50 



Rollins — Ruling Concave Gratings. 



center toward each end, as may be seen at 1 to 10 on the right 

 of the figure, while the angles formed with the surface by the 

 sides of the cuts vary constantly ; moreover, the width of the 

 faces of the same cut is different. The first defect might be 

 corrected by a compensating device attached to the spacing 

 screw nut, but the second and third cannot be with the present 

 methods of ruling. If the block to be ruled were fixed, the 

 diamond point pivoted at a center, A, corresponding with the 

 center of curvature of the grating mechanism, could be arranged 

 to move the diamond through equal arcs, producing equal 



spacing and lines with equal faces and equal angles with the 

 surface, as shown on the left of the figure at 1 to 10. In the 

 case of seven meter gratings the arm would be so long that 

 vibration might be difficult to deal with. Instead of moving 

 the diamond through an arc, the carriage moved by the screw 

 might be pivoted at the center of curvature of the grating and 

 swing through an arc on curved ways ; the nut not being 

 rigidly attached to the carriage, but imparting motion to it by 

 an arm. On account of the vibration factor a different method 

 has been chosen for illustration. The usual screw is used, with 

 a long nut to impart motion to the carriage, which moves on 

 curved ways, instead of on flat ones,, as at present in all ruling 



