Invited paper| Volume 30, ISSUE 4, P397-402, June 2014

# The origins of medical physics

Published:April 07, 2014

## Abstract

The historical origins of medical physics are traced from the first use of weighing as a means of monitoring health by Sanctorius in the early seventeenth century to the emergence of radiology, phototherapy and electrotherapy at the end of the nineteenth century. The origins of biomechanics, due to Borelli, and of medical electricity following Musschenbroek's report of the Leyden Jar, are included. Medical physics emerged as a separate academic discipline in France at the time of the Revolution, with Jean Hallé as its first professor. Physiological physics flowered in Germany during the mid-nineteenth century, led by the work of Adolf Fick. The introduction of the term medical physics into English by Neil Arnott failed to accelerate its acceptance in Britain or the USA. Contributions from Newton, Euler, Bernoulli, Nollet, Matteucci, Pelletan, Gavarret, d'Arsonval, Finsen, Röntgen and others are noted. There are many origins of medical physics, stemming from the many intersections between physics and medicine. Overall, the early nineteenth-century definition of medical physics still holds today: ‘Physics applied to the knowledge of the human body, to its preservation and to the cure of its illnesses’.

## Introduction

Historical origins are often difficult to determine. The discovery of X-rays by Wilhelm Conrad Röntgen in November 1895 offers an event, seductive in its clarity, for the origin of Medical Physics. But this marks only the start of ‘medical radiation physics’ [
• Cohen M.
• Trott N.G.
Radiology, physical science, and the emergence of medical physics.
]. This present short review gives in outline the key developments in medical physics up to the end of the 19th century. The later role of physics and physicists in the introduction of ionizing and non-ionizing radiation into medicine is a subject for a separate historical review.

