Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/634

 614 L I G L I G Fresnel s equally important axes inclined to each other at angles of wave. a n values from to 90. The form of the wave-surface iu such bodies was, at least very approximately, assigned by Fresnel. This forms one of the most brilliant of his Conical many grand discoveries ; and it led to Hamilton s prediction refrae- O f the existence of the two species of conical refraction, which was experimentally verified by Lloyd. Double Fresnel also made the striking discovery that glass and refrac- other simply refracting bodies are rendered doubly refract- tioii j n g w hen in a state of strain. To this Brewster added the stress observation that the requisite strain might be produced by unequal heating instead of by mechanical stress, and also that unannealed glass is usually doubly refractive. Clerk Maxwell in 1873 (Proc. Roy. Soc.) showed that shearing stress in viscous liquids, such as Canada balsam, renders them temporarily doubly-refractive. This subject has been elaborately investigated by Kundt (Pogg. Ann., 1879). The details of these subjects, with those of the polariza tion of light reflected from small particles, the rotatory polarization produced by quartz, sugar, transparent bodies under the influence of magnetism, &c., must be deferred to OPTICS (PHYSICAL). There is, however, one elementary point which must not be omitted here, as it is intimately connected with the wave-theory, that is, the alteration which light undergoes in consequence of the relative motion of the source and spectator in the line of vision. Doppler s When a steamer is moving in a direction perpendicular principle, to the crests of the waves, she will encounter more of them in a given time if her course is towards them than if she were at rest, while, if she be moving in the same direction as the waves, fewer of them will overtake her in a given time than if she were at rest. The same thing is true of sound waves. When an express train passes a level crossing at full speed, the pitch of the steam whistle is higher during the approach to and lower during the recess from the listener at the gate than it would be if the engine were at rest. The successive sound-pulses are emitted at the same intervals as before, but from points successively nearer to or farther from the listener. Hence more or fewer reach his ear in a given time. The principle is precisely the same as that of Burner s observation of the frequency of eclipse of Jupiter s satellites, which we have already given : the number of light-waves which reach the eye per second is increased if the source is approaching, and diminished if it be receding. We are now dealing with a phenomenon which occurs some 600,000,000,000,000 times per second instead of once every forty-two hours. Now, increased wave-frequency, with unaltered velocity of light, certainly implies shorter wave-length and most probably greater refrangibility, and vice versa. There is, undoubtedly, a weak link in this reasoning, due to our ignorance of the true nature of the luminiferous medium and of the species of vibration on which light depends. If we knew some thing definite about the nature of the ether, and the mechanism of its vibrations, this weakness would be at least in part got rid of. Observation has not yet settled the question of the relative motion of bodies, the ether they contain, and the ether in free space. This principle has been applied with success by Huggins and others to find the rate at which we are approaching to or receding from different fixed stars, and the rate of motion in solar cyclones ; and it may even be applied, as was ingeniously suggested by Fox Talbot (B. A. Report, 1871), to determine (from the relative velocities of the components of a double star in the line of sight, measured by its aid) the distance of the star itself from our system. The reader is advised to consult, in connexion with the whole of the second part of this article, the previous article ETIIER. (P. G. T.) LIGHTFOOT, JOHN (1 602-1 G75), an eminent rab binical scholar, was the son of Thomas Lightfoot, vicar of Uttoxeter, Staffordshire, and was born at Stoke-upon-Trent in that county, on March 29, 1602. His school education was received at Morton Green near Congleton, Cheshire, and in June 1617 he entered Christ s College, Cambridge, where he made great progress in Latin and Greek, and was reckoned the best orator among the undergraduates. After taking his bachelor s degree, he became for some time assistant master at Repton in Derbyshire ; at the canonical age he received ordination, and shortly afterwards was appointed curate of Norton-under-Hales in Shropshire. There he attracted the notice of Sir Rowland Cotton, an amateur Hebraist of some attainments it would seem, who made him his domestic chaplain at Bellaport, and was the first to awaken his taste for rabbinical learning. Shortly after the removal of Sir Rowland with his family to London, Lightfoot followed him thither, but for some unexplained reason soon left the capital again, and, visiting his parents at Uttoxeter, took a solemn leave of them, having resolved &quot; to travel beyond the seas.&quot; An unexpected and pressing invitation induced him to change his determination, and to accept a charge at Stone in Staffordshire, where he con tinued for about two years, and where, on May 21, 1628, he married Joyce, daughter of William Crompton of Stone Park, and widow of George Copwood of Delverne, Stafford shire. From Stone he removed to Hornsey near London, for the sake of the library of Sion College, which he often had occasion to consult ; 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. During the summer and autumn of 1630 he lived at Uttoxeter, and in September of that year he was presented by Sir R. Cotton to the rectory of Ashley in Staffordshire, where he continued to discharge his pastoral duties, and to prosecute his rab binical studies, for the next twelve years. For the more uninterrupted pursuit of the latter he is said to have bought a small piece of land near his parsonage, and to have built upon it a small house &quot; containing a study and withdrawing room below, and a lodging chamber above.&quot; &quot; Here he closely followed his said studies with great delight and unwearied diligence, and did choose to lodge here very often, though it were so near to his family and parsonage house.&quot; In June 1642 he left Ashley for London ; the precise occasion of the removal is not known, but probably it arose out of the necessity for personal superintendence of 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 with a dedication to &quot; my dear and loving countrymen of the county of Stafford, and other my friends residing in the city of London.&quot; Soon after his arrival in the capital 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 March 29. It was afterwards published under the title of Elias Eedivivus, 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 judg ment was incumbent on the parliament of his own day. Lightfoot was also one of the original members of the Westminster Assembly, which held its first formal meet-