Thomas Young (scientist)

"A permanent alteration of form limits the strength of materials with regard to practical purposes, almost as much as fracture; since, in general, the force which is capable of producing this effect is sufficient, with a small addition, to increase it till fracture takes place."

"I have resolved to confine my studies and my pen to medical subjects only. For the talents which God has not given me, I am not responsible, but those which I possess, I have hitherto cultivated and employed as diligently as my opportunities have allowed me to do ; and I shall continue to apply them with assiduity, and in tranquillity, to that profession which has constantly been the ultimate object of all my labours."

"I met with an accident about five weeks ago in London, which has prevented my walking ever since, and I think I broke one of the metatarsal bones; this has been a favourable circumstance, for it has increased my literary application in a considerable degree. I have been studying, not the theory of the winds, but of the air, and I have made observations on harmonics which I believe are new. Several circumstances unknown to the English mathematicians which I thought I had first discovered, I since find to have been discovered and demonstrated by the foreign mathematicians; in fact, Britain is very much behind its neighbours in many branches of the mathematics: were I to apply deeply to them, I would become a disciple of the French and German school; but the field is too wide and too barren for me."

"This statement appears to us to be conclusive with respect to the insufficiency of the undulatory theory, in its present state, for explaining all the phenomena of light. But we are not therefore by any means persuaded of the perfect sufficiency of the projectile system: and all the satisfaction that we have derived from an attentive consideration of the accumulated evidence, which has been brought forward, within the last ten years, on both sides of the question, is that of being convinced that much more evidence is still wanting before it can be positively decided. In the progress of scientific investigation, we must frequently travel by rugged paths, and through valleys as well as over mountains. Doubt must necessarily succeed often to apparent certainty, and must again give place to a certainty of a higher order; such is the imperfection of our faculties, that the descent from conviction to hesitation is not uncommonly as salutary, as the more agreeable elevation from uncertainty to demonstration. An example of such alternations may easily be adduced from the history of chemistry. How universally had phlogiston once expelled the aërial acid of Hooke and Mayow. How much more completely had phlogiston given way to oxygen! And how much have some of our best chemists been lately inclined to restore the same phlogiston to its lost honours! although now again they are beginning to apprehend that they have already done too much in its favour. In the mean time, the true science of chemistry, as the most positive dogmatist will not hesitate to allow, has been very rapidly advancing towards ultimate perfection."

"When I was a boy, I thought myself a man. Now that I am a man, I find myself a boy."

"The well known elevation of the pitch of wind instruments, in the course of playing, sometimes amounting to half a note, is not, as is commonly supposed, owing to any expansion of the instrument, for this should produce a contrary effect, but to the increased warmth of the air in the tube."

"It may hereafter be considered how far the excellent experiments of Count Rumford, which tend very greatly to weaken the evidence of the modern doctrine of heat, may be more or less favourable to one or the other system of light and colours."

"It is surprising that so great a mathematician as Dr. Smith could have entertained for a moment, an idea that the vibrations constituting different sounds should be able to cross each other in all directions, without affecting the same individual particles of air by their joint forces: undoubtedly they cross, without disturbing each other's progress; but this can be no otherwise effected than by each particle's partaking of both motions. If this assertion stood in need of any proof, it might be amply furnished by the phenomena of beats, and of the grave harmonics observed..."

"The fundamental doctrines of motion have [herein]... been more immediately referred to axioms simply mathematical than has hitherto been usual; and the application of these doctrines to practical purposes has [herein]... been facilitated."

"The passive of all kinds has been very fully investigated, and many new conclusions have been formed respecting it, which are of immediate importance to the architect and to the engineer, and which appear to contradict the results of some very elaborate calculations."

