5
In agreement with the features of the actual photographs chosen
for the later tests, the principal distance set to instrument for these
bridgings was / = 123,6 mm, the scale of the model Z/f = 3,3 and
the base ratio £/Z = 0,6.
To perform the first bridging it was obviously necessary to carry
out the preliminary adjustment of the control device. This was per
formed, according to the manufacturer’s instructions (2) by verifying
with a centesimal comparator verticality and alignment of the alumi-
nated mirror in respect to the y plate holder and then, on the basis of
the preceding researches, to adjust the grid and levels forming the said
device.
The succession of the various bridging operations by means of the
control device, was of course that suggested by the manufacturers (2).
After effecting the absolute ideal orientation of the first stereogram
formed by the two grids, we recorded the values taken on in the right
hand camera by the three parameters w, 9 and x. Then we introduced in
the lefthand camera the three values recorded before and set the control
device onto the righthand camera and centred the cross-levels by mens
of their relative screws. We then substituted the righthand grid to the
lefthand camera, and, transferring the device onto the lefthand camera,
we checked the centering of the said levels and observed through the
eyepiece the image of the grid reflectd by the mirror.
When the centering of the levels and the grid image reflected by
the mirror presented some discrepancies, we eliminated them by moving
the controls of w, 9, x rotations and recording the new values.
All these operations were repeated 10 times in succession, to make
out as many stereoscopic models.
During this test a very great interest was annexed to examining
the differences noted on the w, 9, x parameters by effect of the
intervention of the control device in transferring the grids from the
righthand to the lefthand camera.
As may be deduced from the following table, they resulted in prac
tice to be within the limits of precision of the scale readings as their
entity reached the first centesimal at the most, i.e. the minimum value
of the graduation interval. The only objection that may be made is
that the relative sign remained more or less constant.