STANDARD MODE
The short radial line represents a beam splitter that simultaneously serves to split the source light into two opposite rays, as well as to receive the two opposite rays and to send them as superimposed beams to the screen, photographic film or other detector to form interference fringes.

INTERFERENCE MODE
The black radial line represents the midpoint between the two images of the source slit that are visible from the position of the photo detector (i.e. the line of equal path length). If the Sagnac apparatus is not rotating, Ballistic, Relativistic, and Aether theories all predict that this would be the position of the center bright fringe of constructive interference.

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Ballistic Theory predicts that in the Sagnac experiment, the counterrotating beams meet at the beam splitter (represented by the short black radial line) with a constant phase relationship that is unaffected by the rate of rotation of the turntable. In other words, Ballistic Theory predicts that the amount of displacement of the interference fringes is unaffected by the rate of rotation. Since, contrary to the predictions of Ballistic Theory, the Sagnac experiment is capable of detecting a fringe displacement which is directly proportional to the rate of rotation, Ballistic Theory is disproven.

After the light has gone around the ring once, the simulation allows you to switch to a view showing how interference fringes form around the center line of the photo detector. 

If you were to peer into an actual Sagnac apparatus from the position of the photo detector, you would see two images of the source slit corresponding to the two paths which the light takes around the Sagnac ring. In the absence of rotation, the position of the central bright fringe of constructive interference will be precisely centered between the two images of the source slit. The spacing between the two images of the source slit determines the spacing between fringes. Widely spaced source images result in narrowly spaced fringes, and vice-versa. In the above simulation, a point to the "magenta" side of the central fringe is closer to the source of the counterclockwise beam; likewise, a point to the "cyan" side of the central fringe is closer to the source of the clockwise beam. Ballistic Theory predicts that the bright fringe of constructive interference will always be precisely centered between the two images of the source slit regardless of the rate of rotation. Note how, in the simulation, differing speeds of rotation have absolutely no influence on the position of the fringe maximum. This prediction of Ballistic Theory is, of course, completely contrary to fact.

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This is, of course, nonsense. In the above simulation, the Sagnac ring is pre-initialized with light waves from the turntable at rest. After starting the simulation, changes in the rotation rate of the turntable are allowed during the first ten seconds. Click on "View Screen" to see how fringes are formed on the photographic plate or screen.

As a first experiment, I suggest letting the simulation run for five seconds, then clicking once on the rotation rate spinner (either up or down). After doing this, one will then see, to the right, a rapid, continuously increasing displacement of the fringes until the total displacement reaches slightly more than one fringe width. The fringes then "snap back" to their original position (i.e. zero displacement) since the light has had a chance to catch up. After reaching steady state with a constant rotation rate, Ballistic Theory predicts that the fringe displacement will be zero.

Ballistic Theory is, of course, disproven once again.

The figure to the right was taken from G. Sagnac, Sur la preuve de la réalité de l'éther lumineux par l'expérience de l'inlerférographe tournant. Comptes Rendus de l'Academie des Sciences (Paris) 157, pp.1410-1413 (1913)

Below is a translated excerpt from the above paper:

The interferometer, already described briefly, is schematically illustrated in the figure: a horizontal rotary table (50 cm in diameter) carries, firmly screwed on it (the adjustment screws being secured by lock screws), all the optical parts as well as the source of light O, a small flashlight with a horizontal metal filament. A microscope objective C₀ projects the image of this filament through a Nicol prism N onto the horizontal slit F in the focal plane of the collimating objective C; m is a reference mirror. The vertically (per Fresnel's convention) polarized parallel beam is divided by an air gap^* beam splitter J, as in the usual interferometer of my research (Comptes rendus, v. 150, p. 1676 (1910)), which I applied to the optical study of the movements of the Earth (Congress of Brussels, Sept. 1910, v. I, p. 207; Comptes rendus, v. 152, p. 310 (1911); Le Radium, 1911, p. 1): the beam T transmitted through the air gap J reflects successively on four mirrors M and traverses the closed loop Ja₁a₂a₃a₄J of area S. The beam R which the same air gap reflects traverses the same circuit in the opposite direction. Returning to J, the beam T, again transmitted, and the beam R, again reflected, are superimposed in the same direction along T2 and R2, and form interference fringes at the principal focus of the lens L on the fine-grained photographic plate pp'.

^*air gap beam splitter = frustrated total internal reflection beam splitter