"The theory of waves has been much simplified, and somewhat extended, and their motions have been illustrated by experiments... A similar method of reasoning has been applied to the circulation of the blood, to the propagation of sound, either in fluids or in solids, and to the vibrations of musical chords; the general principle of a velocity, corresponding to half the height of a certain modulus, being shown to be applicable to all these cases: and a connexion has been established between the sound to be obtained from a given solid, and its strength in resisting a flexure of any kind; or, in the case of ice and water, between the sound in a solid and the compressibility in a fluid state. The doctrine of sound and of sounding bodies in general has also received some new illustrations, and the theory of music and of musical intervals has been particularly discussed."

"With respect to the mathematical part of optics, the curvature of the images, formed by lenses and mirrors, has been correctly investigated, and the inaccuracy of some former estimations has been demonstrated."

"In the department of , the phenomena of halos and parhelia have been explained, upon principles not entirely new, but long forgotten: the functions of the eye have been minutely examined, and the mode of its accommodation to the perception of objects at different distances ascertained: the various phenomena of coloured light have been copiously described, and accurately represented by coloured plates; and some new cases of the production of colours have been pointed out, and have been referred to the general law of double lights, by which a great variety of the experiments of former opticians have also been explained; and this law has been applied to the establishment of a theory of the nature of light, which satisfactorily removes almost every difficulty that has hitherto attended the subject."

"The theory of the tides has been reduced into an extremely simple form, which appears to agree better with all the phenomena, than the more intricate calculations which they have commonly been supposed to require."

"With respect to the cohesion and of liquids, I have had the good fortune to anticipate Mr. Laplace in his late researches, and I have endeavoured to show, that my assumptions are more universally applicable to the facts, than those which that justly celebrated mathematician has employed."

"I have... attempted to throw some new light on the general properties of matter in other forms: and on the doctrine of heat, which is materially concerned in them; and to deduce some useful conclusions from a comparison of various experiments on the elasticity of , on evaporation, and on the indications of s."

"have enumerated, in a compendious and systematical form, the principal facts which have been discovered with respect to galvanic electricity; and I have fortunately been able to profit by Mr. Davy's most important experiments, which have lately been communicated to the , and which have already given to this branch of science a much greater perfection, and a far greater extent, than it before possessed."

"The historical part of the work can scarcely be called new, but several of the circumstances, which are related, have escaped the notice of former writers on the history of the sciences."

"Besides these improvements,... there are others,... which may... be interesting to those... engaged in those departments... Among these may be ranked, in the division of mechanics, properly so called, a simple demonstration of the law of the force by which a body revolves in an ellipsis; another of the properties of al pendulums; an examination of the mechanism of animal motions; a comparison of the measures and weights of different countries; and a convenient estimate of the effect of human labour: with respect to architecture, a simple method of drawing the outline of a column: an investigation of the best forms for arches; a determination of the curve which affords the greatest space for turning; considerations on the structure of the joints employed in carpentry, and on the firmness of wedges; and an easy mode of forming a kirb roof: for the purposes of machinery of different kinds, an arrangement of bars for obtaining rectilinear motion; an inquiry into the most eligible proportions of wheels and s; remarks on the friction of wheel work, and of balances; a mode of finding the form of a tooth for impelling a pallet without friction; a chronometer for measuring minute portions of time; a clock ; a calculation of the effect of temperature on steel springs; an easy determination of the best line of draught for a carriage; an investigation of the resistance to be overcome by a wheel or roller; and an estimation of the ultimate pressure produced by a blow."

"In the hydraulic and optical part, may be enumerated an over flowing lamp; a simplification of the rules for finding the velocity of running water; remarks on the application of force to hydraulic machines; a mode of letting out air from water pipes; an analysis of the human voice; and some arrangements for solar s, and for other optical instruments of a similar nature."

"In the astronomical and physical division of the work, will be found a general rule for determining the correction on account of aberration; a comparison of observations on the ; a table of the order of electrical excitation; a chart of the variation of the compass, and of the ; formulae for finding the heat of summer and winter; remarks on the theory of the winds; and a comparative table of all the mechanical properties of a variety of natural bodies."

