Fig. 13 - L-Shaped Level Bubbles for Registration 
of ^ and LO in the B-8 Plotter 
The slight modifications which we made to the B-8 to permit 
accurate introduction of 'Ÿ and on the left and right plate holders are: 
(a) an extension to the plate holders to permit placing of precise L-shaped 
bubbles; 
(b) adding specially designed L-shaped bubbles to allow accurate introduction 
of tip and tilt in each camera (Fig. 13). Two 20-second bubbles were 
mounted on a stable platform in the shape of the letter L. The bubbles, 
when placed on the plate holder, are parallel to the x and y axes of 
the 9x9 inch diapositives. Each bubble can be tilted in the vertical 
direction. Drums attached to the vertical screws permit direct readings of 
to an accuracy of I e (30 seconds). 
The B-8 is very carefully calibrated. Precise grid plates are used 
to determine the zero readings, T Q and ^ Q , for both the left and the right 
plate holders and also to determine the zero value for general 
It is more convenient to use the level bubbles for the introduction 
and LU than the plotter's drums, because these drums show tip and tilt 
measured with reference to the fixed instrumental axes, whereas the horizon 
camera gives tip and tilt measured with reference to the axes of the aerial 
photograph. Orientation of these axes varies with the drift (swing) of the 
aerial camera at the moment of exposure. Since the level bubbles are located 
directly on the plate holders, they move or swing together with the diapositives 
and the data from the horizon camera can be introduced directly. If, however, 
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