ought to be able to discharge the bodies if first
subjected to the rays, and then passed over the bodies. The apparatus
for performing an experiment to test this prediction is shown in Fig. L,
which serves to illustrate also the manner in which he prevented
electro-static influences of the discharge tube, leading in wires and
induction coil. § 71, near centre. For this purpose he built a large
room in which the walls were of zinc covered with lead. The door for his
entrance and exit was arranged to be closed in an air-tight manner. In
the side wall opposite the door there was a slit 4 cm. wide, covered
hermetically with a thin sheet of aluminum for the entrance of X-rays
from the vacuum tube outside of the room. All the electrical apparatus
connected with the generation of the X-rays was outside of the room. No
force whatever came into the room, therefore, except the X-rays through
the aluminum. § 71. In order to show that air which had been subjected
to the X-rays would discharge a body immediately afterwards upon coming
in contact therewith, he arranged matters so that the air was propelled
by an aspirator. He passed air along a tube made of thick metal so that
the rays could enter only through a small aluminum window near the open
end. At over a distance of 20 cm. from the window was an insulated ball
charged with electricity, and connected to any electroscope or
electrometer. The professor used a Hankel electroscope. No published
sketch was made by Roentgen; therefore, that shown in the figure was
produced by inference from the description. The operation was as
follows: The X-rays passed into the room through the aluminum window,
and then into the metal tube through its aluminum window. When the air
was at rest, the ball was not discharged. When the aspirator was at
work, however, so that the air moved past the aluminum window and past
the ball, the latter was discharged whether electrified positively or
negatively. He modified the operation by maintaining the ball at a
constant potential by means of accumulators, while the air which had
been treated by X-rays was passed by the ball. “An electric current was
started as if the ball had been connected with the wall of the tube by a
bad conductor.” He was not sure whether the air would retain its power
to discharge bodies as long as it remained out of contact with any
bodies. He determined, however, that any slight “disturbance” of the air
by a body having a large surface and not electrified, rendered the air
inoperative. He illustrated this by saying that “If one pushes, for
example, a sufficiently thick plug of cotton-wool so far into the tube
that the air which has been traversed by the rays must stream through
the cotton-wool before it reaches the ball, the charge of the ball
remains unchanged when suction is commenced.” With the cotton-wool
immediately in front of the window, it had no effect, showing,
therefore, that dust particles in the air are not the cause of the
communication of the force of the discharge from the X-rays to the
electrified body. Very fine wire gauze in several thicknesses also
prevented the air from discharging the body when placed between the
aluminum window and the ball within the thick metal tube, as in the case
of the cotton plug. Similar experiments were instituted with dry
hydrogen instead of air, and, as far as he could discern, the bodies
were equally well discharged, except possibly a little slower in
hydrogen. He experienced difficulty in obtaining equally powerful X-rays
at different times. All experimenters are acquainted with this
difficulty. Further, he called attention also to the thin layer of air
which clings to the surface of the bodies, and which, therefore, plays
an appreciable part in connection with the discharge. § 16, near end. In
order to test the matter further as to discharge of electrified bodies,
he placed the same in a highly exhausted bulb and found that the
discharge was in one case, for example, only 1/70 as rapid as in air and
hydrogen at ordinary pressure, thereby serving as another proof that gas
was the intermediate agency. Allowance should be made in all experiments
in connection with the discharging quality of X-rays. The surrounding
gas should be taken into account.
90_a_. APPLICATION OF PRINCIPLE OF DISCHARGE BY X-RAYS.—Professor Robb,
of Trinity College, (_Science_, Apr. 10, ’96), proposed and explained
and practically tested the principle of the discharge of X-rays to
determine the relative transparencies of substances to X-rays. He
plotted a curve in which the co-ordinate represented the charge of the
condenser in micro-coulombs, and the abscissæ the time between charging
and discharging the condenser. The same plan could be adopted, he
suggested, for making quantitative measurements of the intensity of
X-rays from different tubes or the same discharge tube at different
times. J. J. Borgmann, of St. Petersburg, probably was the first to show
that X-rays charged as well as discharged bodies. See _The Elect._,
Lon., Feb. 14, ’96, p. 501. Soon, a similar announcement was made by
Prof. Righi, of Bologna. § 90.
