102 
KIKTLEY F. MATHER 
The telescope tube is inserted in a short sleeve at its mid- 
length, which forms a part of, the transverse axis by means of 
which vertical oscillation is made possible. The telescope is 
mounted revolvably between 180-degree stops in this axis-- 
sleeve and is prevented from turning on its longitudinal axis in 
the process of focusing either by a plunger at the under side or 
by a clamp ring screwed at one end of the sleeve. At either side 
of this axis-sleeve the telescope is surrounded by a ‘Ted metah’ 
collar, accurately turned to the axis of rotation defined by the 
sleeve. These collars are for the support of the striding-level 
by means of which the telescope is brought into the plane of the 
horizon. The striding-level is removable and when not in use is 
held in a corner of the base plate by a binding post. A similar 
post is attached to the top of the axis-sleeve and when the strid- 
ing-level is put in place- for observations, as in figure 1, it is 
snapped down over the shoulder on this post, which merely 
prevents it from falling off in case the alidade is tilted and should 
not support it in any way. The wyes, trued to the same angle, 
at either end of the bubble glass then rest on the metal collars. 
The striding-level itself is a glass vial, partially filled with a 
non-freezing liquid, ground on the inside to the arc of a circle 
with long radius. 
The more uniform this curvature is throughout the length of the 
tube the more regular will be the motion of the bubble, and the greater 
the radius of curvature the greater the sensitiveness of the bubble. 
Within reasonable limits’ the more sensitive the bubble the more perfect 
the work, though a very sensitive bubble may be too unsteady for 
many purposes; on the other hand a sluggish bubble, though it may 
give the appearance of steadiness to an instrument, and an impression 
that it “keeps’’ its adjustment, is incapable of accurate work 
The line tangent to the circular arc of the tube at its middle point, 
or a line parallel to this tangent, is called the axis of the huhhle-tuhe. 
When this axis is horizontal the bubble will be in the center of the tube. 
Should the axis become slightly inclined the bubble will move toward 
the higher end of the tube in proportion to the angle made by the axis 
with the horizon. The glass tube is usually graduated on top by 
marks 0.01 feet (or 2 mm.) apart. The value of a level-bubble is usu- 
