STANDARD - Survey Instrument Antique Center!

83
Ing or astronomical telescopes, unless during an observation the aperture of the
objective is cut down to inch diameter, by means of a diaphragm placed in front of
it, when the image can be seen as sharply defined as those of the erecting telescope;
or the observations must be made with the shade glasses and reflecting prism alone.
7. Using the Colored Shade Glass.
a. Directions. Attach the colored glass shown in fig. 4, to the eye-piece,
to shield the eye from the sun and look directly at it, setting the cross-wires so as
to quarter it.
b. Remarks. This will be found entirely satisfactory when the sun's altitude
is so low as to enable the observer to bring his eye in apposition with the eyepiece
of the telescope with ease.
8. Using- the Diagonal Eye-piece.
a. Directions. Screw on the prism, as shown in fig. 3, to the end of the
common eye-piece. Look directly through the shade-glass, if observing in that
way, turning the prism either way so as to make it convenient to look into it. If
any trouble is experienced in finding the sun with it. let the sun first shine through
the telescope, the colored shade-glass being turned aside, till the brilliant light
perceived in the aperture of this eye-piece shows the telescope to be rightly
directed. Cover the aperture with its shade-glass and proceed.
b. Remarks. By attaching the reflecting prism to the eye-piece of the telescope,
the light is reflected at right angles to the the line of sight of the telescope,
and it thus becomes what is termed a diagonal eye-piece.
This prism can be used for direct observation when the altitude of the sun is
too great to allow the eye to be applied directly to the eye-piece of the telescope,
and not so ^reat as to bring the eye-piece too far over the plate, but through this
range of altitudes the solar screen can be used without the prism, as shown in lig. 2,
and it will usually be found advantageous to do so.
Since the prism in effect withdraws the eye about half an inch further from the
eye-piece of the telescope than its natural position, that being about the distance
traversed by the light in passing through the prism, the high magnifying power
used in C. L. Berger & Sons' transit telescopes makes the use of the reflecting
prism for direct observation a little awkward, and it will usually be found more
satisfactory when using the prism to use the solar screen with it.
9. Using the Reflecting Prism and Solar Screen combined.
a. Directions. Attach the prism, and direct the telescope as in 8. Then,
leaving the aperture of the prism uncovered, adjust the solar screen so as to receive
the images of the sun and the cross-wires, as shown in fig. 1.
b. Remarks. For observing the sun at high altitudes it will be found that in
this, otherwise most difficult of all positions, the use of the solar screen combined
with the prism will enable the engineer to make his observation with the greatest
ease and precision.
10. Making the Observations.
a. Directions. Direct the telescope to the sun, and by means of the slow
motion screws, cause the image of the cross-wires to exactly quarter the sun's image.
Read both circles and record the readings. Eefer the position of the instrument to
some fixed line, and once, after the above work, by another plate reading. Also
note and record the exact instant of time of the observation by the watch.
b. Remarks. This observation with the watch may be used as hereafter
indicated to simplify and lessen the amount of work in making the reductions. A
fair watch of ordinary accuracy is sufficient. The entire work can be carried on
without a watch at all, but it takes some more figuring.
11. Use of the Nautical Almanac.
a. Remarks. In order to use the observations, made as above directed, it
is necessary to find the sun's apparent declination
This is done as directed below.
for the time of observation.
b. Conditions. Let all the algebraic signs be carefully observed throughout
the work. Use the watch time.