An X-ray picture (radiograph) taken by Röntgen

An X-ray is a form of electromagnetic radiation with a wavelength in the range of 10 nanometers to 100 picometers (corresponding to frequencies in the range 30 PHz to 60 EHz). X-rays are primarily used for diagnostic medical imaging and crystallography. X-rays are a form of ionizing radiation and as such can be dangerous. Download high resolution version (512x768, 38 KB)Roentgens X-ray picture of the hand of Alfred von Kolliker, taken 23 January 1896; from [1], which claims copyright. ... Download high resolution version (512x768, 38 KB)Roentgens X-ray picture of the hand of Alfred von Kolliker, taken 23 January 1896; from [1], which claims copyright. ... Wilhelm Röntgen Wilhelm Conrad Röntgen (March 27, 1845 – February 10, 1923) was a German physicist, of the University of Würzburg, who, on November 8, 1895, produced wavelengths of electromagnetic radiation that are now known as x-rays. ... Electromagnetic radiation or EM radiation is a combination (cross product) of oscillating electric and magnetic fields perpendicular to each other, moving through space as a wave, effectively transporting energy and momentum. ... The wavelength is the distance between repeating units of a wave pattern. ... To help compare different orders of magnitudes this page lists lengths between 1 nm and 10 nm (10-9 m and 10-8 m). ... To help compare different orders of magnitude this page lists lengths between 1 pm and 10 pm (10-12 m and 10-11 m). ... An SI prefix is a prefix which can be applied to any unit of the International System of Units (SI) to give subdivisions and multiples of that unit. ... An SI prefix is a prefix which can be applied to any unit of the International System of Units (SI) to give subdivisions and multiples of that unit. ... Medical imaging is the process by which physicians evaluate an area of the subjects body that is not normally visible. ... X-ray crystallography is a technique in crystallography in which the pattern produced by the diffraction of x-rays through the closely spaced lattice of atoms in a crystal is recorded and then analyzed to reveal the nature of that lattice. ... Ionizing radiation is radiation in which an individual particle (for example, a photon, electron, or helium nucleus) carries enough energy to ionize an atom or molecule (that is, to completely remove an electron from its orbit). ...

Physics

X-rays with a wavelength approximately longer than 0.1 nm are called soft X-rays. At wavelengths shorter than this, they are called hard X-rays. Hard X-rays overlap the range of long-wavelength (low energy) gamma rays, however the distinction between the two terms depends on the source of the radiation, not its wavelength: X-ray photons are generated by energetic electron processes, gamma rays by transitions within atomic nuclei. This article is about electromagnetic radiation. ... In physics, the photon (from Greek φοτος, meaning light) is a quantum of excitation of the quantised electromagnetic field and is one of the elementary particles studied by quantum electrodynamics (QED) which is the oldest part of the Standard Model of particle physics. ... Properties The electron (sometimes called negatron; commonly represented as e−) is a subatomic particle. ... The nucleus (atomic nucleus) is the center of an atom. ...

The basic production of x-rays is by accelerating electrons in order to collide with a metal target (tungsten usually). Here the electrons suddenly decelerate upon colliding with the metal target and if enough energy is contained within the electron it is able to knock out an electron from the inner shell of the metal atom and as a result electons from higher energy levels then fill up the vacancy and x-ray photons are emitted.

Detectors

The detection of X-rays is based on various methods. The most commonly known method is the photographic plate, frequently used in hospitals. The X-rays blacken the photographic plate (negative), it is black where the X-rays go through ("soft" parts of the body like organs and skin) and white where the X-rays are stopped ("hard" parts like bone, or contrast product containing iodine injected in blood). Another method is to use a fluorescent plate, e.g. sodium iodide NaI. These methods give no information about the energy of the X-ray photons, just their spatial density. General Name, Symbol, Number iodine, I, 53 Series halogens Group, Period, Block 17 (VIIA), 5, p Density, Hardness 4940 kg/m3, no data Appearance violet-dark grey, lustrous Atomic properties Atomic weight 126. ... General Name, Symbol, Number sodium, Na, 11 Series alkali metal Group, Period, Block 1 (IA), 3, s Density, Hardness 968 kg/m3, 0. ... Definition An iodide ion is an iodine atom with charge -1. ...

