Schaeberle—Collimation Constant of a Transit Circle. 145 
strument, is believed to have some advantages over the ones 
already mentioned. 
such a position that when the telescope is — to the mirror 
is then brought into coincidence with its reflected image by 
slightly rotating either the telescope or the hanging collimator 
about the horizontal axis. 
et m,=the distance of the middle transit wire from its 
reflected image; positive when the image is on that side of the 
wire which is toward the clamp. m,=corresponding distance 
after the collimator has been reversed. c=collimation constant 
—then 
m, +m, 
4 
The collimation constant is therefore positive (for clamp west) 
so long as the algebraic sum of the distances m, and m, is a 
negative quantity. 
If the pivots on which the instrument rotates are not of the 
same diameter, the expression for ¢ will evidently become 
+ 
p being the angle included between the axis of rotation and a 
rectilinear surface-element of the frustum of a cone having for 
ases those circular sections of the pivots which rest upon the 
hire the upper sign being used when the clamp pivot is the 
arger. 
CcC— 
_ On account of the length of the arms of the collimator, 
tmconvenience may be experienced in its reversal unless some 
ments now in activity. The film of silver, if kept properly 
covered when not in use, will last for years before resilvering 
will be necessary. 
Ann Arbor, Mich., Jan. 8, 1883. 