## Iatrophysics

Some links between physics and medicine may be found in the records from ancient civilizations [
• Keevil S.F.
Physics and medicine: a historical perspective.
]. A document from ancient Egypt mentions the treatment of breast abscesses by cauterization, and Hippocrates described how skin temperature distributions could be mapped by using wet clay. Another Greek physician, Herophilus, used a water clock to measure the pulse rate, applying the physics of time metrology to clinical assessment. The Arabic scholar Ibn al-Haytham (Alhazen) (965–1040), demonstrated by logic and experiment that the eye is simply a receiver of light, and does not emit a beam as the Greek scholars had imagined. It would be another 600 years before Kepler added anything new to this understanding, by describing the creation of an inverted image on the retina by the crystalline lens.
My own geographical and temporal point zero for medical physics is much later, in the Italian Renaissance. Santorio Santorio (1561–1636), also known as Sanctorius, was the first to take a measurement technique from physics and apply it successfully in medicine and physiology. The technique was weighing. He was appointed in 1611 as professor of theoretical medicine in Padua and soon published a slim volume, De statica medicina [
• Quincy J.
Medicina statica: being the aphorisms of sanctorius translated into English.
]. He had designed a whole-body scale, with which, for very many years, he had regularly weighed himself and everything that he ate, drank and excreted (Fig. 1). From this study he developed his theory of ‘insensible perspiration’ to account for the difference between material added and material excreted. Many later physiologists would learn the value of physiological measurement through similar weighing experiments [
• Robinson B.
Treatise on the Animal Oeconomy.
,
• Draper J.C.
Experiments on insensible perspiration.
]. His view of the body as a machine became a widely used metaphor in the succeeding decades.
The next important figure was the Italian physicist Giovanni Borelli (1608–79), who was the first to make a serious attempt to place mechanistic ideas of the body on a firm mathematical footing. The word that later became associated with these ideas is iatrophysics, ‘physics applied to medicine’, but only used in the narrow sense of a physiology that explained all the workings of the body in purely mechanistic terms.
Borelli was born in Naples on 28 January 1608. He studied mathematics in Rome and spent time as professor of mathematics at the University of Messina. In 1656, Borelli was appointed as professor of mathematics at the University of Pisa. During his 12-year stay there he made important contributions to mathematical astronomy, showing that the orbits of the moons of Jupiter were elliptical, and tracking the parabolic trajectory of a comet. Both Newton and Huygens recognized the importance of Borelli's anticipation of a gravitation force to explain these movements.
His reputation as an astronomer was established. Yet, at nearly 50 years old, he launched himself into a completely new area of study. He established his own anatomical laboratory, obtained human cadavers to dissect and brought in live animals and birds for vivisection. By the time Borelli left Pisa in 1668 he had accumulated all the experimental material that he required for his book on mathematics and physics as applied to physiology. His final years were spent in Rome where his De Motu Animalium [
• Borelli G.A.
] was published shortly after he died in 1679.
Borelli approached his subject as a mathematical physicist, not a physician. He knew that physics had converted star-gazing into astronomy, using a combination of improved experimental observation and mathematical analysis. He set about applying the same principles to physiology. Borelli's vocabulary describing the actions of living beings was exclusively mechanical: forces and moments, gravity and weight, contraction and expansion, volumes and velocities, swelling, binding and wrinkling, effervescence, mixing, and scraping. His analogies were mechanical too; pulleys and scales, goatskin bottles, sieves and balls of string. He gave an extensive analysis of the movements of muscles, and the forces they exert, when walking, lifting, flying and swimming. He included numerous detailed illustrations (Fig. 2). His was an entirely new quantitative approach to what later became known as biomechanics. Borelli also explored the causes of the internal motions of animals and correctly challenged several currently-held views. He described lung expiration as a passive process. He states that ‘respiration was not instituted to cool and ventilate the flame and heat of the heart’. He used the new alcohol thermometers of the Accademia del Cimento to show that the intra-cardiac temperature of a live stag was no different from the temperature within any other organ.
However, he struggled to describe how muscles actually work. Borelli did his best with his mechanical models but, perhaps unsurprisingly, got it wrong. According to Borelli, muscle fibres swell and become harder and tighter, which causes a contraction between the ends of the muscle. This is caused by bubbles formed when nervous juice is shaken out into the muscle fibres. Later physicists also had views about this challenging problem. Isaac Newton speculated that nervous action might be mediated by the aether. Bryan Robinson, a Dublin doctor and enthusiastic Newtonian, saw Newton's aether acting in both nerves and muscles [
• Robinson B.
Treatise on the Animal Oeconomy.
]. Newton also proposed that sight resulted from vibrations in the aether in the optic nerve, caused when a stream of high velocity light particles strikes the retina. At a time before electricity was understood, this was the only means available to Newton by which he could express concepts of nervous activity.
The next major contributions came from two outstanding eighteenth century mathematicians, who were also close friends, Daniel Bernoulli (1700–1782) and Leonhard Euler (1707–1783). Bernoulli studied medicine and then, like his father, became an academic mathematician. In 1753 he made perhaps his most significant contribution to physiology, with his pupil Daniel Passavent, by estimating the work done by the heart, calculating that ‘the daily work done by the heart is equivalent to that required to raise a weight of 144,000 pounds to a height of one foot’ (about 24,000 m kg) [
• Buess H.W.
Harvey and the fountain of modern haemodynamics by Albrecht von Haller.
]. Bernoulli thus anticipated by many decades the emergence of the concept of energy.
In 1775 Euler published an essay in which he set out the one-dimensional equations for the conservation of mass and momentum in a distensible tube. Here we find the first serious application of differential calculus to a problem in physiology. In Euler's original formulation,
$(ⅆsⅆt)+(ⅆ·νsⅆz)=0$
(1)

$2g(ⅆpⅆz)+ν(ⅆνⅆz)+(ⅆνⅆt)=0$
(2)

where s is the cross-sectional area, v the average velocity, p the pressure, g the reciprocal of the density of blood, t is the time and z the axial distance. This was a step-change in the application of mathematical physics to the inner workings of the body. Leibniz and Newton had provided new tools with which to study bodies in motion, and here we can see the first moves towards their use for physiology [
• Euler L.
Principia pro motu sanguinis per arterias determinando.
].

