e until the middle of the seventeenth century。 The first powerful telescope of this type was made by Huygens and his brother。 It was of twelve feet focal length； and enabled Huygens to discover a new satellite of Saturn； and to determine also the true explanation of Saturn's ring。 It was Huygens； together with Malvasia and Auzout； who first applied the micrometer to the telescope； although the inventor of the first micrometer was William Gascoigne； of Yorkshire； about 1636。 The micrometer as used in telescopes enables the observer to measure accurately small angular distances。 Before the invention of the telescope such measurements were limited to the angle that could be distinguished by the naked eye； and were； of course； only approximately accurate。 Even very careful observers； such as Tycho Brahe； were able to obtain only fairly accurate results。 But by applying Gascoigne's invention to the telescope almost absolute accuracy became at once possible。 The principle of Gascoigne's micrometer was that of two pointers lying parallel； and in this position pointing to zero。 These were arranged so that the turning of a single screw separated or approximated them at will； and the angle thus formed could be determined with absolute accuracy。 Huygens's micrometer was a slip of metal of variable breadth inserted at the focus of the telescope。 By observing at what point this exactly covered an object under examination； and knowing the focal length of the telescope and the width of the metal； he could then deduce the apparent angular breadth of the object。 Huygens discovered also that an object placed in the common focus of the two lenses of a Kepler telescope appears distinct and clearly defined。 The micrometers of Malvasia； and later of Auzout and Picard； are the development of this discovery。 Malvasia's micrometer； which he described in 1662； consisted of fine silver wires placed at right…angles at the focus of his telescope。 As telescopes increased in power； however； it was found that even the finest wire； or silk filaments； were much too thick for astronomical observations； as they obliterated the image； and so； finally； the spider…web came into use and is still used in micrometers and other similar instruments。 Before that time； however； the fine crossed wires had revolutionized astronomical observations。 〃We may judge how great was the improvement which these contrivances introduced into the art of observing；〃 says Whewell； 〃by finding that Hevelius refused to adopt them because they would make all the old observations of no value。 He had spent a laborious and active life in the exercise of the old methods； and could not bear to think that all the treasures which he had accumulated had lost their worth by the discovery of a new mine of richer ones。〃＇1＇ Until the time of Newton； all the telescopes in use were either of the Galilean or Keplerian type； that is； refractors。 But about the year 1670 Newton constructed his first reflecting telescope； which was greatly superior to； although much smaller than； the telescopes then in use。 He was led to this invention by his experiments with light and colors。 In 1671 he presented to the Royal Society a second and somewhat larger telescope； which he had made； and this type of instrument was little improved upon until the introduction of the achromatic telescope； invented by Chester Moor Hall in 1733。 As is generally known； the element of accurate measurements of time plays an important part in the measurements of the movements of the heavenly bodies。 In fact； one was scarcely possible without the other； and as it happened it was the same man； Huygens； who perfected Kepler's telescope and invented the pendulum clock。 The general idea had been suggested by Galileo； or； better perhaps； the equal time occupied by the successive oscillations of the pendulum had been noted by him。 He had not been able； however； to put this discovery to practical account。 But in 1656 Huygens invented the necessary machinery for maintaining the motion of the pendulum and perfected several accurate clocks。 These clocks were of invaluable assistance to the astronomers； affording as they did a means of keeping time 〃more accurate than the sun itself。〃 When Picard had corrected the variation caused by heat and cold acting upon the pendulum rod by combining metals of different degrees of expansibility； a high degree of accuracy was possible。 But while the pendulum clock was an unequalled stationary time…piece； it was useless in such unstable situations as； for example； on shipboard。 But here again Huygens played a prominent part by first applying the coiled balance…spring for regulating watches and marine clocks。 The idea of applying a spring to the balance…wheel was not original with Huygens； however； as it had been first conceived by Robert Hooke； but Huygens's application made practical Hooke's idea。 In England the importance of securing accurate watches or marine clocks was so fully appreciated that a reward of L20；000 sterling was offered by Parliament as a stimulus to the inventor of such a time…piece。 The immediate incentive for this offer was the obvious fact that with such an instrument the determination of the longitude of places would be much simplified。 Encouraged by these offers； a certain carpenter named Harrison turned his attention to the subject of watch…making； and； after many years of labor； in 1758 produced a spring time…keeper which； during a sea…voyage occupying one hundred and sixty…one days； varied only one minute and five seconds。 This gained for Harrison a reward Of L5000 sterling at once； and a little later L10；000 more； from Parliament。 While inventors were busy with the problem of accurate chronometers； however； another instrument for taking longitude at sea had been invented。 This was the reflecting quadrant； or sextant； as the improved instrument is now called； invented by John Hadley in 1731； and independently by Thomas Godfrey； a poor glazier of Philadelphia； in 1730。 Godfrey's invention； which was constructed on the same principle as that of the Hadley instrument； was not generally recognized until two years after Hadley's discovery； although the instrument was finished and actually in use on a sea…voyage some months before Hadley reported his invention。 The principle of the sextant； however； seems to have been known to Newton； who constructed an instrument not very unlike that of Hadley； but this invention was lost sight of until several years after the philosopher's death and some time after Hadley's invention。 The introduction of the sextant greatly simplified taking reckonings at sea as well as facilitating taking the correct longitude of distant places。 Before that time the mariner was obliged to depend upon his compass； a cross…staff； or an astrolabe； a table of the sun's declination and a correction for the altitude of the polestar； and very inadequate and incorrect charts。 Such were the instruments used by Columbus and Vasco da Gama and their immediate successors。 During the Newtonian period the microscopes generally in use were those constructed of simple lenses； for although compound microscopes were known； the difficulties of correcting aberration had not been surmounted； and a much clearer field was given by the simple instrument。 The results obtained by the use of such instruments； however； were very satisfactory in many ways。 By referring to certain plates in this volume； which reproduce illustrations from Robert Hooke's work on the microscope； it will be seen that quite a high degree of effectiveness had been attained。 And it should be recalled that Antony von Leeuwenboek； whose death took place shortly before Newton's； had discovered such micro…organisms as bacteria； had seen the blood corpuscles in circulation； and examined and described other microscopic structures of the body。
XIV。 PROGRESS IN ELECTRICITY FROM GILBERT AND VON GUERICKE TO FRANKLIN We have seen how Gilbert； by his experiments with magnets； gave an impetus to the study of magnetism and electricity。 Gilbert himself demonstrated some facts and advanced some theories； but the system of general laws was to come later。 To this end the discovery of electrical repulsion； as well as attraction； by Von Guericke； with his sulphur ball； was a step forward； but something like a century passed after Gilbert's beginning before anything of much importance was done in the field of electricity。 In 1705； however； Francis Hauksbee began a series of experiments that resulted in some startling demonstrations。 For many years it had been observed that a peculiar light was seen sometimes in the mercurial barometer； but Hauksbee and the other scientific investigators supposed the radiance to be due to the mercury in a vacuum； brought about； perhaps； by some agitation。 That this light might have any connection with electricity did not； at first； occur to Hauksbee any more than it had to his predecessors。 The problem that interested him was whether the vacuum in the tube of the barometer was essential to the light； and in experimenting to determine this； he invented his 〃mercurial fountain。〃 Having exhausted the air in a receiver containing some mercury； he found that by allowing air to rush through the mercury the metal became a jet thrown