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.