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 procedure through several delicate observations, such as the difference in refraction between dry and humid air.

It has seemed to me that a mode of observation based on this principle is the only one that reveals the changes in speed due to motion. It consists in producing interference fringes with two light rays, after they have passed through two parallel tubes in which air and water may float at great speeds and in opposite directions. The special goal that I have tried to attain has needed several innovations, which I will indicate.

Great difficulties were encountered relative to the light intensity. The tubes, with an interior diameter of 5.3 mm, had to be traversed by the light near their center and not near their sides. Thus, the two slits had to be more elongated than usual, and, consequently, the light intensity at the point of origin of the fringes was very low.

This inconvenience was overcome by placing a convergent lens behind the slits. Then the fringes were observed at the point of the beam junction where the light intensity is fairly great.

Since the length of the tubes was fairly large, 1.487 m, it was feared that any difference in temperature or pressure between the two tubes would initiate a considerable displacement of the fringes, which in turn could completely mask the displacement due to motion.

This difficulty has been obviated by means of a telescope having a mirror at its focal point. This way each beam is forced to traverse the two tubes successively, so that both beams covered exactly the same distance, but in opposite direction. The effects produced by pressure or temperature are thus compensated. I have satisfied myself, through several experiments, that the compensation is complete, and regardless of any changes in density or temperature introduced upon the medium in one of the tubes, the fringes keep their same exact position. In this type of arrangement, the fringes should be 4