52 



Rollins — Hiding Concave Gratings. . 



tion of the arrow 2, as shown by the dotted lines, bringing 

 different parts of the block, BK, under the ruling diamond, 

 D. This arrangement requires no compensating device, except 

 for the periodic errors of the screw, which may be present in 

 any form of ruling machine. In the case of a concave spher- 

 ical grating this mechanism would make the lines with equal 

 spaces, while if a section were cut through the grating, as in 

 figure 1, the grooves would all have the same angles with the 

 surface. It will always be mechanically impractical to make a 



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machine in which this is true through the whole ruled surface 

 of a concave spherical grating, but if the surface of the grat- 

 ing is concave cylindrical then the plan shown in figure 5 is 

 capable of overcoming all the defects mentioned in the first 

 part of the paper. Another difficulty with the present methods 

 of ruling concave spherical gratings remains to be considered, 

 namely, the diamond must rule both up and down hill. No 

 diamond does this well, consequently the forward and back 