"The arrangement of the whole work is probably different... from any other... the extent of the subjects... rendered it necessary to preserve a... methodical and uniform system; and it is presumed, that this arrangement will be... of some value, especially in a work calculated to serve as a key, by means of which, access may be obtained to all the widely scattered treasures of science; and which will enable those... desirous of extending their researches in any particular department, to obtain expeditiously all the information that books can afford them."

"[T]he lectures... may be expected to remain tolerably commensurate to the state of the sciences for a much longer period; since, in investigations so intimately connected with mathematical principles, the essential improvements will always bear a very small proportion to the number of innovations. ...the references, which it contains, are... sufficient to lead those, who may consult the passages quoted, to the works of every author of eminence that has treated of the respective subjects."

"I have... begun to collect materials for a work... relating to every department of medical knowledge: ...it will be comparatively more concise than these lectures, in proportion to what has been said and written respecting physic, but, I hope, much more complete, with regard to all that is known with certainty, and can be applied with utility."

"There is no study more difficult than that of physic: it exceeds, as a science, the comprehension of the human mind: and those who blunder onwards, without attempting to understand what they see, are often very nearly on a level with those, who depend too much on imperfect generalisations, applied to facts, which can scarcely be subjected to any well marked analogy. Hence it may happen, that talents and labour may become useless for want of a proper direction... To assist in furnishing the student with a sufficient direction... is the principal object of this work."

"Physic is one of those departments, in which there is frequent necessity for the exercise of an incommunicable faculty of judgment, and a sagacity, which may be called transcendental, as extending beyond the simple combination of all that can be taught by precept. Nor is there any other mode of cultivating these powers, than by pursuing a much more extensive range of elementary study, than appears, to a common and superficial observer, to be in any way connected with the immediate objects of the profession."

"I may here refer to a curious mathematical calculation by Dr. Thomas Young, to the effect, that if three words coincide in two different languages, it is ten to one they must be derived in both cases from some parent language, or introduced in some other manner. "Six words would give more," he says, "than seventeen hundred to one, and eight near 100,000, so that in these cases the evidence would be little short of absolute certainty.""

"Reflexion, refraction, the formation of images by lenses. the mode of operation of the eye, the spectral composition and recomposition of the different kinds of light the invention of the reflecting telescope, the first foundations of colour theory, the elementary theory of the rainbow pass us by in procession, and finally come his of the colours of thin films as the origin of the next great theoretical advance, which had to await, over a hundred years, the coming of Thomas Young."

"To complete the theory of reflexion and refraction on the undulatory hypothesis, it will be necessary to show what becomes of those oblique portions of the secondary waves, diverging in all directions from every point of the reflecting or refracting surfaces... which do not conspire to form the principal wave. But to understand this, we must enter on the doctrine of the interference of the rays of light,—a doctrine we owe almost entirely to the ingenuity of Dr. Young, though some of its features may be pretty distinctly traced in the writings of Hooke, (the most ingenious man, perhaps, of his age,) and though Newton himself occasionally indulged in speculations bearing a certain relation to it. But the unpursued speculations of Newton, and the appercus of Hooke, however distinct, must not be put in competition, and, indeed, ought scarcely to be mentioned with the elegant, simple, and comprehensive theory of Young,—a theory which, if not founded in nature, is certainly one of the happiest fictions that the genius of man has yet invented to group together natural phenomena, as well as the most fortunate in the support it has unexpectedly received from whole classes of new phenomena, which at their first discovery seemed in irreconcileable opposition to it. It is, in fact, in all its applications and details one succession of felicities insomuch that we may almost be induced to say, if it be not true, it deserves to be so. The limits of this Essay, we fear, will hardly allow us to do it justice."

"Whether we regard the depth of Dr. Young's learning, the extent of his research, the accuracy of his statements, or the beauty and originality of his theoretical views, in whatever way we contemplate these Lectures, our admiration is equally excited. They embody a complete system of Mechanical Philosophy drawn from original sources, and illustrated by a hand capable of reducing them to the most perfect subjection. Unlike other popular writers, who... either take the sciences at second hand, or content themselves simply with... adopting the hypotheses of more distinguished philosophers, Dr. Young travelled over the whole literature of science, and whilst we are astonished at the rich store of materials which he has collected, we find nothing more prominent than the impress of his own acute and powerful mind."

