56 W. A. Rogers—New Diffraction Ruling Engine. 
the codperation of Mr. Chas. V. Woerd, the — super: 
intendent of the Waltham Watch Factory. My warmest thanks 
are also due to the manager and treasurer of the Company, 
Mr. . Robbins, for consenting to a a task some- 
what outside of the r regular work of the fa 
To Mr. Woerd I committed the sha she hi of the 
details of the new machine, after deciding upon the general 
principles of its construction. 
e new machine is now so nearly completed that some pro- 
visional work has already been done with it. It would be pre- 
mature to say that it will rule a grating absolutely without 
measurable errors. The resolution of solar lines must be the 
a.) In the usual construction of a precision-screw, it is the 
custom to grind and polish the threads with fine emery by 
means of a lead nut, after the thread has been cut as perfectly 
as possible in the lathe. A definite relation then exists be- 
tween the threads and the centers on which they are cut. Now 
in grinding and polishing, the lead nut is usually held by the 
hand as it traverses forward and backward, and the test of 
the uniformity of the threads is entirely one of feeling. But 
during this operation the relation between the threads and the cen- 
ters may be entirely changed, since the action of the lead nut no 
longer bears any fixed relation to the centers. Hence when the 
screw is mounted wit respect to its centers, we may always 
expect systematic errors of various kinds. 
In the construction of the screw of the Waltham machine, 
an attempt has been made to avoid the errors introduced in 
this way. The bottom of the thread was, at the suggestion of 
Mr. Woerd, entirely cut away, giving entire freedom in the 
action of the emery upon the faces of the threads. The screw 
rests in semi-circular bearings, and is kept in position by its 
own weight. The conical ends are made of tempered steel. 
One end presses against a polished ee oe eoaene exactly 
erpendicular to the axis of the screw t in ponte 
by means of a steel spring bolt working i pi other end. 
The nut is made a part of the moving bed-plate, and rests 
directly upon the screw. 