## The origin of medical physics

The first use of the term Medical Physics (or to put it more accurately, Physique médicale) was in Paris in 1778. The term was introduced by the general secretary of the Société royale de médecine, Félix Vicq d'Azir (1748–1794) [
• Parent A.
Félix Vicq d'Azir: anatomy, medicine and revolution.
]. Physics was explicitly included in the work of this society alongside the other basic sciences such as botany, natural history and chemistry, and reinforced in the title of its journal, Les Mémoires de médecine & de physique médicale. The contents were separated into sections, including ‘Observations of general physics applied to medicine’. Fig. 3 shows two decorative headings from the companion publication, L'Histoire de la société royale de médecine, demonstrating the complete change in political emphasis that was caused by the Revolution.
Amongst the papers we find Mauduyt de la Varenne (1732–1792) making a critical study into the medical uses of electricity [
• Mauduyt
Mémoire sur les différentes manières d'administer l'électricité, et observations sur les effets qu'elles ont produits. Extrait des Mémoires de la société royale de médecine.
]. The therapeutic use of electricity was then relatively recent, following the demonstration of charge storage in a Leyden Jar by Peter Musschenbroek in 1746. Physicists had quickly started to explore its possible medical applications. In Paris, Abbé Jean-Antoine Nollet (1700–1770) soon published his observations on the biological effects of electricity [
• Nollet Abbé
Recherches sur les Causes particulaires des Phénoménes électriques.
]. Echoing Sanctorius' experiment, Nollet showed that cats and birds all lost weight after electrification (Fig. 4), ascribing this to increased ‘insensible perspiration’. He offered this possible therapeutic method to his medical colleagues, adding ‘If the doctor's art does not reap the full benefit from what is apparently promised by a physicist, may I at least be forgiven for having believed in it, because it seemed to be true’. Jean Jallabert (1712–1768), professor of mathematics and of philosophy in Geneva, was among several who reported successful treatment of paralysis using electric shocks [
• Jallabert
Experiences sur l'electricite, avec quelques conjectures sur la cause de les effets.
]. Many, however, including Nollet, found the clinical response to be highly variable. By the time of Maudyut's review, the Italian physicist Tiberius Cavallo (1759–1809), working in London, was recommending the use of Lane's ‘electrometer’, a calibrated spark-gap, to control the strength of the shock and so improve the consistency of therapeutic outcomes [
• Cavallo T.
An essay on the theory and practice of medical electricity.
,
• Franklin B.
Description of an electrometer invented by Mr Lane; with an account of some experiments made by him with it; in a letter to Benjamin Franklin L.L.D., F.R.S.
].
The storming of the Bastille on 14 July 1789 was the trigger for events that saw France spiral into bankruptcy, chaos and terror, when everything and everyone associated with the old regime was swept away. A law passed on 7 August 1793 closed down all French academies and literary societies, including the Société royale de médecine. Lavoisier, in prison awaiting execution, wrote “(If) the physicist, … by the new avenues of possibility that his researches open up, does no more than extend human life by a few years, even a few days, he may aspire to the glorious title of benefactor of mankind” [
• Fife G.
The terror – the shadow of the guillotine: France 1792–1794.
].
When Vicq d'Azir died shortly thereafter, he left a document that would, in due course, set Paris on the road to become the leading centre for medical training and research in Europe for the first half of the nineteenth century [
• Société royale de médecine
Nouveau plan de constitution pour la médecine en France.
]. His plan recommended that basic sciences, including medical physics, should be an essential part of medical training. The plan was just a proposal, however, only a consultation document. As the Revolution in France developed, deeper political events caused the reorganisation of medical training to disappear from the agenda. Some scientists steered a careful course through the chaos. One such was the Comte de Fourcroy (1755–1809). In 1791 he launched a short-lived journal, La Médecine eclairée par les sciences physiques. In the introduction to the first issue he lays out his own vision: “The study of medicine always starts with the study of physics. It is not possible to be a doctor without being a physicist.”
Jean-Noel Hallé (1754–1822) may reasonably be named as the founding father of medical physics [
,
• Dechambre
Dictionnaire encyclopédique des sciences médicale Ser 4.
] (Fig. 5). In December 1794 he was appointed as the professor of medical physics and hygiene at the new École de santé (School of Health) in Paris. Before the Revolution, Fourcroy had invited him to set out his suggestions for a course in hygiene [
• Hallé
Exposition of a plan of a complete treatise in hygiene.
]. His was a vision within which health was as much to do with the man-made physical environment as it was with diet and exercise. Medical physics has nowadays become associated with the diagnosis and treatment of disease. For Hallé, physics was equally associated with the preservation and enhancement of a healthy life.
According to one biographer his lectures ‘concentrated principally on those phenomena of the animal body that can be reduced to the known laws of physical science’ [

Cuvier G. Recueil des éloges historiques lus dans les séances publiques de l'institut royale de France. t.3 1820 à 1827; 345–360.