"Thomas Young... attained equal eminence by his discoveries in connection with the undulatory theory of light, in which he was the first to assert the principle of interference, and that of transverse vibrations, and by his discovery of the key to the system of hieroglyphics. ...The remarkable fact that Young, of whom Helmholtz says that he came a generation too soon, remained scientifically unrecognised and popularly almost unknown to his countrymen, has been explained by his unfortunate manner of expression and the peculiar channels through which his labours were announced to the world. His frequently unintelligible style, his obscure and inelegant mathematics, the habitual incognito which he preserved, his modesty in replying to attacks, and his general want of method in enunciating his ideas, contrast very markedly with the writings of some of his rivals, especially in France..."

"[S]everal great names contributed, by the authority they commanded, to oppose Young's claims to originality and renown. Lord Brougham, shielded by the powerful anonymity of the ',' and ostentatiously parading the authority of Newton, submitted the views of Young to a ruthless and unfair criticism, the popular influence of which Young probably never overcame. The great authority on optics, Brewster, who has enriched that science by such a number of experiments and observations of the first importance, never really adopted the theories of Young and Fresnel. In... the science of hieroglyphics, the authority of Bunsen decided against Young and for the Frenchman Champollion. But this decision, which did so much to obscure the merits of Young, was founded on an insufficient knowledge of the dates of Young's publications."

"Thus to the plain man there may be no metaphor in... Thomas Young's "kinetic energy" and Michelangelo's figure of Leda. Placed in their customary contexts these present nothing to him but the face of literal truth. To the initiated, however, who are aware of the "gross original" senses as well as the now literal senses , they may become metaphors. There are no metaphors per se...."

"It is now more than twenty years since I somewhat rashly undertook to write the Life of Dr. Young. ...The undertaking was consequently abandoned, and it was proposed to transfer it to other hands; but it was not found easy to secure the services of a person who possessed sufficient scientific knowledge to enable him to write the life of an author whose works were so various in their character and not unfrequently so difficult to understand and analyse, as those of Dr. Young."

"In the year 1801, Young accepted the office of Professor of Natural Philosophy at the Royal Institution, which had been established in the year preceding, chiefly by the exertions of the well known Sir Benjamin Thompson, Count Rumford. ...After managing the affairs of the Institution for a few months, and commencing the editing of its Journal, he quarrelled with some of the directors and abandoned the scheme altogether. The conducting of the Journal was thenceforward entrusted to the joint care of Dr. Young and his colleague, Mr. Davy, at that time Professor of Chemistry, in whose hands and in those of his not less distinguished successor, Mr. Faraday, the chemical laboratory of the Institution has become the most celebrated in Europe. Dr. Young's first lecture was delivered on the 20th of January, 1802, and the last on the 17th of May. The whole number of lectures given during this Session was thirty-one, which was increased, by the introduction of new subjects in the following year, to sixty... his great work, entitled "A Course of Lectures on Natural Philosophy and the Mechanical Arts," which was published four years afterwards. They are divided into three parts, containing twenty lectures each. The 1st, including Mechanics, theoretical and practical ; the 2d, Hydrostatics, Hydrodynamics, Acoustics, and Optics ; the 3rd, Astronomy, the Theory of the Tides, the Properties of Matter, Cohesion, Electricity and Magnetism, the Theory of Heat and Climatology. They form altogether the most comprehensive system of Natural Philosophy, and of what the French call Physics, that has ever been published in this country; equally remarkable for precision and accuracy... and for the addition or suggestion of new matter or new views in almost every department of philosophy. ... We have heard it remarked, that no writer, on any branch of science which the lectures treat of, can safely neglect to consult them, so rich is the mine of knowledge which they contain; and it is a well known fact, that many important propositions and discoveries have been more or less clearly indicated in them, which have only been recognized or pointed out when other philosophers discovered them independently, or announced them as their own."

