430 



KNOWLEDGE • 



[March 17, 1882. 



ELECTROMAGNETIC TFIEOKY OF LIGHT. 



IN n ri'cciit iminln'riif Km>wi.ki>cik you wrotci : — " A rorrodponclcnt 

 who fpvt'K 114 III) imin<\ tiHkH uk to oxplniii the* nlfctro-miif^ntir 

 thoorv of li^fhl. Wc know of no Hiich llioory. The writor, who 

 BAyN tho iMuluhitory thoory in font l>om(? Hwullowi'ii up piocomonl 

 by tin' clortro-niuj^ic'tic, knowH vnry httUi iihoiit tho tnnttor. Thn 

 oviiK'noi' for tho iindiiliitory thoory ih niinply ovorwholniinjf." 



Most will (iffroo in your two hiHt ronohiHionH ; hut H8 there in nn 

 el««ctroinn(^nptir theory of lijfht, po»»ilily iffnornuco of itn oxislenco 

 on tho pftrt of tlie moat nnmixoinnt man I know ("Oh Lord, «ir ! 

 Why there 't (ii'r%'oii well ngain." — Kn.J, may justify complinnco 

 with your namolesH corri'Hponilent'd reiiuent fornn explanation of 

 it. It ig. in fnrt, an undulatory theory, and only a mmiiticntion in 

 detail of the theorj' of lluyKhonH. Clork Maxwell supt'osted the 

 electro-mafrnetif theory a« a mounn of doterniininp the relation 

 Ijotweeu the jdienomena of elertro-magnotism and tlioso of tight, 

 hasod upon the asHumption that each of tJiese is due to certain 

 modes of motion in tho all-pervading "ether" of space, tho 

 phenomeiui of electric currents and magnets being due to streams 

 and whorls, or other bodily movements in tho substance of the 

 other ; while light is duo to vibrations to and fro in it. Dr. Thomp- 

 son writes on this subject as follows : — " Hero is evidence for think- 

 ing that mngnotism is a phenomenon of rotation, there being a 

 rotation of linmelhiufi around an axis lying in tho direction of the 

 magnetisation. Such a thoory would explain the rotation of the plane 

 of polarisation of a ray passing through a magnetic field. For a ray 

 of plane-polarised light may be conceived of as consisting of a pair 

 of (oppositely) circularly-polari.sed waves, in which the right-handed 

 rotation in one ray is periodicallycounteracted by an equal left-handed 

 rotation in the other ray, and if such a motion were imparted to a 

 medium in which thero were superposed a rotation (such as we 

 conceive to take place in every magnetic field) about the same 

 direction, one of these circularly-polarised rays would be accelerated 

 and the other retarded, so that, when they were again compounded 

 into a single plane-polarised ray, this plane would not coincide with 

 the original plane of polarisation, but would be apparently turned 

 round through an angle proportional to the superposed rotation. 

 An electric (iisi)lacement produces a magnetic force at right angles 

 to itself ; it also produces (by the peculiar action known as induction) 

 an electric force which is propagated at right angles, both to the 

 electric displacement and to the magnetic force. Now it is known 

 that in the propagation of light, the actual displacements or vibra- 

 tions which constitute the so-called ray of light are executed in 

 directious at right angles to the direction of propagation. Tliis 

 analogy is an important point in the theory, and immediately 

 suggests the question whether the respective rates of propagation 

 are the same. Now, the velocity of propagation of electro-magnetic 

 induction is that velocity " v," which represento the ratio between 

 the electrostatic and the electro-magnetic units, and which (in air) 

 is believed to be 2'9857 x 10'" centimetres per second. And the 

 velocity of light (in air) has been repeatedly measured (by Fizeau, 

 Cornu, ilichelson, and others), giving as the approximate value 

 2-9992 X 10" centimetres per second. The close agreement of these 

 figures is at least remarkable. Amongst other mathematical de- 

 ductions from the theory may be mentioned the following : — (1.) all 

 true conductors of electricity must be opaque * to light ; (II.) for 

 transparent media, the spceilic inductive capacity ought to be equal 

 to tho square of the index of refraction. Kxperiments by Gordon, 

 Boltzmann, and others, show this to be approximately true for 

 waves of very great wave-length. The values are shown below. 

 For gases tho agreement is even closei 



K. 



Flint Glass .3162 



Bisulphide of Carbon... 1-812 



Sulphur (mean) 4151 



Paraffin 2-32 



2796 

 2-606 

 4-02t 

 2-33 

 A. K. EoLi.iT. 



OUR ANCESTORS. 



ME. GRANT ALLEN, at p. 351 of Know-ledok, in describing 

 Palu'olitliic men, says their flint implements are found buried 

 under tho concreted floors of caves. This no doubt is true, but ho 

 goes on to say that, " since tho days of Palaeolithic men, Britain has 



• In some crystalline bodies which conduct electricity better in 

 one direction than in another, the opacity to light differs corre- 

 spondingly. Coloured crystals of Tourmaline conduct electricity 

 hotter across tho long axis of the crystal than along that axis. 

 Such crystals are much more opaque to light passing along the axis 

 than to light passing acros.s it. And, in the case of rays traversing 

 the crystal across the axis, tho vibrations across the axis are more 

 completely absorbed than those parallel to tho axis : whence it 

 follows thai the transmitted light will bo polarised." 