]. This was demonstrated in the course syllabus [
• Hallé
• Pinel
Cours de physique médicale et d'hygiene. Plan general de l'enseignement dans l'école de santé de Paris.
]. There was a complete absence of Borelli's micro-mechanical analogies to explain secretion, digestion and excretion. The failure of such models was now considered to be complete, and explanations were being sought in chemistry and vitalism. Instead, he concentrated on the strengths of physics: mechanics applied to musculo-skeletal movement, physics of the circulation, the eye and the ear. He added general sections on the principles of applying physics to medicine, and on the core skill of animal experimentation. A section on meteorology derived from the long-established view that illness is related to weather conditions. There were sections on the effects of heat, light, electricity and magnetism on the body. His interest in urban hygiene appears in a section on fireplace design.
Eventually a definition of medical physics emerged, in the 1814 revised edition of Nysten's medical dictionary [
• Nysten P.-H.
Dictionnaire de Médecine, et des Sciences accessoires à la Médecine.
]. This definition is remarkable for its completeness, accuracy and conciseness:Physics applied to the knowledge of the human body, to its preservation and to the cure of its illnesses. (Physique appliquée à la connaisance du corps humain, à son conservasion et à la guerison de ses maladie).
Absent are the conceptual restrictions associated with iatrophysics. The horizon is lifted high above the narrow constraints of physiological mechanics. Moreover, even with the passage of over two centuries since it was composed, this definition remains as general and as true today as it was then. Only the focus for emphasis has altered. Nineteenth century medical physics was dominated by physiological physics, ‘physics applied to the knowledge of the human body’. During the twentieth century the emphasis moved towards the use of physical techniques in the diagnosis and cure of illness. Modern trends in medicine are re-emphasising the importance of a healthy life-style and environment. With an increasingly ageing population, it may be expected that the 21st century will see the full range of physical concepts and methods being applied more and more to the preservation of health, in addition to the present focus on cure of its ailments. This may range from a deeper involvement in non-ionizing radiation protection, providing biophysical judgements to interpret epidemiological ‘evidence’, through an increasing emphasis on rehabilitation engineering in all its facets, to the quantification of cellular scale forces on the cytoskeleton, with their bio-physical responses.
One small section in Hallé's course concerned animal electricity, which was the latest hot topic in medical physics since Luigi Galvani (1737–1798) published his studies in 1791. Hallé led a study into Galvanism, one of the most comprehensive independent reviews at that time [
• Hallé
Des premières expèriences faites en floreal & prairial de l'an 5, par la commission nommée pour examiner & verifier les phenomènes du galvanisme.
]. Then, shortly after Alessandro Volta (1745–1827) had demonstrated his electric pile in 1800, Hallé built his own and compared electric treatment using it with that from an electric shock [
• Sue P.
Histoire du Galvanism.
].
By the time Hallé died in 1822, medical physics had a name, a formal definition, and a clear content. But new subjects can only be considered complete if they have a continuous longevity, independent of the lives and contributions of individual scientists. This was to became true for medical physics, first in Paris, and then elsewhere in France, in other European countries and finally across the Atlantic. Hallé's successor, Pierre Pelletan, took the first full chair of medical physics, separating the subject from hygiene. Pelletan's successor, Jules Gavarret (1809–1890), made many contributions during his extraordinary career as a medical physicist. For example his first book, published in 1840, was the first to introduce the concept of inferential statistics for the analysis and comparison of medical therapies [
• Gavarret J.
Principes généraux de statistique médicale.
]. His influence facilitated the spread of medical physics to the other medical centres in France.

## Text-books on medical physics

Pelletan published the first physics textbook for medical students in 1824 [
• Pelletan fils
Traité élémentaire de physique générale et médicale.
]. In the third, 1838, edition Pelletan anticipated not only medical physics but also medical physicists: ‘It is to be hoped that a certain number of men will occupy themselves specifically with finding associations or with discovering all the possible relationships that exist between physics and the other medical sciences’.
Soon, similar physics texts for medical students were being published by doctors throughout Europe, in Spain [
• Lopez J.M.
Lecciones elementales de Física experimental con aplicación a la medicina y a las artes.
], in Britain [
• Bird G.
Elements of natural philosophy; being an experimental introduction to the physical sciences.
], and in Germany [

Heidenreich FW. Elemente einer medizinischen Physik. Erstes Buch Physik, 1843 Leipzig.

