in Buling his Test-Plates. By William A. Bogers. 83 
Row are these Lines accurately spaced f 
The ordinary way is to give to the head of the screw, which 
carries the plate to be ruled, the desired movement over equal 
intervals by means of a ratchet and pawl : but this method is open 
to the two objections, that one is limited to the number of teeth cut 
on the disk, or to an even combination of them ; and also, that all 
errors of the gear-cutter with which the ratchet was originally cut 
are transferred directly to the rulings, with the addition of other 
errors arising from want of centering, &c. 
I have employed for this purpose the following device, which, 
as far as I am aware, is new in its application : A rigid arm two 
feet in length vibrates upon a shaft set exactly in a line with the 
precision screw. At one end a magnet, fitted to the curvature of 
the head of the screw, is attached by eight pivots in such a way as 
to give parallel motion with respect to the arm. The outer portion 
of the head of the screw consists of a rim of soft iron, which operates 
as an armature to draw the magnet to it when the circuit is com- 
pleted. The other end of the arm works between two stops, one of 
which is adjustable. The action, then, is this: the circuit being 
completed, the magnet becomes firmly attached to the head of the 
screw, and by the movement of the arm from one stop to the other, 
it is carried over a given space. The circuit being broken, the arm 
during the reverse movement carries the magnet with it without 
disturbing the precision screw. In order to guard against every 
possibility of disturbance, a second magnet holds the head of the 
screw in place while the first one is moving back to prepare for 
the next increment of motion. By varying the distance between 
the stops, any desired motion whatever, within certain limits, can 
be given to the screw. From repeated experiments, it is found that 
about twenty movements of the arm for iqIqq of a revolution of 
the screw-head can be made without varying more than one from 
this number. 
If now the lower stop is replaced by a wheel made to revolve 
simultaneously with the head of the screw, and if to the periphery 
of this wheel a curvature is given corresponding to the known 
errors of the screw, it is obvious that the screw can be made to 
correct its own errors. Thus, if at any point in its revolution the 
screw gives too small intervals, the periphery of the wheel must be 
filed away enough to increase the space ruled by the amount of 
the error. I am indebted to Professor Joseph Winlock, the late 
Director of Harvard College Observatory, for he suggestion of 
this elegant method of overcoming the residual errors of the 
screw. 
