November 2, 1896.] 



KNOWLEDGE. 



261 



■wbich sets out from the sun on its journey to the earth is 

 passed on to us intact, no part, apparently, being held back 

 from us by the ether. 



The ether is all-pervading as well as all-encompassing, 

 for ethereal wave motions are passed on through solids and 

 liquids at a velocity far exceeding that proper to the 

 elasticity of the substances themselves. This is readily 

 shown in the case of bodies transparent to light, and can 

 also be shown by special devices in the case of bodies 

 transparent to the longer waves, which are detected by 

 their heating efifect. Other bodies which cannot be pene- 

 trated by these means are transparent to the very long 

 waves produced by electric surgings. That the medium 

 which transmits electric waves is the same as that which 

 transmits illuminating rays is inferred from the fact that 

 the velocity of electric waves is the same as the velocity of 

 light. Moreover, electric waves can be polarized, showing 

 that they are waves of transverse displacement. 



No evidence has been obtained of the transmission of 

 longitudinal waves in ether, from which it seems that 

 ether is incompressible, or nearly so. Were it otherwise 

 one would expect to get compression waves of ether when 

 light is reflected. 



The use of lenses is to modify the wave front of lighting 

 waves. Tbeir transmission is slower in glass than in air, 

 the ether in a sol'd being weighted or clogged, or else 

 having its stiffness relaxed. Light which passes through 

 the thick part of a lens is retarded more than that which 

 passes through the thin part, and the wave which emerges 

 from a convex lens has therefore a concave front. The 

 area of the wave front consequently contracts to a point 

 at a certain distance from the hns, and here the amplitude 

 of the wave, and consequently the illuminating effect, is 

 enormously increased. 



The mode of vibration of the ether in the plane at right 

 angles to the direction of propagation of the wave cannot 

 properly be said to be knon-n, for we are ignorant of the 

 ultimate constitution of ether. There is no doubt, however, 

 that the motions are siiccrssfidhj irjn-i'si'nftil by the ordinary 

 wave theory of light, which assumes a mechanical dis- 

 placement such as occurs m the vibrations of the rigid 

 sorts of molecular matter. According to the wave theory 

 of light, the ether receives from a glowing solid, motions 

 which vary from instant to instant. There is no regularity 

 in the succession of these motions, owing to the confused 

 jostling of the particles of the heated solid. Consequently, 

 if we consider the vibrations in a fixed plane cutting the 

 ray of light at right angles, the excursions of the ether 

 particles in successive instants undergo such permutations 

 as the following. At the first instant the particle is 

 moving clockwise in a circular orbit. This changes to an 

 ellipse of constantly increasing eccentricity, which presently 

 closes up till it becomes a straight line. At this moment 

 the particle has the rectilinear harmonic motion which 

 best satisfies the ordinary conception of a " vibration." 

 Presently, however, the line opens out into an ellipse, in 

 which the motion may be counter-clockwise, and this 

 broadens out into a circle with counter-clockwise motion 

 which closes up again, and so on through the whole round 

 of changes. This is ordinary light. If the orbit of the 

 ether particle be fixed, we have polarized light. A 

 polarized ray excites in the eye the ordinary sensation of 

 light, but the interaction of rays of polarized light is 

 distinct from the interaction between rays of ordinary 

 light. 



Suppose ordinary light to pass perpendicularly through 

 a slice of a non-isotropic crystal cut paralKl to a plane 

 which contains two axts of diilbrcnt elasticity. Any force 

 acting in a direction other than that of an axis can- 



not set up a vibration in such direction, for the force 

 of restitution does not act in the direction of displacement. 

 The motion is therefore resolved in the directions of the 

 two axes, and two waves are propagated independently 

 through the crystal, the vibration in one being a simple 

 harmonic motion in the direction of the first axis, and in 

 the other a similar motion in the direction of the other 

 axis. Each ray in the crystal is a ray of rectilinearly 

 polarized light. If one ray be stopped in its passage 

 through the crystal, the other, on emergence, gives us a 

 ray of rectilinearly polarized light travelling in air, the 

 direction of vibration remaining after emergence just as it 

 was fixed in the crystal. If another slice of crystal, 

 similarly prepared so that only light vibrating in the 

 direction of the one axis can pass, be placed in the path of 

 the ray emerging from the first crystal, then, if the position 

 of the second slice be similar to that of the first, the ray 

 passes freely through. If the second slice be then rotated 

 about the axis of the ray, less and less light passes through, 

 until, when the slice has been rotated through a right 

 angle, the light is completely stopped. 



If both the rectilinearly polarized rays be allowed to 

 emerge from a slice of crystal, they may compound 

 together to produce a ray of elliptically or circularly 

 polarized light. In this the excursion of the ether particle 

 is a circle or ellipse in which the particle follows a fixed 

 orbit. 



In such waves of light the motion of the particle 

 resembles the motion of the particle in a water wave, 

 except that the plane of motion is at right angles to the 

 direction in which the wave travels. 



THE LIVERPOOL MEETING OF THE BRITISH 

 ASSOCIATION. 



LIVERPOOL is not an ideal city from an irsthetic 

 point of view, but a considerable amount of scien- 

 tific work has been carried on there in the past, 

 and its University College is a great centre of 

 "light and leadi ig," with a professoriate of world- 

 wide renown, actively engaged in the advancement of 

 knowledge. The sixty-sixth annual meeting of the liritish 

 Association could hardly be anything else but a success 

 when held in a town owning so many men of scientific 

 eminence. So far as numbers go, the meeting ranks high 

 in the history of the Association. The largest meeting 

 was held in JIanchester in 1878, the total then being three 

 thousand eight hundred and thirty-eight ; Newcastle comes 

 next with an attendance of three thousand three hundred 

 and thirty-five in is('.;:i, and the Liverpool meeting reached 

 practically the same figure. 



The scientific meetings of the Association were held 

 this year in ten sections, viz. : — (A") Mathematical and 

 Physical Science, (B) Chemistry, (C) Geology, (I)) Zoology, 

 (E) Geography, [F) Economic Science and Statistics, (G) 

 ^Mechanical Science, (II) Anthropology, (I) Physiology, (K) 

 Botany. The president of each of tbese sections delivered 

 an address, and fifty or sixty papers were read and discussed 

 in each section. Several evtning lectures and discourses 

 were also delivered during the meeting, so that it is hopeless 

 for us to attempt to dtsciibe the work accomplished. All 

 we can do is to give a statement of the general features 

 of the meeting, and call attention to some of tlie most 

 interesting points raised. 



Sir Joseph Lister, the Prrsident of the .Association this 

 year, is also President of the Royal Society. The work for 

 which his name is honoured is the antiseptic method of oper- 

 ating and treating wounds, and it constitutes the greatest 



