-<£•
M 1
v.
170
plate-holders are provided with a special mark that has to correspond
with a similar mark on the carrier. For the same reason each carrier is
provided with a special set of plate-holders.
The adjustment of the fixed glass-plate can be carried out with a
maximum of accuracy
when the line AB passes
through D. In general
there will be small discre
pancies which will, for
our purpose, be taken into
account. The adjustment
is separately executed for
two directions that are
perpendicular to each
other. Fig. 2 illustrates the
successive steps during the
execution.
K represents the cross
on the glass-plate which
has to be brought to coin
cidence with the (invis
ible) rotation-centre D.
At the first adjustment the measuring-mark is put as near as
possible to K and the position of the index to the divided ruler by
which the displacement can be read, be l1. Then the plate-holder is
rotated over 200 grades and M is once more as near as possible trans
ported to K. The reading be /2.
When now the carrier is set to a reading 1/2 Ui '2)» M must be
right over D. Now K can in one direction be adjusted to D and the
whole process can be repeated after rotation of the plate-holder over
100 grades.
Since the adjustment of the glass-plate must be performed by hand
it will be safe to check the first half of the operation after the execu
tion of the second.
It will be possible to use the obtained result for the adjustment of
the measuring-mark to the rotation-centre so that line AB will pass
through D. In that case the adjustment of the next plate-holders will
be much simpler.
To estimate the accuracy of the just described method the following
experiment was carried out. The glass-plate of a plate-holder was
replaced by a grid-plate, the centre of which was treated as the point
that had to coincide with D. Discrepancies between D and the line
AB were not adjusted so that results were obtained that are com
parable with practical circumstances.
The adjustment was repeated 20 times for two of the available
triangulators. To be able to judge the results of each setting the
directions were measured to the four nearest grid-points (1 to 4 in
fig. 3) by using the normal circle. From the so obtained angles
0,1
M
M represents the measuring-mark.
1st adj.
2nd
3rd
•40,
D„<?.
Reading
m 2 ReadincjJ?2
Readinqi(?,+£.)
4th M4
K4+
Fig. 2
