STANDARD - Survey Instrument Antique Center!

34
The Pancratic, or Changeable Power Eye-piece for the
Erecting Telescope
This feature, long in use in commercial articles such as field-glasses, binoculars, etc.,
can be applied, in case of surveying instruments, to eye-pieces of erecting telescopes
only, and is then desirable only in exceptional cases. Its use in a surveying instrument
cannot be considered a distinct gain, since it complicates the mechanism of the
telescope even in its simplest form by the introduction of a greater number of
pieces, besides adding to the weight of the eye-piece. In practical use it requires one
more operation than the regular eye-piece, but unless a person is thoroughly familiar
with it, not infrequently two more operations are required when focussing on an
object.
In using it the first operation will be to set the magnifying power, by means of
the movable lens, into the first position marked for it ; the second, to sharply focus the
wires ; and the third, to bring the object into view. Whenever the power has been
changed, by accident or otherwise, the wires will have to be brought again into focus
by moving the whole eye-piece in the usual manner. Owing to the greater complexity
of this eye-piece some of the lenses are not readily accessible for cleaning in the field or
in the office, so that greater care is required to preserve clearness of vision. These
lenses are likely therefore to be coated with a film after exposure, thus defeating the
very object for which they are designed, viz., to give additional light by the use of a low
power. Moreover, when applied to a transit the focal length of the object-glass has to
be shortened by about one-half inch, thereby directly lessening the power unless the
height of the standards be increased, which of course makes the instrument more topheavy.
These are distinct disadvantages in a telescope. The power chosen for the telescope
of a field instrument is generally the one best fitted for it and therefore should be
permanent. To make it changeable in order to reduce this power lessens the degree of
accuracy with which a measuring telescope can be pointed at a distant object, and will
thwart the intention of the maker, who harmonized the power of the telescope with
the sensitiveness of the levels and reading of the graduations. So, to reiterate, it is
the discerning power of a telescope, obtained by a normal aperture combined with a
sharp definition and high magnification of the object, that will make it possible to
quickly and accurately read graduated rods and see staffs at great distances. A low
power, therefore, would not reveal the capabilities of the instrument, and on this account
the desire is more often expressed to increase the normally large power as
explained above in order to read close at long range rather than to have it lessened.
The aim of the makers is to keep surveying instruments simple in design and free
from incumbrances, so that the observer's whole attention may be given to the work
before him. While we do not recommend the use of these variable power eye-pieces,
when desired they can be applied to the larger size transits and levels when made to
order.
The Graduations.
Engineers' transits have various graduations on their circles, according to the
requirements of the different branches of civil engineering. These various graduations
are read by opposite verniers, which may be either single or double. American
instruments have usually double opposite verniers, commonly reading the
circle to single minutes or to thirty seconds. For a higher grade of work, required
in the larger cities and on extended land surveys, they should, however* read to
twenty or ten seconds according to size of circle.
Transits intended for triangulation should have only single opposite verniers and
one row of figures clockwise from to 360.
All instruments desired with graduations differing from those specified in this cat-
alogue under each style and size, will be made to order only.
The customary graduations of C. L. Berger & Sons' instruments are as follows :
The circle divided to half degrees, the verniers reading to single minutes, see Fig. 1.
t i < u <
twenty minutes, " " " " thirty seconds,
"
Fig. 2.