104 
KIRTLEY F. MATHER 
Fixed to the opposite end of the transverse axis and attached 
just outside of the left hand standard is a graduated arc, gen- 
erally of about 130 degrees duration, by means of which vertical 
angles are measured. It is therefore referred to as the vertical 
arc. It moves past a shorter arc of similar curvature, which 
carries the vernier scale and is adjustable by means of a hori- 
zontal screw working through the left hand standard in much 
the same way as does the micrometer screw on the right standard. 
The base plate is firmly attached to the telescope standards 
and is primarily intended to form a straight edge, parallel with 
the line of sight through the telescope and maintaining that 
fixed relation no matter in what direction the instrument is 
pointed. To facilitate the use of the straight edge, the right 
side of the plate is beveled and graduated in linear measure; it is 
known as the fiducial edge. In the miniature alidades there is 
attached to one corner of the base plate a compass box, or de- 
clinatoire, housing a compass needle which has a possible oscilla- 
tion of about 10 degrees. The compass is not intended for the 
reading of angles of departure from magnetic north, but solely 
for the designation of the magnetic meridian; therefore the only 
markings are the zero points at either end of the needle. The 
needle is either of the tubular or the steel bar variety, mounted 
generally on a sapphire-tipped pivot, and provided with clamp- 
ing device and adjustable balance. The latter must be adjusted 
by the user for different latitudes by sliding it along the needle 
until it is properly balanced. A declinatoire so constructed as 
to make the needle easily accessible is therefore preferable. 
There is also fixed to the base plate, generally near the objective 
end of the telescope, a hulVs eye level by means of which the plane 
table on which the alidade is resting may be quickly brought 
into an approximately level attitude. 
An extra” attachment which has proved to be a great time- 
(and therefore money-) saver is the parallel rule, shown in 
figure 1. A brass straight-edge, | inch wide and as long as the 
alidade base, carries two brass bars, 1 or IJ inch long, pivoted 
near either end, with the pivot centers in a line parallel to the 
straight edge. Set in the free end of each short bar is a round 