Initially, most common detection methods were based on the ionisation of gases, as in the Geiger-Müller counter: a sealed cylinder with a polymer window contains a gas, and a wire, and a high voltage is applied between the cylinder (cathode) and the wire (anode). When an X-ray photon enters the cylinder, it ionises the gas which becomes conducting, creating a current flow (a kind of flash); this peak of current is detected and is called a "count". An antique Victoreen civil defense survey meter (a type of low accuracy geiger counter). ... Diagram of a copper cathode The electrode of an electrochemical cell at which reduction occurs is referred to as the cathode (from the Greek word κάθοδος = going down). In an electrolytic cell the cathode is negatively charged and in a galvanic cell the cathode is positively charged. ... An anode (from the Greek άνοδος = going up) is the positive electrode in an electrolytic system or circuit. ...

When the high voltage between anode and cathode is decreased, the detector is no longer saturated, and the height of the current peak is proportional to the energy of the photon; it is thus called a "proportional counter". Most of times, the cylinder is not sealed but is constantly fed with "fresh gas", is thus called a "flow counter". This proportionality property allows filtering the "interesting" peaks from the noise and other photons, but the resolution in energy is not enough to determine the energy spectrum; such a feature requires a diffracting crystal to first separate the different photons, the method is called wavelength dispersive X-ray spectroscopy (WDX or WDS). The noun spectrum (plural: spectra) has a variety of meanings. ... Diffraction is the apparent bending and spreading of waves when they meet an obstruction. ... The Wavelength dispersive X-ray spectroscopy is a method used to determine the energy spectrum of a X-ray radiation. ... The Wavelength dispersive X-ray spectroscopy is a method used to determine the energy spectrum of a X-ray radiation. ...

Some materials such as NaI can "convert" an X photon to a visible photon; an electronic detector can be built by adding a photomultiplier. These detectors are called "scintillators" or "scintillation counters". Photomultipliers, or photomultiplier tubes (PMT) are extremely sensitive detectors of light in the ultraviolet, visible and near infrared. ... A scintillator is a device or substance that absorbs high energy (ionizing) electromagnetic or charged particle radiation then, in response, fluoresces photons at a characteristic Stokes-shifted (longer) wavelength, releasing the previously absorbed energy. ... A scintillation counter measures ionizing radiation. ...

Since the 1970s, new semiconductor diode detectors have been developed (silicon or germanium doped with lithium, Si(Li) or Ge(Li)). X-ray photons are converted to electron-hole pairs in the semiconductor, and are collected to detect the X-rays. When the temperature is low enough (the detector is cooled by Peltier effect or best by liquid nitrogen), it is possible to directly determine the X-ray energy spectrum; this method is called energy dispersive X-ray spectroscopy (EDX or EDS); it is often used in small X-ray fluorescence spectrometers. These detectors are often called "solid detectors". Events and trends Although in the United States and in many other Western societies the 1970s are often seen as a period of transition between the turbulent 1960s and the more conservative 1980s and 1990s, many of the trends that are associated widely with the Sixties, from the Sexual Revolution... A semiconductor diode detector is a semiconductor detector made of germanium that has found broad application during recent decades, in particular for gamma and X-ray spectrometry and as particle detectors. ... General Name, Symbol, Number silicon, Si, 14 Series metalloid Group, Period, Block 14 (IVA), 3, p Density, Hardness 2330 kg/m3, 6. ... General Name, Symbol, Number germanium, Ge, 32 Series metalloids Group, Period, Block 14 (IVA), 4 , p Density, Hardness 5323 kg/m3, 6 Appearance greyish white Atomic properties Atomic weight 72. ... General Name, Symbol, Number Lithium, Li, 3 Series Alkali metal Group, Period, Block 1(IA), 2, s Density, Hardness 535 kg/m3, 0. ... The Peltier-Seebeck effect, or thermoelectric effect, is the direct conversion of heat differentials to electric voltage and vice versa. ... General Name, Symbol, Number Nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15 (VA), 2 , p Density 1. ... The energy dispersive X-ray spectroscopy (EDX or EDS) is a method used to determine the energy spectrum of X-ray radiation. ... EDX is a three letters abbreviation that can mean in chemical analysis: Energy dispersive X-ray spectroscopy in computer science: it is a data register in the 32 bits Intel architecture, see IA-32; This is a disambiguation page — a navigational aid which lists other pages that might otherwise share... In X-ray fluorescence (XRF) a material is exposed to X-rays with a relatively high energy. ...