90_A_. BORGMANN AND GERCHUN’S EXPERIMENTS. ACTION OF THE X-RAYS ON
ELECTRO-STATIC CHARGES AND (LA DISTANCE EXPLOSIVE.) _Comptes Rendus_,
Feb. 17, ’96; from _Trans._, by Louis M. Pignolet.—A positively charged
zinc disk connected to an electroscope lost its charge almost instantly
and acquired a negative charge. When the charge on the zinc disk was
negative, the loss was much slower and was not complete, a certain
charge remaining. When the rays fell upon two small platinum balls
connected to the terminals of an induction coil but separated beyond its
sparking distance, sparking took place between them, showing that
X-rays, like ultra-violet rays, increase the sparking distance of static
charges.
90_b_. RIGHI’S EXPERIMENTS. BODIES IN THE NEUTRAL OR NEGATIVE STATE,
POSITIVELY ELECTRIFIED BY X-RAYS. _Comptes Rendus_, Feb. 17, 1896. From
Trans. by Louis M. Pignolet.—The measurements were made by this eminent
Italian physicist, with a Mascart electrometer connected with the bodies
upon which the X-rays impinged and enclosed in a grounded metallic case
(Faraday cylinder) provided with an aluminum window for the entrance of
the rays. A metallic disk connected with the electrometer lost its
charge rapidly whether positive or negative.
§ 99_S_. Initial positive charges were not completely dissipated;
negative charges were not only completely dissipated but the bodies
acquired positive charges. Disks in the neutral state were charged
positively by the X-rays the same as takes place with ultra-violet rays.
The final positive potential was greater for copper than for zinc and
still greater for retort carbon (“_le carbon de cornue_”) 90_c_. at end.
The various results are not conflicting if the particular materials are
taken into accounts. 90_c_ at end.
90_c_. The experiments of Prof. Minchin, an expert in such measurements,
are properly described here, in that they seem to clear up the
superficial ambiguity. He formulated the conclusion (_The Elect._, Lon.,
Mar. 27, ’96, p. 736) thus:—“The X-rays charge some bodies positively
and some negatively, and whatever charge a body may receive by other
means, the X-rays change it, both in magnitude and sign, to the charge
which they independently give to the body.” Thus, in the case of
magnesium, if the same is first positively charged by any suitable
means, then will the X-rays not only discharge it, but electrify it
negatively, while if this metal is first negatively charged, the X-rays
either diminish or increase the discharge. It must be remembered,
however, that this is not true with all metals, for he found that gold,
silver, copper, platinum, iron, aluminum, bismuth, steel and antimony,
are all positively electrified.
90_d_. BENOIST & HERMUZESCU’S EXPERIMENT. NEGATIVE CHARGES DISSIPATED
FASTER THAN POSITIVE BY X-RAYS. RATE DEPENDS UPON ABSORPTION. LAW
FORMULATED. _Comptes Rendus_, Feb. 3, Mar. 17 and April 27, ’96. They
observed that the rays dissipated entirely the charge of electrified
bodies in their path, and that negative charges were dissipated more
rapidly than positive. § 99_Q_. They also noticed the discharge augments
with the opaqueness of the body and that the effect is more considerable
with two thin superposed sheets than with one. In experimenting upon the
influence of the discharge of the gaseous dielectric in which the bodies
were located, they formulated the following law. The rapidity of the
dissipation of the electric charge of an electrified body under the same
condition varies as the square root of the density of the gas
surrounding the body. The dissipation of the electric charge depends
upon the nature of the electrified body, due to a sort of absorbing
power (§ 99_M_) connected with the opaqueness of the body and upon the
nature of the surrounding gas, due to the density of the gas or when
passing from one gas to another. (From trans. by Louis M. Pignolet.)
91. Before Roentgen published in his second paper of Mar. 9, ’96, an
account of his focus tube, the Kings College published a description of
an exactly similar one, represented in the cut. See _Elec. Rev._, Lon.,
Mar. 13. ’96, p. 340. The cathode is concave and the anode is formed of
platinum and is plane and at such an angle that the X-rays generated, §
63_b_., on diffusion of internal cathode rays, will be thrown out
through the thin walls of the bulb. § 55 and 57. As the rays emanate
from a point, the shadows are much clearer, especially in conjunction
with powerful rays permitting several feet between the object and the
tube. Mr. Shallenberger was among the first, and was the first as far as
the author knows (_Elect. World_, Mar. 7, ’96, see cut reproduced) to
originate the use of an X-ray focus tube.