"Dr. Young, by his own confession, and for reasons... alluded to, was not adapted for a popular lecturer. His style was too compressed and laconic, and he had not sufficient knowledge of the intellectual habits of other men, to address himself prominently to those points of a subject where their difficulties were likely to occur. If... delivered nearly in the form in which they are printed, [the lectures] must have been generally unintelligible even to well-prepared persons, notwithstanding all the assistance which models, drawings, and diagrams could afford."

"It was the kindred science of sound which had suggested to Young his principle of interference, and he was under a similar obligation to the same science for the suggestion of the principle which formed the first step in the solution of the great problem of double refraction."

"We propose... to call the attention of our readers to some of the more remarkable Memoirs, or Philosophical Essays, of Dr. Young, which have not elsewhere been noticed; selecting those which are distinguished... or which are otherwise calculated to show the extraordinary capacity which he possessed of solving the most difficult problems in the applications of mathematics to natural philosophy, by processes apparently the most inadequate to the purpose. He never confined himself to the beaten track of a systematic investigation. We find in his writings no symmetrical formula or analytical refinements. There is no seeking after generalities, when the particular question which he has in hand does not require them; whilst every expedient is freely resorted to, however irregular and unusual, if it serves the purpose which he has in view. Important and difficult steps are passed over as manifest, terms are neglected as insignificant, analogies take the place of proofs, and we are surprised to find ourselves at the end of an investigation, even within the limits of space which would commonly be deemed hardly sufficient to master the difficulties which meet us at the beginning. But his rare sagacity hardly ever deserts him; and though he has occasionally been led to hasty and premature conclusions, or committed mistakes in numerical calculations, from the brevity and rapidity of his processes, yet nothing can be more surprising than the general soundness of his views of mechanical principles and their applications, and the correctness both of his philosophical and numerical results."

"A Memoir on Hydraulics, printed in the Philosophical Transactions for 1808, was introductory to another in the same Collection for the following year, on the Functions of the Heart and Arteries. The connection between these subjects was considered by him... sufficiently close to give them both a professional character, and thus to exempt them from the restriction which he had imposed upon the class of publications which alone should be allowed to appear under his own name. ...Few persons can be found ...with a union of acquirements so remote from each other as to be able to prosecute an inquiry of this nature, or to judge of the correctness of the conclusions to which it leads; but as such it was exactly suited to Dr. Young, who delighted in questions so obscure and difficult, where his various knowledge and bold spirit of speculation had full room for their exercise."

"The propriety of the selection which was made by the Institute of France, of Wollaston, Davy, and Young, as the most eminent representatives of English science in that age, was disputed by very few of their contemporaries... If Young held the lowest place in the order of precedency then, he unquestionably occupies the highest now. The most brilliant achievements of Davy, whether considered singly or collectively, are probably surpassed in importance by the discovery and demonstration of the interference of light; but whilst the first received the prompt and unhesitating acknowledgment of the scientific world and at once secured for their author the honours and rewards which were due to his merits, the second, even after emerging from a long period of misrepresentation and neglect, had to make its way, step by step... against the opposition of adverse and long established theories, supported by the authority of the two greatest men known to the scientific history of the past and the present age; and it only received a tardy and reluctant recognition—and that rather by implication than avowedly—when near the close of his life, the was awarded by the Royal Society to Fresnel, who completed the structure of which Dr. Young had laid the foundations. If we refer to his other scientific works, embracing so wide a range of subjects, and some...—more especially his essays on the tides and the cohesion of fluids—so remarkable for the boldness and originality of their treatment, we shall find that they were rarely read and never appreciated by his contemporaries, and even now are neither sufficiently known nor adequately valued: whilst if justice was awarded more promptly and in more liberal measure... to his hieroglyphical labours, these also were singularly unfortunate... by coming into collision with adverse claims which were most unfairly and unscrupulously urged in his own age, and not much less so... in very recent times. The great variety also of his titles to commemoration as a classical scholar and archaeologist, a medical writer, an optician, a mathematician, or a physical philosopher, increases the difficulty of judging his relative rank amongst men of celebrity, whether they were his contemporaries or not: for the position which he might not venture to claim... to any single department of human knowledge, might be readily conceded to him when his combined labours were taken into consideration."