It is commonly thought that X-rays are invisible to the human eye, and for almost all everyday uses of X-rays this may seem true, however, very strictly speaking, it is actually false. In special circumstances, X-rays are in fact visible to the "naked eye". An effect first discovered by Brandes in experimentation a short time after Röntgen's landmark 1895 paper; he reported, after dark adaptation and placing his eye close to an X-ray tube, seeing a faint "blue-gray" glow which seemed to originate within the eye itself.[1] (http://www.orau.org/ptp/articlesstories/invisiblelight.htm) Upon hearing this, Röntgen reviewed his record books and found he in fact, also saw the effect. When placing an X-ray tube on the opposite side of a wooden door Röntgen saw the same blue glow seeming to emanate from the eye itself, but thought his observations were spurious due to the fact that he only saw the effect when he used one type of tube. Later he realized that the tube which created the effect was the only one which produced X-rays powerful enough to make the glow plainly visible and the experiment was thereafter repeated readily. The fact that X-rays are actually faintly visible to the dark-adapted naked eye has largely been forgotten today is probably due to the lack of desire to repeat what we would now see as a recklessly dangerous and harmful experiment with ionizing radiation. It is not known what the exact mechanism in the eye is which produces the visibility and it could be due to either conventional detection (excitation of rhodopsin molecules in the retina), direct excitation of retinal nerve cells, or secondary detection via, for instance, X-ray induction of phosphorescence in the eyeball and then conventional retinal detection of the secondarily produced visible light. Diagram of a human eye. ... Wilhelm Röntgen Wilhelm Conrad Röntgen (March 27, 1845 – February 10, 1923) was a German physicist, of the University of Würzburg, who, on November 8, 1895, produced wavelengths of electromagnetic radiation that are now known as x-rays. ... From Latin ex- + -periri (akin to periculum attempt). ... Ionizing radiation is radiation in which an individual particle (for example, a photon, electron, or helium nucleus) carries enough energy to ionize an atom or molecule (that is, to completely remove an electron from its orbit). ... Categories: Biochemistry stubs | G protein coupled receptors | Sensory receptors | Pigments ... Phosphorescence is a radiative transition involving a change in the spin multiplicity of a molecule. ...

Medical uses

X-rays can reveal the details of bones and teeth

Since Röntgen's discovery that X-rays can identify bony structures, X-rays have been developed for their use in medical imaging. Radiology is a specialised field of medicine that employs radiography and other techniques for diagnostic imaging. Indeed, this is probably the most common use of X-ray technology. Photo by dene. From www. ... Medical imaging is the process by which physicians evaluate an area of the subjects body that is not normally visible. ... Radiology is the branch of medical science dealing with the medical use of x-ray machines or other such radiation devices. ... Medicine is a branch of health science concerned with restoring and maintaining health. ... Radiography is the creation of radiographs, photographs made by exposing a photographic film or other image receptor to X-rays. ... Medical imaging is the process by which physicians evaluate an area of the subjects body that is not normally visible. ...