Iioen Kubmergeil beneath Roveral hundred feet of noo." Surely Mr. 

 Grant Allen can hardly expect your readers to acce|it this stat4.-mcDt. 

 That (ircat Britain may have been submerged Kinco the eurlior 

 Paluyilithic men lived upon what in now Britain is, perhaps, possible, 

 but tliat Britain has been under the sea siDco tho Palu.-ulithio 

 men of tlio caves lived here is, I think, a stretch uf the writer's 

 imagination. Close to where I live iu London, implemcntiferoua 

 gravels and sands are common, in one groat de^iosit of loam and 

 sand at Stoko Newington, Palu,'olithic implements arc only four 

 feet or less beneath tho surface, and tho surface is loam or sand 

 full of fresh water shells, with no trace whatever of the sea in any 

 part. These Palu'olithic deposits have never been under the ses. 

 Mr. Allen thero describes the mon as low-browed, fierco-jawcd, 

 crouching creatures, inferior to the existing Australians or Andamail 

 islanders, Ac, rather a complete and sweeping description, when il 

 is remembered that no complete skeleton of any individoal o), 

 Paheolithic times has yet been seen, and not enough scattered boneij 

 aro known sufTicient to build up a slngli' skeleton. ■ 



I should feel inclined to question the correctness of the greater par) 

 of the paragraph in which the above statements occur, but howeveij 

 low-browed and fierce-jawed these " black fellows " may have been! 

 1 can make one statement about the works of the Palaeolithic mecj 

 who lived where north-east London now is, that few people will b<J 

 inclined to deny who have ever seen the objects I refer to, and tha 

 statement is, the flint implements the Palipolithic men have lef 

 behind them here, are as perfect works of art in their way as any 

 thing ever made by the best artificers of ancient or modem timei 

 The implements found here are often models of symmetrical beant; 

 indicating eyes perfectly trained to the appreciation of true curve 

 and beautiful and appropriate forms. Tho workmanship of the» 

 tools shows tho work of skilful and delicate finger.-i, workroanshi 

 that to the present day simply defies the efforts towards imitatio 

 of the most accomplished modern forgers. 



That the time is immensely distant since the older Palaeolith; 

 implements were made, and that the older tools are generally rer 

 rude, 1 quite admit, but Paheolithic times lasted for a long perio* 

 and tho gravels containing the largest number of Palaeolithic iiv 

 plements have certainly never been nnder the sea since the impl- 

 ments were made. Woethinotox G. Sihth. 



Intellige.nce in Animals. — About three years ago I was livi^ i 

 in a street in Birmingham. At the back of the houses ran strips t ] 

 garden, or half garden and half courtj-ard, separated from oB 

 another at the further end by party walls about 4 ft. 6 in. hig. 

 The grating over the drain in our court was broken, and rats one 

 or twice paid us a visit, and once the cat vigorously pursued one U 

 over the court. The cat was not full grown and did not manage D ' 

 catcli the rat. which retired out of the cat's reach beneath a wate- 

 butt raised on single bricks. Pussy watched and waited forawhif, 

 but obtaining no help from human friends, suddenly jumped up ad 

 dashed over the party wall and in less than two minutes retumd 

 full speed with another cat, eager both of them apparently fora 

 renewed chase. Unfortunately the rat had meanwliile discreety 

 retired to the drain. A cat which I had in Birmingham, and anotbr 

 which I have now, both entirely untaught, have a trick of rattlig 

 the handles of the doors with their paws when wishing cither o ' 

 enter or leave the room. When in the room Kith us she, after pawig ^ 

 the handle, looks round beseechingly to us. — A. M. F. 



Reasoning in Animals. — llany most interesting cases have beo J 

 given in Knowleoge on this subject. Will you allow me to suppc] 

 ment one which, I think, surpasses all yet given. One of or 

 seamen took a large, white-coated bitch at Petropauloski. in Kai- 

 schatka. and she became a great favourite with the crew. Subs- 

 quently the ships were ordered to China, and layoff the Englia 

 Hongs at Canton. It w.as customary to allow the bitch to swii'J 

 ashore for a run, and she used to swim back to the ship afterward, 

 anil be hauled on board by placing herneck and fore-legs in a bowlinj-i? 

 knotted noose. But the tide running furiously in that part of tbij 

 river, the dog hadnochance of making the vessel by pushing off froi^ 

 the shore ahead of tho ship, but used to come down to a low wha" 

 opposite, and watch for bits of wood, &c., floating, to see which '»f»-<l 

 the tide ran, and then running up in a contrary direction, wonly 

 jump in, and come down with the tide to the ship. On one occasio'' 

 she was seen to watch for some time ; but, as it happened, nothin- 

 floated by to give her an indication of the direction of the tidi'J 

 Whereupon, slie was observed to drop down on one foreleg, ant 

 the planks of the wharf being almost Ie%-el with the water, hangth 

 free paw over into the water, by which means, apparently, sh' 

 obtained tho knowledge of the tide's direction, and, running n ■ 

 against it, came off, as usiml, quite safely. She was one of th* 

 finest water dogs 1 ever saw. I well remember her jumping ol',^ 

 our gangway at Aden, 16 feet from the water line. — Pebcival Aj 

 POTHEROILL, B.A., F.R.A.S. 