"The first publication by Young of his theory of color appeared in a entitled, "On the Theory of Light and Colors," which Young read before the Royal Society on Nov. 12 1801. ... The fact that Young, the founder of the undulatory theory of light, in this Bakerian Lecture, in which it has been said that he laid the foundations of that doctrine, should set forth his views in a series of postulates followed by citations from the writings of Newton, to give them weight and proof, may justly surprise those who have trusted to the second-hand information derived from carelessly-complied text books and from hastily prepared popular lectures. But then, where would be the pugilistic charm of the popular lecturer on the undulatory theory of light, if Newton, his champion, the violent defender of the emanation cause, should decline to enter as a contestant? ... Young's hypothesis imagines each sensitive point of the retina to contain particles capable of vibrating in perfect unison to those vibrations causing three principal colors (red, yellow, and blue, in this the first publication of his hypothesis) "and that each of the particles is capable of being put in motion, less or more forcibly, by undulations differing less or more from a perfect unison." This would suppose such a triple molecular constitution of each nerve fibril as to cause the three species of its constituent molecules (or the atoms forming the molecules) to be in tune with the three rates of vibration corresponding respectively to the undulations of the ether causing red, yellow, and blue. He afterward says: "and each sensitive filament of the nerve may consist of three portions, one for each principal color." We have here a conception of the mode of action of an ætherial vibration on the retinal nerve fibrils which has not been described by those who have given accounts of Young's theory of color. ...the statements made by Young in the foregoing paper concerning his color hypothesis were entirely hypothetical not having been based on any observation or experiment either of his own or of others..."

"The next publication by Young on his theory of color... a paper read by him before the , on July 1, 1802... "An account of some cases of the production of colours, not hitherto described." ... Young changed his three elementary color-sensations from red, yellow, and blue, to red, green, and violet, in consequence of Dr. Wollaston's correction of the description of the prismatic spectrum." ... Wollaston... only observed imperfectly the dark lines of the spectrum, now known as Fraunhofer's lines, but he imagined he saw a spectrum... divided into four distinct and separated "primary divisions." He at once inferred and erroneously that Newton's analysis... was false; that no orange or yellow exists... but between the red and the blue there exists only a "yellowish green." ...Young made a similar but even greater error in his description... I imagine that when Wollaston's sharp eye caught the glimpse of the divided spectrum he naturally thought... that the dark lines were the dividing lines of the pure simple colors of the solar spectrum. ... Young in finally selecting red, green and violet as the three elementary color-sensations was not, as Helmholtz states, guided in their choice "by the consideration that the extreme colors of the spectrum occupied the privileged positions," but selected those colors on hearing of Wollaston's supposed complete analysis of the sun's light into red, greenish blue and violet colors, separated from each other in the spectrum by dark spaces."

"We hear no more from Young about his theory of colors until 1807, when he published the first volume of his celebrated work, "A Course of Lectures on Natural Philosophy and the Mechanical Arts." ...Young gives a concise statement of his views on the analysis of the sensations of color and supports these views with conclusive experiments with rotating colored discs; but, strange to say, he omits from this account... all mention of the physiological explanation of it which he gave in the Bakerian Lecture of 1801. ... [I]n the Natural Philosophy we read that, "The sensations of various kinds of light may also be combined in a still more satisfactory manner by painting the surface of a circle with different colours... and causing it to revolve with such rapidity, that the whole may assume the appearance of a single tint, or of a combination of tints, resulting from the mixture of the colours." These experiments were evidently first made by Young; and are fully described in the text and perfectly illustrated... in the plates of Young's work. These experiments have been carefully repeated by Helmholtz, Maxwell, and others, and of their general accuracy there is no doubt. We can readily imagine the delight with which Young must have viewed these beautiful experiments, which, however, together with other truths unfolded by him, were destined to remain unnoticed, "until a later generation, by slow degrees, arrived at the discovery of his discovery.""