The use of X-rays are especially useful in the detection of pathology of the skeletal system, but are also useful for detecting some disease processes in soft tissue. Some notable examples are the very common chest X-ray, which can be used to identify lung diseases such as pneumonia, lung cancer or pulmonary oedema, and the abdominal X-ray, which can detect ileus (blockage of the intestine), free air (from visceral perforations) and free fluid (in ascites). In some cases, the use of X-rays is debatable, such as gallstones (which are rarely radiopaque) or kidney stones (which are often visible, but not always). Also, Traditional plain X-rays pose very little use in the imaging of soft tissues such as the brain or muscle. Imaging alternatives for soft tissues are computed axial tomography (CAT or CT scanning), magnetic resonance imaging (MRI) or ultrasound. Grays illustration of a human femur, a typically recognized bone. ... In medicine, the term soft tissue refers to tissues that connect, support, or surround other structures and organs of the body. ... Pneumonia (the ancient Greek word for lungs) is defined as an infection involving the alveoli of the lungs. ... Lung cancer is a malignant tumour of the lungs. ... Intestinal obstruction can be partial or complete blockage of the intestine, either large or small. ... The intestine is the portion of the alimentary canal extending from the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine. ... In medicine, gallstones are crystalline bodies formed within the body by accretion or concretion of normal or abnormal bile components. ... Radiopaque is matter that does not allow a certain amount of electromagnetism to pass through. ... Kidney stones are solid accretions (crystals) of dissolved minerals in urine found inside the kidneys or ureters. ... In the anatomy of animals, the brain, or encephalon, is the supervisory center of the nervous system. ... A top-down view of skeletal muscle Muscle is a contractile form of tissue. ... CT apparatus in a hospital Computed axial tomography (CAT), computer-assisted tomography, computed tomography, CT, or body section roentgenography is the process of using digital processing to generate a three-dimensional image of the internals of an object from a large series of two-dimensional X-ray images taken around... Magnetic Resonance Image Magnetic resonance imaging (MRI) is a method of creating images of the inside of opaque organs in living organisms as well as detecting the amount of bound water in geological structures. ... Medical ultrasonography is an ultrasound-based imaging diagnostic technique used to visualize internal organs, their size, structure and their pathological lesions. ...

X-rays are also used in "real-time" procedures such as angiography or contrast studies of the hollow organs (e.g. barium enema of the small or large intestine) using fluoroscopy. Angioplasty, medical interventions of the arterial system, rely heavily on X-ray-sensitive contrast to identify potentially treatable lesions. Angiography or arteriography is a medical imaging technique in which an X-ray picture is taken to visualize the inner opening of blood filled structures, including arteries, veins and the heart chambers. ... A barium enema, also called a lower gastrointestinal series, is a medical procedure used to examine and dignose problems with the human large intestines. ... The fluoroscope is a medical instrument used by physicians to view the internal organs of the body best described as a motion X-ray. Like an x-ray machine it takes an image of the interior of the body, but unlike the x-ray it uses a powerful radiation source... Angioplasty is the mechanical dilation of an artery that have been obstructed, generally due to atheroma (the lesion of atherosclerosis). ...

Radiotherapy, a curative medical intervention, now used almost exclusively for cancer, employs higher energies of radiation. Radiation therapy (or radiotherapy) is the medical use of ionizing radiation as part of cancer treatment to control malignant cells (not to be confused with radiology, the use of radiation in medical imaging and diagnosis). ... When normal cells are damaged or old they undergo apoptosis; cancer cells, however, avoid apoptosis. ...

History

Among the important early researchers in X-rays were Professor Ivan Pului, Sir William Crookes, Johann Wilhelm Hittorf, Eugene Goldstein, Heinrich Hertz, Philipp Lenard, Hermann von Helmholtz, Nikola Tesla, Thomas Edison, Charles Glover Barkla, Max von Laue, and Wilhelm Conrad Röntgen. Sir William Crookes (June 17, 1832 – April 4, 1919) was an English chemist and physicist. ... Among the important early researchers in X-rays were Sir William Crookes, Johann Wilhelm Hittorf, Eugene Goldstein, Heinrich Hertz, Philipp Lenard, Hermann von Helmholtz, Thomas Edison, Nikola Tesla, Charles Barkla, and Wilhelm Conrad Roentgen. ... Heinrich Hertz Heinrich Rudolf Hertz (February 22, 1857 - January 1, 1894), was the German physicist for whom the hertz, the SI unit of frequency, is named. ... Philipp Eduard Anton von Lenard (born in Bratislava on June 7, 1862 – died May 20, 1947 in Messelhausen) was a physicist and the winner of the Nobel Prize for Physics in 1905 for his research on cathode rays and the discovery of many of their properties. ... Hermann Ludwig Ferdinand von Helmholtz (August 31, 1821 – September 8, 1894) was a German physician and physicist. ... Nikola Tesla (July 9/July 10, 1856 - January 7, 1943) was a physicist, inventor, and electrical engineer of unusual intellectual brilliance and practical achievement. ... Thomas Alva Edison Thomas Alva Edison (February 11, 1847 – October 18, 1931) was an American inventor and businessman who developed many important devices. ... Charles Glover Barkla (June 7, 1877 – October 23, 1944) was an English physicist. ... Max von Laue (October 9, 1879 - April 24, 1960) was a German physicist, who studied under Max Planck. ... Wilhelm Röntgen Wilhelm Conrad Röntgen (March 27, 1845 – February 10, 1923) was a German physicist, of the University of Würzburg, who, on November 8, 1895, produced wavelengths of electromagnetic radiation that are now known as x-rays. ...

