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512 k'lCJURDSON'S BLVHBOOK OF PROJECTION
the "lens tube," "lens barrel," "slit assembly," "optical system" — all of which names are applied to this same piece of equipment by different manufacturers. It is shown in Figure 118 as a horizontal metal cylinder running from the exciter lamp shield to the sound gate.
To the left of the lens tube, in both pictures, is the exciting lamp. In Figure 118 the lamp is hidden behind its metal shield. (2) The action of the lens tube upon the exciting light is clearly diagrammed in Figure 117. The moving film appears in Figure 117 as a vertical line. In Figure 118 the film shows up white (blank leader was used to make the photograph). It is firmly clasped by the "sound gate" or "tension pad" at the point where it passes through the light coming from the lens tube. The photo-electric cell shown in the diagram cannot be seen in the picture. It is located in the photo-cell compartment just to the right of the sound gate.
(3) The sound gate holds the film in such fashion that it is free to slide downward. It is pulled downward, of course, by the sprocket that shows below and just to the left of the gate. But the pressure on the film as it passes through the gate is such that it cannot move in any other direction except downward. It cannot move forward or back out of the focal point, shown in Figure 117 as the place where the light rays from the slit assembly's objective lenses are focussed.
What the Photo-Cell Sees
(4) Assume that an opaque portion of the sound track cuts off the exciting light from the photo-cell, and is followed, a moment later, as the film moves downward, by a transparent portion. (See Figures 101 and 102). Assume that opaque and transparent portions of the sound track replace each other in the path of the exciting light a thousand times a second, as the film moves downward. In that case the photo-cell sees a light turned on and off a thousand times a second. A human eye would not see that. A human eye would be deceived by the phenomenon of "persistence of vision," which makes moving pictures possible, into seeing a steadily-burning light of