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 observed at the same point of departure of the beams; sunlight is admitted laterally, and directed toward the tubes by its reflection on a mirror. The beams, after this double passage through the tubes, return to set up their interference pattern a short distance beyond the mirror traversed by them originally. It is here that the fringes are observed with the aid of an ocular having a divided scale.

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 fringes are then as large as they would be if the slits were really a lot closer. Also, there is no appreciable loss in intensity, and, on the contrary, the intensity can be greatly increased by increasing the light source. The size of the fringes can be varied at will by changing the inclination of the glass, and so it is possible to obtain the fringe size most convenient for observing the displacement with precision.

I want to describe now the arrangement of the tubes and apparatus to set the water in motion.

The two tubes, placed side by side, were closed at either end with a single piece of glass glued into place with gum lac, in a position perpendicular to the common direction. Near each end a branch, shaped as a rounded elbow, establishes communication with a larger tube that goes to the bottom of a flask. Consequently, there were four flasks connected to the ends of the two tubes. 5