"There were giants abroad in the world of science in the early days of our century, Herschel, Lagrange, and Laplace; Cuvier, Brongniart, and Lamarck; Humboldt, Goethe, Priestley—what need to extend the list?—the names crowd upon us. But among them all there was no taller intellectual figure than that of a young Quaker who came to settle in London and practise the profession of medicine in the year 1801. The name of this young aspirant to medical honors and emoluments was Thomas Young. He came fresh from professional studies at Edinburgh and on the Continent, and he had the theory of medicine at his tongue's end; yet his medical knowledge, compared with the mental treasures of his capacious intellect as a whole, was but as a drop of water in the ocean. Incidentally the young physician was prevailed upon to occupy... the chair of Natural Philosophy at the , which Count Rumford had founded, and of which Davy was then Professor of Chemistry—the institution whose glories have been perpetuated by such names as Faraday and Tyndall, and which the Briton of to-day speaks of as the "Pantheon of Science.""

"As early as 1793, when he was only twenty, Young had begun to communicate papers to the of London, which were adjudged worthy to be printed in full in the Philosophical Transactions; so it is not strange that he should have been asked to deliver the before that learned body the very first year after he came to London. The lecture was delivered November 12, 1801. Its subject was "The Theory of Light and Colors," and its reading marks an epoch in physical science; for here for the first time was brought forward convincing proof of that undulatory theory of light... which holds that light is not a corporeal entity, but a mere pulsation in the substance of an all-pervading ether, just as sound is a pulsation in the air, or in liquids or solids. Young had... advocated this theory at an earlier date, but it was not until 1801 that he hit upon the idea which enabled him to bring it to anything approaching a demonstration. It was while pondering over the familiar but puzzling phenomena of colored rings into which white light is broken when reflected from thin films—...—that an explanation occurred to him which at once put the entire undulatory theory on a new footing. With that sagacity of insight which we call genius, he saw of a sudden that the phenomena could be explained by supposing that when rays of light fall on a thin glass part of the rays being reflected from the upper surface other rays reflected from the lower surface might be so retarded in their course through the glass that the two sets would interfere... By following up this clew with mathematical precision, measuring the exact thickness of the plate and the space between the different rings of color, Young was able to show mathematically what must be the length of pulsation for each of the different colors of the spectrum. He estimated that the undulations of red light... must number about 37,640 to the inch, and pass any given spot at a rate of 463 millions of millions of undulations in a second, while the extreme violet numbers 59,750 undulations to the inch or 735 millions of millions to the second."

"Young... examined the colors that are produced by scratches on a smooth surface, in particular testing the light from "Mr. Coventry's exquisite micrometers," which consist of lines scratched on glass at measured intervals. These microscopic tests brought the same results as the other experiments. The colors were produced at certain definite and measurable angles, and the theory of interference of undulations explained them perfectly, while, as Young affirmed... no other theory hitherto advanced could expIain them at all. Taking all the evidence together Young declared that he considered the argument he had set forth in favor of the undulatory theory of light to be sufficient and decisive."

"This doctrine of interference of undulations was the absolutely novel part of Young's theory. The all compassing genius of Robert Hooke had... very nearly apprehended it more than a century before, as Young himself points out, but... even with the sagacious Hooke it was only a happy guess... and utterly ignored by all others. Young did not know of Hooke's guess until he himself had fully formulated the theory, but he hastened then to give his predecessor all the credit that could possibly be adjudged his due... To Hooke's contemporary, Huyghens, who was the originator of the general doctrine of undulation as the explanation of light, Young renders full justice also. For himself he claims only the merit of having demonstrated the theory which these and a few others of his predecessors had advocated without full proof."