332 H. M. JOHNSON 



Each grating is held in a metal ring, R 1 , R 2 . The two rings 

 bear on each other. Each ring is cut with a shoulder extending 

 from the concave cylindrical surface, to which the grating is 

 cemented. The shoulder is high enough that the two gratings 

 are separated by about 1-64 inch, which is sufficient to prevent 

 their rubbing, but not enough to cause disturbance from parallax 

 at the distance at which the instrument is used. The ruled 

 surfaces of the gratings are placed interior. Two lugs, L 1 and 

 L 2 , are screwed into the holder H about 110° apart at the exterior 

 circumference of the rings, and serve as bearings for the rings in 

 rotating. Two stops, RS 1 and RS 2 , the former only of which 

 is shown in the blueprint, are attached to the convex cylindrical 

 surface of the rings as bearings for the yoke Y, by the action of 

 which the rings are rotated. This yoke is moved by a lever L 

 connected with the yoke by a free double-pointed pin P and by 

 the spring Sp 2 , which serves to keep the points of P in their 

 respective cup bearings. The direction of movement of Y is 

 tangential to the rings. A spring Sp 1 is attached to hooks affixed 

 to the convex surfaces of the rings, so that the stops RS 1 and RS 2 

 are always held snugly against the tips of Y. Thus any movement 

 of the yoke moves the gratings through an equal angle. 



The longitudinal movement of the yoke Y is limited by the 

 position of two stops, St 1 and St 2 . St 1 is made with a long threaded 

 sleeve, through which passes the shaft of the micrometer screw 

 Scr. The base of this stop is quite wide, and bears on the plane 

 surface of the holder H with its entire nether surface. Its ten- 

 dency to rotate with the screw is thus made negligibly small. 

 The sleeve of St 1 passes through a circular opening in Y the center 

 of which is in the axis of the screw and of Y. The fit is rather 

 close, just permitting free longitudinal movement of Y over the 

 sleeve. The stop St 2 limits the movement of Y in the opposite 

 direction. The base of this stop also bears on the plane surface 

 of H. The shoulder of St 2 against which Y is drawn is made with 

 a circular opening large enough to permit clear passage of the 

 shaft of Scr and easy passage of the sleeve of St 1 . The base of 

 St 2 carries two slots, through which pass two strong screws that 

 fasten St 2 to the holder H at the setting desired. To change the 

 setting of St 2 these screws are loosened, St 2 drawn against Y, 

 and the whole moved by the micrometer screw until St 2 reaches 

 its proper position. Then the screws are tightened and the yoke 



