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Rh But when a flash of light like that measured passes through a refracting medium, the front waves of the flash are continually dying away, as shown at the end of the figure, and the place of each is taken by the wave following. A familiar case of this sort is seen when a stone is thrown into a pond. The front waves die out one at a time, to be followed by others, each of which goes further than its predecessor, while new waves are formed in the rear. Hence the group, as represented in the figure by the

larger waves in the middle, moves as a whole more slowly than do the individual waves. When the speed of light is measured the result is not the wave-speed as above defined, but something less, because the result depends on the time of the group passing through the medium. This lower speed is called the group-velocity of light. In a vacuum there is no dying out of the waves, so that the group-speed and the wave-speed are identical. From Michelson’s experiments it would follow that the retardation was about of the whole speed. This would indicate that in carbon bisulphide each individual light wave forming the front of a moving ray dies out in a space of about 15 wave-lengths.

.—For Foucault’s descriptions of his experiments see Comptes Rendus (September 22 and November 24, 1862), and Recueil de Travaux Scientifiques de Léon Foucault (2 vols., 4to, Paris, 1878). Cornu’s determination is found in Annales de l’Observatoire de Paris, Mémoires, vol. xiii. The works of Michelson and Newcomb are published in extenso in the Astronomical Papers of the American Ephemeris, vols. i. and ii.

LIGHTFOOT, JOHN (1602–1675), English divine and rabbinical scholar, was the son of Thomas Lightfoot, vicar of Uttoxeter, Staffordshire, and was born at Stoke-upon-Trent on the 29th of March 1602. His education was received at Morton Green near Congleton, Cheshire, and at Christ’s College, Cambridge, where he was reckoned the best orator among the undergraduates. After taking his degree he became assistant master at Repton in Derbyshire; after taking orders he was appointed curate of Norton-under-Hales in Shropshire. There he attracted the notice of Sir Rowland Cotton, an amateur Hebraist of some distinction, who made him his domestic chaplain at Bellaport. Shortly after the removal of Sir Rowland to London, Lightfoot, abandoning an intention to go abroad, accepted a charge at Stone in Staffordshire, where he continued for about two years. From Stone he removed to Hornsey, near London, for the sake of reading in the library of Sion College. His first published work, entitled Erubhin, or Miscellanies, Christian and Judaical, penned for recreation at vacant hours, and dedicated to Sir R. Cotton, appeared at London in 1629. In September 1630 he was presented by Sir R. Cotton to the rectory of Ashley in Staffordshire, where he remained until June, 1642, when he went to London, probably to superintend the publication of his next work, A Few and New Observations upon the Book of Genesis: the most of them certain; the rest, probable; all, harmless, strange and rarely heard of before, which appeared at London in that year. Soon after his arrival in London he became minister of St Bartholomew’s church, near the Exchange; and in 1643 he was appointed to preach the sermon before the House of Commons on occasion of the public fast of the 29th of March. It was published under the title of Elias Redivivus, the text being Luke i. 17; in it a parallel is drawn between the Baptist’s ministry and the work of reformation which in the preacher’s judgment was incumbent on the parliament of his own day.

Lightfoot was also one of the original members of the Westminster Assembly; his “Journal of the Proceedings of the Assembly of Divines from January 1, 1643 to December 31, 1644,” now printed in the thirteenth volume of the 8vo edition of his Works, is a valuable historical source for the brief period to which it relates. He was assiduous in his attendance, and, though frequently standing almost or quite alone, especially in the Erastian controversy, he exercised a material influence on the result of the discussions of the Assembly. In 1643 Lightfoot published A Handful of Gleanings out of the Book of Exodus, and in the same year he was made master of Catharine Hall by the parliamentary visitors of Cambridge, and also, on the recommendation of the Assembly, was promoted to the rectory of Much Munden in Hertfordshire; both appointments he retained until his death. In 1644 was published in London the first instalment of the laborious but never completed work of which the full title runs The Harmony of the Four Evangelists among themselves, and with the Old Testament, with an explanation of the chiefest difficulties both in Language and Sense: Part I. From the beginning of the Gospels to the Baptism of our Saviour. The second part From the Baptism of our Saviour to the first Passover after followed in 1647, and the third From the first Passover after our Saviour’s Baptism to the second in 1650. On the 26th of August 1645 he again preached before the House of Commons on the day of their monthly fast. His text was Rev. xx. 1, 2. After controverting the doctrine of the Millenaries, he urged various practical suggestions for the repression with a strong hand of current blasphemies, for a thorough revision of the authorized version of the Scriptures, for the encouragement of a learned ministry, and for a speedy settlement of the church. In the same year appeared A Commentary upon the Acts of the Apostles, chronical and critical; the Difficulties of the text explained, and the times of the Story cast into ''annals. From the beginning of the Book to the end of the Twelfth'' ''Chapter. With a brief survey of the contemporary Story of the'' Jews and Romans (down to the third year of Claudius). In 1647 he published The Harmony, Chronicle, and Order of the Old Testament, which was followed in 1655 by The Harmony, Chronicle, and Order of the New Testament, inscribed to Cromwell. In 1654 Lightfoot had been chosen vice-chancellor of the university of Cambridge, but continued to reside by preference at Munden, in the rectory of which, as well as in the mastership of Catharine Hall, he was confirmed at the Restoration. The remainder of his life was devoted to helping Brian Walton with the Polyglot Bible (1657) and to his own best-known work, the Horae Hebraicae et Talmudicae, in which the volume relating to Matthew appeared in 1658, that relating to Mark in 1663, and those relating to 1 Corinthians, John and Luke, in 1664, 1671 and 1674 respectively. While travelling from Cambridge to Ely (where he had been collated in 1668 by Sir Orlando Bridgman to a prebendal stall), he caught a severe cold, and died at Ely on the 6th of December 1675. The Horae Hebraicae ''et Talmudicae impensae in Acta Apostolorum et in Ep. S. Pauli'' ad Romanos were published posthumously.

The Works of Lightfoot were first edited, in 2 vols. fol., by G. Bright and Strype in 1684; the Opera Omnia, ''cura Joh. Texelii'', appeared at Rotterdam in 1686 (2 vols. fol.), and again, edited by J. Leusden, at Franeker in 1699 (3 vols. fol.). A volume of Remains was published at London in 1700. The ''Hor. Hebr. et Talm''. were also edited in Latin by Carpzov (Leipzig, 1675–1679), and again, in English, by Gandell (Oxford, 1859). The most complete edition is that of the Whole Works, in 13 vols. 8vo, edited, with a life, by R. Pitman (London, 1822–1825). It includes, besides the works already noticed, numerous sermons, letters and miscellaneous writings; and also The Temple, especially as it stood in the Days of our Saviour (London, 1650).

See D. M. Welton, John Lightfoot, the Hebraist (Leipzig, 1878),

LIGHTFOOT, JOSEPH BARBER (1828–1889), English theologian and bishop of Durham, was born at Liverpool on the 13th of April 1828. His father was a Liverpool accountant. He was educated at King Edward’s school, Birmingham, under James Prince Lee, afterwards bishop of Manchester, and had as contemporaries B. F. Westcott and E. W. Benson. In 1847 Lightfoot went up to Trinity College, Cambridge, and there read for his degree with Westcott. He graduated senior classic and 30th wrangler, and was elected a fellow of his college. From 1854 to 1859 he edited the Journal of Classical and Sacred Philology. In 1857 he became tutor and his fame as a scholar grew rapidly. He was made Hulsean professor in 1861, and shortly afterwards chaplain to the Prince Consort and honorary chaplain in ordinary to the queen. In 1866 he was Whitehall