[Illustration:
TYPICAL FOCUS TUBE.
]
91_a_. APPARATUS EMPLOYED.—Prof. Roentgen paid tribute to Tesla, by
alluding to the advantages resulting from the use of the Tesla condenser
and transformer. In the first place, he noticed that the discharge
apparatus became less hot, and that there was less probability of its
being pierced. Again the vacuum lasted longer, at least in the case of
his particular apparatus. Above all, stronger X-rays were produced.
Again careful adjustment of the vacuum was not as necessary as with the
Ruhmkorff coil.
92. X-RAYS AND LONGITUDINAL VIBRATIONS.—Prof. Roentgen did not consider
X-rays and ultra-violet rays to be of the same nature, although they
produced many common effects. The X-rays, as he found, by the above
related experiments, behaved quite differently from the ultra-violet
rays, which are highly refrangible, practically all subject to
reflection, capable of being polarized, and absorbed according to the
density of the absorbents. For valid reasons, the X-rays cannot be
infra-red rays. While he does not affirm any theory, yet he suggests the
theory of longitudinal waves for explaining the properties of X-rays.
(This was not suggested again in his second announcement.) He stated
that the hypothesis needs a more solid foundation before acceptance. The
reason why Roentgen termed the energy X-rays is simply because X in
algebra represents an unknown quantity.
[Illustration:
SHALLENBERGER APPARATUS AND FOCUS TUBE. § 91.
]
93. At the Johns Hopkins University, U. S., in 1884, Sir William
Thomson, (Kelvin) delivered a lecture in which he argued that the
production of longitudinal vibrations, by electrical means, is
reasonable and possible of occurrence. J. T. Bottomly, in _Nature_, Lon.
Feb., (see also _Elect. Eng._, N.Y., Feb. 19, ’96, p. 187) called
attention to this lecture as being of interest in view of Roentgen’s
suggestion about longitudinal vibrations. Lord Kelvin called attention
to what had been developed in connection with the electro-magnetic
theory of light and referred to his own work in 1854, in connection with
the propagation of electric impulses along an insulated wire surrounded
by gutta percha, but he said that at that time no one knew the relation
between electro-static and electro-magnetic units. The part of the
lecture referring particularly to the possibility of longitudinal waves
in luminiferous ether by electrical means reads as follows. “Suppose
that we have at any place in air, or in luminiferous ether (I cannot now
distinguish between the two ideas) a body that, through some action we
need not describe, but which is conceivable, is alternately, positively
and negatively electrified; may it not be that this will give rise to
condensational waves? Suppose, for example, that we have two spherical
conductors united by a fine wire, and that an alternating E. M. F. is
produced in that fine wire, for instance, by an alternate current
dynamo-electric machine, and suppose that sort of thing goes on away
from all other disturbance—at a great distance up in the air, for
example. The result of the action of the dynamo-electric machine will be
that one conductor will be alternately, positively and negatively
electrified, and the other conductor negatively and positively
electrified. It is perfectly certain, if we turn the machine slowly,
that in the air in the neighborhood of the conductors, we shall have
alternately, positively and negatively directed electric force with
reversals of, for example, two or three hundred per second of time, with
a gradual transition from negative, through zero to positive, and so on;
and the same thing all through space; and we can tell exactly what the
potential and what the electric force are at each instant at any point.
Now, does any one believe that, if that revolution were made fast
enough, the electro-static law of force, pure and simple, would apply to
the air at different distances from each globe? Every one believes that
if the process can be conducted fast enough, several million times, or
millions of millions times per second, we should have large deviations
from the electro-static law in the distribution of electric force
through the air in the neighborhood. It seems absolutely certain that
such an action as that going on would give rise to electrical waves.
Now, it does seem to me probable that these electrical waves are
condensational waves in luminiferous ether; and probably it would be
that the propagation of these waves would be enormously faster than the
propagation of ordinary light waves.” Notice that the above was written
twelve years prior to Roentgen’s discovery.
94. Prof. Schuster, in _Nature_, Lon., Jan. ’96, stated that the great
argument against the supposition of waves of very small length lies in
the absence of refraction, but questioned whether this objection is
conclusive. He further stated: “The properties of the ether may remain
unaltered within the greater part of the sphere of action of a molecule.