]. A growing awareness developed that physics could not be ignored, and might even be of practical benefit. In the Royal Institution in London, Michael Faraday (1791–1867) had given doctors a new and more manageable means of generating electricity for medical therapy. The Berlin physiologist Emil du Bois-Reymond (1818–1896) had shown unequivocally the electrical nature of nerve conduction and muscle contraction. Microscopic cellular structures, previously unseen and unknown, were being viewed using new achromatic lens combinations.
It was a challenge, though, in these books aimed at medical students, to keep an appropriate balance between basic physics and its new medical applications. Soon, two books were published in which the authors focussed specifically on physics applied to physiology and medicine. For the first we return to Borelli's Pisa, to a book published in 1844 by the professor of physics at the university there, Carlos Matteucci [
• Matteucci C.
Lezioni sui fenomeni fisico-chimici dei corpi viventi.
]. He had been studying the electrical discharges of the torpedo fish, and subsequently the electromechanical behaviour of frog muscle. The book resulted from a course of lectures on physics applied to living beings, requested by the Tuscan government. Matteucci claimed, ‘This is perhaps the first time that a course of lectures, under this name, has been introduced into medico-physical education’. Nor was Matteucci's book only about bioelectricity, as might be imagined, although he did devote about one third of it to this topic. In addition, he included chapters on the biophysical processes of osmosis, absorption and digestion, on nutrition related to animal heat, on the circulation and on vision, speech and hearing.
The second of these earliest true medical physics books was by the eminent German physiologist Adolf Fick (1829–1901) [
• Fick A.
Die medizinische Physik.
]. As a medical student he had come under the influence of the physiologist Carl Ludwig (1816–1895) who, after Fick's graduation, appointed him as prosector (demonstrator) in anatomy in Zurich. Not long after his arrival there in 1852, Fick published his classic mathematical analysis of diffusion [
• Fick A.
Über diffusion.
]. Still in his mid-twenties, he then accepted an invitation to write a supplement on medical physics for Müller-Pouillet’s Lehrbuch der Physik, a widely-used physics text-book. He covered diffusion, skeletal mechanics, haemodynamics, sound, heat, optics and electricity, each one including contributions from his own original research, for example, on muscle contraction, wave motion in elastic tubes, the work of the heart, respiratory and circulatory sounds, astigmatism, colour vision, and the chemical origins of animal heat.
In Fick's last chapter we can see one of the reasons for the dominance of German physiology at this time. Here he described nine instruments for physiological measurement: the planimeter, Ludwig's kymograph, the sphygmograph, the microscope, Helmholtz's ophthalmoscope and chronometer, the stereoscope, du Bois-Reymond's galvanometer and his induction apparatus for inducing tetanus. These physical instruments revolutionised physiological measurement, giving doctors the tools for measurement that would alter diagnostic medicine for ever. In 1868 Fick moved to Würzburg to become professor of physiology, staying there for the remainder of his eminent career. It was here, in 1870, that Fick proposed a technique that remains one of the standard methods by which cardiac output may be measured [
• Fick A.
Über die Messung des Blutquantums in den Herzventrikeln.
].
Medical physics failed to gain much purchase in Britain and the USA during this period. There may have been a language problem. Before 1800, the word physics was scarcely used in English, largely restricted to translations from Latin, German or French texts. The English term was ‘natural philosophy’. It was much easier for continental scientists to find phrases such as physique médicale and medizinische physik than it was for British scientists and doctors to encapsulate the broad reach of links between the two disciplines under a single title. This semantic problem did not inhibit the scientific development of each separate intersection of physics with medicine, but it did serve to delay its coherence into a separate discipline in English-speaking countries. Neil Arnott was the first to use the term medical physics in English, in 1827. Through his international best-selling book on popular science [
• Arnott N.
Elements of physics or natural philosophy, general and medical, explained independently of technical mathematics.
] he created a public understanding of the intimate link between physics and medicine. But the claim that he was the first medical physicist [
• Lenihan J.
Neil Arnott – the first medical physicist.
] is not really supported by the evidence.
In 1855, the year before Fick's Die medizinische Physik appeared in press, Gavarret published a book on animal heat under a general medical physics title [
• Gavarret J.
Physique médicale – de la Chaleur produite par les Êtres vivants.
