124 



KNOWLEDGE 



[June 1, 1894. 



at the beginning of this century, supposed that the motion 

 of the ether was not affected by the passage of gross 

 matter through it. but that the ether passed through solid 

 masses such as the earth or the planets fi-eely like the 

 wind through a grove of trees, or water through a wide- 

 spaced net dragged through it. The earth, according to 

 him, moved on its path, but the ether within it and around 

 it remained fixed in space. Experiments have been made 

 to test this question of the relative motion of the ether, 

 but it still remains imsettled. Some experiments seem to 

 point to one conclusion, some to the other. 



The phenomenon of aberration, which consists in a 

 periodic displacement of the position of a star as seen fi-om 

 the earth, the period of the displacement being a year, is 

 usually explained as due to the composition of the velocity 

 of light with the velocity of the earth in its orbit. Owing 

 to aberration, in order to observe a star in the direction 

 A c or B i> a telescope must be pointed in 

 the direction a d, if in the figui-e a c 

 represents the velocity of light and a b 

 the velocity of the earth. The apparent 

 position of a star is always in front of its 

 true position. A ray entering the telescope 

 tube at D will travel with the velocity of 

 light and reach the other end of the tele- 

 scope while this latter has travelled with 

 the earth from a to b, so that the direction 

 of the ray in the telescope is along n a. 

 Thus aberration is satisfactorily explained. 

 But if this is the true explanation, the 

 amount of aberration must depend on 

 the medium which is contained in the 

 tube if the ether is 

 moving matter, for 

 light considered is 

 the medium filling the telescope. 



The effect of filling the telescope tube with different 

 substances has been tried by Sir G. B. Airy at Greenwich, 

 and he found that the aberration was the same whether 

 the tube contained air or water. Now, in the wave theory 

 the index of refraction of a substance is the ratio of the 

 velocity of light in vacuum to its velocity in the substance, 

 and as the index of refraction of water is one and one- 

 third, the velocity of light in air is four- thirds of its 

 velocity in water, and the aberration with a telescope 

 containing water shoidd be four-thirds of its value under 

 ordinary conditions, on the supposition that ether is not 

 dragged along with moving matter. 



To account for the fact that the aberration remains 

 constant it has been suggested that moving matter carries 

 the ether, or a portion of it, along with it. M. Fizeau has 

 made experiments to investigate how far the velocity of 

 light is affected by the motion of the medium, /.( ., the 

 matter through which it passes. If we imagine a luminous 

 object and an observer to be fixed in position relatively to 

 the ether, then if the medium intervening is moving 

 towards the observer and it carries the ether with it, the 

 velocity of the waves towards the observer will be increased, 

 but if the ether does not move with the matter of the 

 medium the velocity of light will not be affected by the 

 motion. 



In Fizeau's experiment a pencil of rays from a narrow 

 slit was sent through two parallel tubes containing water 

 and reflected back again to the observer. The water was 

 made to flow through the tubes, in an opposite direction 

 in each, with a velocity of over twenty feet per second ; 

 and one half of the beam of light went through one of the 

 tubes travelling in the direction of the flow of water and 

 back through the other still in the direction of How, while 



not affected by the 

 the velocity of the 

 its velocity through 



the other half of the light went in a direction against the 

 flow of the water, entering through the second tube aud 

 returning by the first. The rays of light which had 

 traversed similar paths were brought to a focus by a lens, 

 and if the velocities of the rays were affected by the 

 moving water they would be affected in opposite directions, 

 and thus a difference in the time of transit would be 

 caused and a phase difference produced which should give 

 rise to interference fringes. By altering the direction of 

 the stream of water in the tubes displacement of the 

 fringes should take place if the ether is dragged along by 

 the water, and this is what Fizeau found was the case. 

 The fringes were displaced towards the right or towards 

 the left of the observer according as, in the tube placed at 

 the right of the observer, the movement of the water was 

 towards the observer or in the opposite direction. 



Interference consists in the crest of one wave coinciding 

 with the trough of another and thus producing an absence 

 of resultant effect, or in two crests coinciding and thus 

 producing heightening of effect. When a beam of light is 

 split into two portions which are made to traverse slightly 

 differing paths, and these two portions are made to meet 

 again and overlap, in some places crests may fall on 

 troughs and cause there an absence of Ught, and thus 

 lines of darkness or interference fringes may be produced. 

 This is the principle involved in Fizeau's experiment, and 

 by measuring the amount of shifting of the interference 

 bands he obtained a measure of the change produced in 

 the velocity of the light by the motion of the medium 

 traversed. From his experiment he concluded that 

 Fresnel's theory was correct, according to which the 

 ether is partly dragged along by the surrounding matter 

 in proportion to its refractive index, so that the velocity of 

 the matter need not be taken into account, or added to or 

 subtracted from the velocity of the light, but that the 

 excess of the ether within the medmm over that outside is 

 alone carried along. According to Fresnel's theory, if V 

 is the velocity of light, and i- the velocity in a transparent 

 medium, and the index of refraction of the medium is «, 

 then the velocity of light in the direction of the motion of 



I — c. This closely agrees 



the medium will be V H j — 



with Fizeau's result. 



Messrs. Michelson and Morley, in America, a few years 

 ago made experiments to test if there was any relative 

 motion between the luminiferoua ether and the earth, and 

 the conclusion they came to was that if there was any it 

 was small — that is, that the ether is carried along with the 

 earth on its course through space. This result is in 

 contradiction to the ordinarily accepted explanation of 

 aberration. The nature of the experiment consisted in 

 sending light along two paths, one in the direction of the 

 earth's motion, and the other at right angles to it, and by 

 reflection causing the rays to fall on the same point, and 

 produce interference owing to the difference in path. By 

 sending the ray which went in the direction parallel to that 

 of the earth's motion first with this motion and then 

 against it, displacement of the interference fringes should 

 take place if there is relative motion between the earth 

 and the ether, and the amount of the displacement which 

 would occur can be calculated. Mr. Michelson found that 

 the displacement was probably less than the fortieth part 

 of what it would have been if the ether was at rest and 

 the earth moved relatively to it, and certainly less than 

 the twentieth of this value. The observations were made 

 with great accuracy, and Michelson reckoned that he could 

 detect an alteration of one part m four thousand millions, 

 if it existed. This is equivalent to detecting an error of 

 less than one-thousandth of an inch in fifty miles, and 