The number of molecules lying within a wave length of ordinary light is
not greater than the number of motes which lie within a sound wave, but,
as far as I know, the velocity of sound is not materially affected by
the presence of dust in the air. Hence there seems nothing impossible in
the supposition that light waves, smaller than those we know of, may
traverse solids with the same velocity as a vacuum. We know that
absorption bands greatly affect the refractive index in neighboring
regions; and as probably the whole question of refraction resolves
itself into one of resonance effects, the rate of propagation of waves
of very small lengths does not seem to me to be prejudged by our present
knowledge. If Roentgen rays contain waves of very small length, the
vibrations in the molecule which respond to them, would seem to be of a
different order of magnitude from those so far known. Possibly, we have
here the vibration of the electron with the molecule, instead of the
molecule carrying with it that of the electron.”
95. Prof. J. J. Thomson showed how it was possible that “longitudinal
waves can exist in a medium containing moving charged ions, and in any
medium, provided the wave length is so small as to be compared with
molecular dimensions, and provided the ether in the medium is in motion.
It follows from the equation of the electro-magnetic field that the
ether is set in motion in a varying electric field. These short waves
would not be refracted, but in this respect they do not differ from
transverse waves, which on the electro-magnetic theory would not be
refracted if the wave length were comparable with molecular distances.”
From _Elect. Eng._, N.Y., Mar. 18, ’96, p. 286, in reference to a paper
before the _Cam. Phil. So._
96. One of the very first questions asked in reference to a discovery is
as to its practical utility. Already, we have important applications in
one of the most humane directions, and that is in connection with
diagnosis. Sciagraphs can also be employed in schools for the purpose of
education, in some departments of anatomy, etc. The interest that it
excites and the amusement that it affords are not to be overlooked, for
anything in the nature of recreation possesses utility. However, we may
greatly thank all experimenters who have investigated the subject, and
who have not left its development alone to Roentgen; for predictions as
to the utility of a discovery, however, apparently exaggerated, are very
often proved, by subsequent developments, to have been underrated. Upon
this point Prof. Boltzmann, in _Zeit. Elect._, Jan. 15, ’96, see also,
_The Elec._, Lon., Jan. 31, ’96, p. 447, stated, “If we remember to what
discoveries the most insignificant new natural phenomenon, such as the
attraction of small objects by rubbed amber, of iron by the lode-stone,
the convulsive twitches of a frog’s leg due to electric discharges, the
influence of the electric current upon the magnetic needle,
electro-magnetic induction etc., has led us, one can imagine to what
applications an agent will be turned, which a few weeks after its
discovery has given rise to such surprising results.”
97. Soon after hearing, (about the first of Feb. ’96,) of the Roentgen
discovery, it occurred to the author to carry on experiments with
fluorescence, but finding that it was inconvenient to work in a
perfectly dark room, and, recognizing that black cardboard had
practically no effect upon absorbing the X-rays, he devised a sciascope
(_daily papers_, Feb. 13, and _Elect. Eng._, Feb. 19) which he
afterwards learned was independently invented and used at about the same
time by Prof. William F. Magie, of Princeton University, (see _Amer.
Jour. Med. Sci._, Feb. 7, ’96 and Feb. 15, ’96) and by Prof. E.
Salvioni, of Italy under the name of cryptoscope, (see _Med. Sur. Acad._
of Perugia, Italy, Feb. 8, ’96.) In about a month afterwards (_Elect.
Eng._, N.Y., Apr. 1, ’96, p. 340) the instrument (with phosphorescent
calcic tungstate § 13. in place of fluorescent barium platino cyanide)
was again published under the name of the Edison fluoroscope. There are
probably many other claimants—some professor in London—name forgotten.
They all consist of a tapering tube with a sight hole at one end and a
fluorescent screen in the other, which is closed by opaque card board.
(Frontispiece at Chap. X). For the sake of conformity, the words
sciagraph and sciagraphy and similar derivatives, and in view of the
meaning of the radical definitions, have been employed throughout the
book. The objection to the word fluoroscope is that the instrument is
practically universally employed in seeing the shadows of objects,
otherwise invisible to the naked eye, rather than to test fluorescence.
The name sciascope was early suggested by Prof. Magie. For those who
wish to make a screen, the author may state that he obtained a good one
by mixing pulverized barium platino cyanide with varnish and spreading
the mixture over a sheet of tracing cloth.
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