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Let us give A'B' the same value as AB, that is to say, let us increase the velocity in the second case in order that the time of exposure shall be the same; we shall at once see that in the first case the object will be completely uncovered for only a very short time, while in the second the exposure will be perfect for a very appreciable period.
But the perpendicular to the motion of the crank pin is the line of the crank itself, and consequently is revolving about O with an angular velocity represented by AOa. The motion of A' is in the direction A'B', but its velocity at the instant is zero. Hence, drawing a vertical line at A', limited by the prolongation of aO, we have A'a' for the motion of the instantaneous axis.
The length DE of the curve traced by the point P will evidently be equal to A'B', the stroke of the engine, and that again to AB, the throw of the crank. The highest position of P will be that shown in the figure, determined by placing the crank vertically, as OC. At that instant the motions of C and C' are horizontal, and being inclined to CC' they must be equal.
As the shutter continues its travel the point, B', appears in its turn and becomes illuminated like the point, A'. At this moment the objective is completely uncovered; the image, A'B', is perfect, and possesses its maximum intensity.
The solution is logical, then, and will permit of obtaining excellent negatives. Let us now examine how the image, A'B', is formed. The point, A, appears first, and becomes lighter and lighter up to the moment at which all the rays that emanate from the point, A, are unveiled. The point, B', is not yet visible.
The time of exposure which is absolutely active, we propose to call effective time of exposure in contradistinction to the total time of the same. The more we increase the value of C'D', that is to say, that of the effective time, the more the ratio, C'D'/A'B', will approximate to unity, and the nearer we shall reach perfection.
Then the point, A', gradually becomes obscured and disappears; and the same is the case with all parts of A'B'. The image is developed progressively from A' to B', and makes its impression upon the sensitive plate successively a fact which, as may be conceived, may have its importance. Upon placing the guillotine shutter in the optical center of the objective, what will occur?
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