]. It was an extensive revue of the deep physiological challenge; how do animals create and maintain their body temperature? It was a time when the general concept of energy and its conservation was being discussed by Europe's most able scientists. In his seminal 1847 analysis, Über die Erhaltung der Kraft, Hermann von Helmholtz (1821–1894) had already suggested that chemical force-equivalents, appropriate for investigating metabolic processes, could be identified, comparable with similar force-equivalents for thermal and electrical forms of energy [
• Helmholtz H.
On the conservation of force; a physical memoir.
]. By 1866, Gavarret was offering an advanced course to doctors in Paris, which he called Physique biologique, in which he further developed these ideas [
• Gavarret J.
Physique biologique. Les Phenomenes physiques de la vie.
]. He declared work (travaille) to be as universal as mass, and its conservation to be as true for living as for inorganic bodies. Gavarret was marshalling his arguments for a final push against those physiologists who still retained a belief in ‘an independent directing force in the body’. He called it ‘a mere useless hypothesis … that the vitalist school invoke to explain the phenomena of nutrition and development’.
Finally, it was not a British author who wrote the first book on medical physics in English, but an American. This was John Christopher Draper (1835–1885), a New York professor, and his book, A Text-Book of Medical Physics for the use of Students and Practitioners of Medicine, was published the year he died [
• Draper J.C.
A text-book of medical physics for the use of students and practitioners of medicine.
]. A scientist of many interests, he had been teaching physiology and chemistry in New York for many years. Draper starts his preface, ‘The fact that a knowledge of Physics is indispensable to a thorough understanding of Medicine has not yet been as fully realized in this country as in Europe’. A review for the American Medical Association reported: ‘The first work on medical physics in this country, (this book) is the forerunner of a new era in medical education’, adding ‘Thus far our colleges may, with but few exceptions, be said to have ‘thrown physics to the dogs’’ [
• Anon A.
Text-book of medical physics.
].
During the last decade of the nineteenth century there was a major re-launch of physics applied to medical treatment, following a century of intermittent developments in therapeutic medical electricity. Clearly, the origin of radiotherapy lies in Röntgen's discovery of X-rays in 1895, the discovery of radioactivity by Henri Becquerel the following year, and in Marie Curie's subsequent discovery of radium. By concentrating on these dramatic events it is easy to forget that other medical applications of physics have their origins at this same time. The therapeutic use of infrared and ultraviolet radiation was pioneered by the Nobel prize-winning Danish doctor Niels Finsen (1860–1904) [
• Finsen N.R.
Phototherapy. With an appendix on the light treatment of Lupus Tr. JH Sequeria.
]. In 1893 he introduced ‘red room’ treatment for smallpox lesions, and in 1895 he showed that lupus vulgaris, a disfiguring form of tuberculosis, responded to treatment with UV. Thus, phototherapy was born. A second new therapy arose from the discovery of radio waves in 1888 by Heinrich Hertz (1857–1894). Arséne d'Arsonval (1851–1940), professor of biophysics at the Collége de France in Paris, was investigating the biological responses of electrical currents and the problem of industrial deaths from electrocution. From the knowledge he gained he introduced the first clinical high-frequency heat therapy unit, in the Hôtel Dieu, in 1895. Thus, modern electrotherapy was born. Even medical ultrasound can trace it origin to this same period, with the discovery of piezoelectricity by Jacques and Pierre Curie in 1881 [
• Curie J.
• Curie P.
Développement, par pression, de l'électricité polaire dans les cristaux hémièdre à faces inclinées.
,
• Duck F.A.
The Curie papers on piezoelectricity.
].

## Summary

Medical physics is a multi-facetted gem, reflecting all the colours of physics into all the departments of medicine. It has illuminated the origins of many clinical specialities: cardiology, ophthalmology, orthopaedics, neurology, audiology, physiotherapy, radiology. Other specialities, medical statistics, nutrition, environmental health, dermatology, and even general surgery, have sources here. The origins of physics in modern medical imaging and treatment emerged at the end of a century in which physiological physics had previously dominated; medical physics teaching became formalized at the end of the eighteenth century, preceded by the start of medical electricity; ideas of biomechanics and iatrophysics first emerged during the seventeenth century. The origins of medical physics are as diverse as its long, rich and deep heritage: much of the story remains to be explored and retold.

## Acknowledgement

In preparing this article I have drawn strongly on the text of my book Physicists and Physicians: A History of Medical Physics from the Renaissance to Röntgen, (Institute of Physics and Engineering in Medicine, York UK, 2013. 310pp). I am very grateful to the IPEM for permission to use this text. Equations ((1), (2)) in this paper have been corrected from the book, and I am grateful to the reviewer for pointing out the error.

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Radiology, physical science, and the emergence of medical physics.
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