Physicist Johann Hittorf (1824 - 1914) observed tubes with energy rays extending from a negative electrode. These rays produced a fluorescence when they hit the glass walls of the tubes. In 1876 the effect was named "cathode rays" by Eugene Goldstein. Later, English physicist William Crookes investigated the effects of energy discharges on rare gases, and constructed what is called the Crookes tube. It is a glass vacuum cylinder, containing electrodes for discharges of a high voltage electric current. He found, when he placed unexposed photographic plates near the tube, that some of them were flawed by shadows, though he did not investigate this effect. In 1892, Heinrich Hertz began experimenting and demonstrated that cathode rays could penetrate very thin metal foil (such as aluminium). Philip Lenard, a student of Heinrich Hertz, further researched this effect. He developed a version of the cathode tube and studied the penetration by X-rays of various materials. Philip Lenard, though, did not realize that he was producing X-rays. In electronics, a vacuum tube (American English) or (thermionic) valve (British English) is a device generally used to amplify a signal. ... 1876 is a leap year starting on Saturday. ... 1892 was a leap year starting on Friday (see link for calendar). ... General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13 (IIIA), 3, p Density, Hardness 2700 kg/m3, 2. ...

In April 1887, Nikola Tesla began to investigate X-rays using high voltages and vacuum tubes of his own design, as well as Crookes tubes. From his technical publications, it is indicated that he invented and developed a special single-electrode X-ray tube, which differed from other X-ray tubes in having no target electrode. He stated these facts in his 1897 X-ray lecture before the New York Academy of Sciences. The principle behind these devices is nowadays called the bremsstrahlung process, in which a high-energy secondary X-ray emission is produced when charged particles (such as electrons) pass through matter. By 1892, Tesla performed several such experiments, but he did not categorize the emissions as what were later called X-rays, instead generalizing the phenomenon as radiant energy. He did not publicly declare his findings nor did he make them widely known. His subsequent X-ray experimentation by vacuum high field emissions led him to alert the scientific community to the biological hazards associated with X-ray exposure. April is the fourth month of the year in the Gregorian Calendar and one of four with the length of 30 days. ... 1887 is a common year starting on Saturday (click on link for calendar). ... 1897 was a common year starting on Friday (see link for calendar). ... Bremsstrahlung, German for braking radiation, is electromagnetic radiation produced by the acceleration of a charged particle, such as an electron, when deflected by another charged particle, such as an atomic nucleus. ... 1892 was a leap year starting on Friday (see link for calendar). ... Radiant energy is energy that is transported by waves. ...

Hermann von Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Röntgen made his discovery and announcement. It was formed on the basis of the electromagnetic theory of light (Wiedmann's Annalen, Vol. XLVIII). However, he did not work with actual X-rays.

On November 8, 1895, Wilhelm Röntgen, a German scientist, began observing and further documenting X-rays while experimenting with vacuum tubes. Röntgen, on December 28, 1895, wrote a preliminary report "On a new kind of ray: A preliminary communication". He submitted it to the Würzburg's Physical-Medical Society journal. This was the first formal and public recognition of the categorization of X-rays. Röntgen referred to the radiation as "X", to indicate that it was an unknown type of radiation. The name stuck, although (over Röntgen's great objections), many of his colleagues suggested calling them Röntgen rays. They are still referred to as Röntgen rays in many languages (for example, German: Röntgenstrahlen; Danish: Røntgenstråling, Russian: Рентген). Röntgen received the first Nobel Prize in Physics for his discovery. November 8 is the 312th day of the year (313th in leap years) in the Gregorian Calendar, with 53 days remaining. ... 1895 was a common year starting on Tuesday (see link for calendar). ... December 28 is the 362nd day of the year (363rd in leap years) in the Gregorian Calendar, with 3 days remaining. ... 1895 was a common year starting on Tuesday (see link for calendar). ... Würzburg is a city in Bavaria, Germany. ... List of Nobel Prize laureates in Physics from 1901 to the present day. ...

