Hippolyte Fizeau

“ 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. ”

“ The fringes remain very clear while the water is in motion. They move parallel to themselves, without disturbances, in amounts perceptibly proportional to the speed of the water. At a speed of two meters per second, the displacement is already fairly noticeable, and is perfectly measurable at water speeds between 4 and 7 meters per second. ”

“ 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 speed near the center is much greater than the median speed, which in turn is greater than the speed near the walls.
Also, the slits, placed in front of each tube to admit the light rays that will eventually cause the interference, are located in the middle of the circular end of the tubes, so that the rays traverse the central zones where the speed of the water should be greater than the median speed. ”

“ 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. ”

“ The air speed was calculated from the pressure and the dimensions of the tubes, according to the known laws pertaining to gas flow. The found value was controlled by the known capacity of the bellows, and the speed at which it had to be actuated In order to produce a fairly constant pressure at the entrance to the tubes. It was relatively easy to obtain air speeds of up to 23 meters per second and, on rare occasions, much greater speeds of uncertain value.
No appreciable displacement of the fringes was detected in any of the experiments. ”