vity of any number of interdependent bodies cannot rise higher than the point from which it falls； he reached correct conclusions as to the general principle of the conservation of vis viva； although he did not actually prove his conclusions。 This was the first attempt to deal with the dynamics of a system。 In this work； also； was the true determination of the relation between the length of a pendulum and the time of its oscillation。 In 1681 he returned to Holland； influenced； it is believed； by the attitude that was being taken in France against his religion。 Here he continued his investigations； built his immense telescopes； and； among other things； discovered 〃polarization；〃 which is recorded in Traite de la Lumiere； published at Leyden in 1690。 Five years later he died； bequeathing his manuscripts to the University of Leyden。 It is interesting to note that he never accepted Newton's theory of gravitation as a universal property of matter。
XI。 NEWTON AND THE COMPOSITION OF LIGHT Galileo； that giant in physical science of the early seventeenth century； died in 1642。 On Christmas day of the same year there was born in England another intellectual giant who was destined to carry forward the work of Copernicus； Kepler； and Galileo to a marvellous consummation through the discovery of the great unifying law in accordance with which the planetary motions are performed。 We refer； of course； to the greatest of English physical scientists； Isaac Newton； the Shakespeare of the scientific world。 Born thus before the middle of the seventeenth century； Newton lived beyond the first quarter of the eighteenth （1727）。 For the last forty years of that period his was the dominating scientific personality of the world。 With full propriety that time has been spoken of as the 〃Age of Newton。〃 Yet the man who was to achieve such distinction gave no early premonition of future greatness。 He was a sickly child from birth； and a boy of little seeming promise。 He was an indifferent student； yet； on the other hand； he cared little for the common amusements of boyhood。 He early exhibited； however； a taste for mechanical contrivances； and spent much time in devising windmills； water…clocks； sun…dials； and kites。 While other boys were interested only in having kites that would fly； Newtonat least so the stories of a later time would have us understandcared more for the investigation of the seeming principles involved； or for testing the best methods of attaching the strings； or the best materials to be used in construction。 Meanwhile the future philosopher was acquiring a taste for reading and study； delving into old volumes whenever he found an opportunity。 These habits convinced his relatives that it was useless to attempt to make a farmer of the youth； as had been their intention。 He was therefore sent back to school； and in the summer of 1661 he matriculated at Trinity College； Cambridge。 Even at college Newton seems to have shown no unusual mental capacity； and in 1664； when examined for a scholarship by Dr。 Barrow； that gentleman is said to have formed a poor opinion of the applicant。 It is said that the knowledge of the estimate placed upon his abilities by his instructor piqued Newton； and led him to take up in earnest the mathematical studies in which he afterwards attained such distinction。 The study of Euclid and Descartes's 〃Geometry〃 roused in him a latent interest in mathematics； and from that time forward his investigations were carried on with enthusiasm。 In 1667 he was elected Fellow of Trinity College； taking the degree of M。A。 the following spring。 It will thus appear that Newton's boyhood and early manhood were passed during that troublous time in British political annals which saw the overthrow of Charles I。； the autocracy of Cromwell； and the eventual restoration of the Stuarts。 His maturer years witnessed the overthrow of the last Stuart and the reign of the Dutchman； William of Orange。 In his old age he saw the first of the Hanoverians mount the throne of England。 Within a decade of his death such scientific path…finders as Cavendish； Black； and Priestley were bornmen who lived on to the close of the eighteenth century。 In a full sense； then； the age of Newton bridges the gap from that early time of scientific awakening under Kepler and Galileo to the time which we of the twentieth century think of as essentially modern。
THE COMPOSITION OF WHITE LIGHT In December； 1672； Newton was elected a Fellow of the Royal Society； and at this meeting a paper describing his invention of the refracting telescope was read。 A few days later he wrote to the secretary； making some inquiries as to the weekly meetings of the society； and intimating that he had an account of an interesting discovery that he wished to lay before the society。 When this communication was made public； it proved to be an explanation of the discovery of the composition of white light。 We have seen that the question as to the nature of color had commanded the attention of such investigators as Huygens； but that no very satisfactory solution of the question had been attained。 Newton proved by demonstrative experiments that white light is composed of the blending of the rays of diverse colors； and that the color that we ascribe to any object is merely due to the fact that the object in question reflects rays of that color； absorbing the rest。 That white light is really made up of many colors blended would seem incredible had not the experiments by which this composition is demonstrated become familiar to every one。 The experiments were absolutely novel when Newton brought them forward； and his demonstration of the composition of light was one of the most striking expositions ever brought to the attention of the Royal Society。 It is hardly necessary to add that； notwithstanding the conclusive character of Newton's work； his explanations did not for a long time meet with general acceptance。 Newton was led to his discovery by some experiments made with an ordinary glass prism applied to a hole in the shutter of a darkened room； the refracted rays of the sunlight being received upon the opposite wall and forming there the familiar spectrum。 〃It was a very pleasing diversion；〃 he wrote； 〃to view the vivid and intense colors produced thereby； and after a time； applying myself to consider them very circumspectly； I became surprised to see them in varying form； which； according to the received laws of refraction； I expected should have been circular。 They were terminated at the sides with straight lines； but at the ends the decay of light was so gradual that it was difficult to determine justly what was their figure； yet they seemed semicircular。 〃Comparing the length of this colored spectrum with its breadth； I found it almost five times greater； a disproportion so extravagant that it excited me to a more than ordinary curiosity of examining from whence it might proceed。 I could scarce think that the various thicknesses of the glass； or the termination with shadow or darkness； could have any influence on light to produce such an effect； yet I thought it not amiss； first； to examine those circumstances； and so tried what would happen by transmitting light through parts of the glass of divers thickness； or through holes in the window of divers bigness； or by setting the prism without so that the light might pass through it and be refracted before it was transmitted through the hole； but I found none of those circumstances material。 The fashion of the colors was in all these cases the same。 〃Then I suspected whether by any unevenness of the glass or other contingent irregularity these colors might be thus dilated。 And to try this I took another prism like the former； and so placed it that the light； passing through them both； might be refracted contrary ways； and so by the latter returned into that course from which the former diverted it。 For； by this means； I thought； the regular effects of the first prism would be destroyed by the second prism； but the irregular ones more augmented by the multiplicity of refractions。 The event was that the light； which by the first prism was diffused into an oblong form； was by the second reduced into an orbicular one with as much regularity as when it did not all pass through them。 So that； whatever was the cause of that length； 'twas not any contingent irregularity。 〃I then proceeded to examine more critically what might be effected by the difference of the incidence of rays coming from divers parts of the sun； and to that end measured the several lines and angles belonging to the image。 Its distance from the hole or prism was 22 feet； its utmost length 13 1/4 inches； its breadth 2 5/8； the diameter of the hole 1/4 of an inch； the angle which the rays； tending towards the middle of the image； made with those lines； in which they would have proceeded without refraction； was 44 degrees 56'； and the vertical angle of the prism； 63 degrees 12'。 Also the refractions on both sides of the prismthat is； of the incident and emergent rayswere； as near as I could make them； equal； and consequently about 54 degrees 4'； and the rays fell perpendicularly upon the wall。 Now； subducting the diameter of the hole from the length and breadth of the image； there remains 13