In 1895, Thomas Edison investigated materials' ability to fluoresce when exposed to X-rays, and found that calcium tungstate was the most effective substance. Around March 1896, the fluoroscope he developed became the standard for medical X-ray examinations. Nevertheless, Edison dropped X-ray research around 1903 after the death of Clarence Madison Dally, one of his glassblowers. Dally had a habit of testing X-ray tubes on his hands, and acquired a cancer in them so tenacious that both arms were amputated in a futile attempt to save his life[2] (http://www.ratical.org/radiation/KillingOurOwn/KOO6.html). 1895 was a common year starting on Tuesday (see link for calendar). ... Thomas Alva Edison Thomas Alva Edison (February 11, 1847 – October 18, 1931) was an American inventor and businessman who developed many important devices. ... For alternative meanings, see March (disambiguation). ... 1896 was a leap year starting on Wednesday (see link for calendar). ... 1903 has the latest occurring solstices and equinoxes for 400 years, because the Gregorian calendar hasnt had a leap year for seven years or a century leap year since 1600. ...

In 1906, physicist Charles Barkla discovered that X-rays could be scattered by gases, and that each element had a characteristic X-ray. He won the 1917 Nobel Prize in Physics for this discovery. 1906 was a common year starting on Monday (see link for calendar). ... Charles Glover Barkla (June 7, 1877 – October 23, 1944) was an English physicist. ... 1917 was a common year starting on Monday (see link for calendar). ... List of Nobel Prize laureates in Physics from 1901 to the present day. ...

The use of X-rays for medical purposes (to develop into the field of radiation therapy) was pioneered by Major John Hall-Edwards in Birmingham, England. In 1908, he had to have his left arm amputated owing to the spread of X-ray dermatitis[3] (http://www.birmingham.gov.uk/xray). Radiation therapy (or radiotherapy) is the medical use of ionizing radiation as part of cancer treatment to control malignant cells (not to be confused with radiology, the use of radiation in medical imaging and diagnosis). ... The city from above Centenary Square. ... Royal motto: Dieu et mon droit (French: God and my right) Englands location within the UK Official language English de facto Capital London de facto Largest city London Area  - Total Ranked 1st UK 130,395 km² Population  - Total (2001)  - Density Ranked 1st UK 49,138,831 377/km² Religion... 1908 is a leap year starting on Wednesday (link will take you to calendar). ...

In the 1950s X-rays were first harnessed to produce an X-ray microscope. Millennia: 1st millennium - 2nd millennium - 3rd millennium Events and trends Technology United States tests the first fusion bomb. ... An X-ray microscope uses electromagnetic radiation in the soft X-ray band to produce images of very small objects. ...

In the 1990s the Chandra X-Ray Observatory (http://chandra.harvard.edu/) was launched, allowing the exploration of the very violent processes in the universe which produce X-Rays. Unlike visible light, which is a relatively stable view of the universe, the x-ray universe is unstable, it features stars being torn apart by black holes, galactic collisions, and novas, neutron stars that build up layers of plasma that then explode into space. Events and trends Technology Explosive growth of the Internet; decrease in the cost of computers and other technology Reduction in size and cost of mobile phones leads to a massive surge in their popularity Year 2000 problem (commonly known as Y2K) Microsoft Windows operating system becomes virtually ubiquitous on IBM...

See also

• X-ray crystallography
• X-ray astronomy
• X-ray machine
• X-ray microscopy
• Geiger counter
• N-ray

X-ray crystallography is a technique in crystallography in which the pattern produced by the diffraction of x-rays through the closely spaced lattice of atoms in a crystal is recorded and then analyzed to reveal the nature of that lattice. ... X-ray astronomy is an observational branch of astronomy, which deals with the study of X-ray emission from celestial objects. ... Bertha Röntgens hand, one of the first x-rays The x-ray machine is a machine used to produce x-rays. ... X-ray microscopy is a type of microscopy which uses X-rays for image production. ... An antique Victoreen civil defense survey meter (a type of low accuracy geiger counter). ... The so-called N rays (or N-rays) were a phenomenon described by French scientist Ren -Prosper Blondlot but subsequently shown to be illusory. ...