Hippolyte Fizeau

“ I conclude, then, that this hypothesis does not agree with experiment. We shall next see that, on the contrary, the third, or Fresnel's hypothesis, leads to a value of the displacement which differs very little from the result of observation.
We know that the ordinary phænomena of refraction are due to the fact that light is propagated with less velocity in the interior of a body than in a vacuum. Fresnel supposes that this change of velocity occurs because the density of the æther within a body is greater than that in a vacuum. ”

“ If the body is set in motion, only a part of the interior ether is set in motion with the body; this is the portion that has a greater density than that of the surrounding ether. The density of this mobile portion is expressed by D' - D. The other portion of the ether, which remains immobile while the motion takes place, has a density D.
What is the speed of propagation of the waves in this type of medium — a portion in motion and a portion motionless — supposing, for simplicity's sake, that the body moves in the direction of propagation of the waves? ”

“ The double transit of the light was for the purpose of augmenting the distance traversed in the medium in motion, and further to compensate entirely any accidental difference of temperature or pressure between the two tubes, from which might result a displacement of the fringes, which would be mingled with the displacement which the motion alone would have produced ”

“ But it has been possible to give the apparatus such a high degree of symmetry that there were no apparent density differences in the two tubes during the flow of air. Thus, the double observation would not have been necessary. Nonetheless, this second observation has been done for added assurance, and to allay the fear that the displacement being looked for could have been accidentally compensated by a small difference in density, which would have masked it completely.
In spite of these precautions, no fringe displacement by air motion could be found. ”

“ The greatest velocity at our command is unquestionably that of the earth in its orbit. At noon, during the period of the solstices, for instance, the direction of this motion is horizontal and from east to west; from this it follows that when a plate of glass receives a ray of light coming from the west, it ought to be considered as really moving to meet the ray with the immense velocity of 31,000 metres per second. ”

“ It follows from this, that if the fringes are not displaced when light traverses air in motion, there should, on the contrary, be a sensible displacement when the experiment is made with water, the index of refraction of which is very much greater than that of air. ”

“ Another advantage of the double trajectory by the beam is that of increasing the probable effect of motion; it is just as if the tubes were twice as long.
This arrangement also permits the use of a very simple means to make the fringes larger than they should be at the distance separating the two slits (this distance was 9 millimeters) . This is done by placing a very thick glass in front of one of the slits inclined in such a way that, because of refraction effects, the two slits would appear very close to each other. ”

“ First, Fresnel and, more recently, Doppler, Stokes, Challis and several others have published papers on this subject, but it does not seem that any of the theories so far proposed have been able to completely satisfy the physicists. Because of a lack of definite knowledge about the properties of the luminous ether and its relationship to ponderable matter, it has been necessary to introduce hypotheses, among which are those which are more or less probable, but none that can be considered as proven. ”

“ By causing the inclination of the glass to vary, the breadth of the bands may be varied at pleasure, and thus the magnitude most convenient for precisely observing their displacement may be readily given to them. ”

“ In fact, the greatest influence to be feared was that of preoccupation while measuring small quantities which involved a great deal of estimation. I believe that the results I have obtained have been free of this cause of error. ”

“ By making water move in the two tubes at the same time and in contrary directions in each, it will be seen that the effects should be added. This double current having been produced, the direction may be again reversed simultaneously in the two tubes, and the effect would again be double. ”

“ In this manner a displacement due to motion would have been detected with certainty, even if it had been accompanied by an accidental displacement due, for instance, to some defect of symmetry in the diameters or orifices of the tubes, whence would result an unequal resistance to the passage of air, and consequently a difference of density. ”

“ This inconvenience was made to disappear by placing a convergent lens behind the two slits; the bands were then observed at the point of concourse of the two rays, where the intensity of light was very considerable. ”