Counting the Rulings of a Diffraction Grating. 717 



against the Standard Invar Metre of the National Physical 

 Laboratory which has been repeatedly verified at Sevres. 

 This is at the rate of 5,682*57 spacings to the cm. or 14,433*7 

 to the inch at 16°*0 C, which may be compared with 

 Brashear's certified value of 14,438 rulings to the inch, 

 temperature unknown. 



For Mr. Gilford's grating the mean spacing value over 

 the whole of the grating was determined, but there would of 

 course be no difficulty in obtaining its value over any parti- 

 cular region should local variation be suspected. 



In most of the accompanying photographs (taken wi ha 

 magnification of 690, which is reduced in the reproductions 

 to about 450) the crystalline structure of the speculum metal 

 shows up strongly. , ■ fi 



Fig. 1 (PI. XV.) shows the remarkably straight edge formed 

 by the points where the ruling diamond was set down at the 

 beginning of each stroke. The other and far more irregular 

 edge where the diamond was lifted from the metal is seen in 

 fig. 2. The two sides of each furrow in the metal and the 

 remains of the " cuttings '* can be plainly seen at the ends of 

 some of the longer rulings. It is evident that the rulings are 

 not very light ones, and that the original flat surface of the 

 speculum has been completely replaced by a succession of 

 fully developed V-shaped furrows with no intervening plane 

 surface. This renewal of the surface is further illustrated 

 by fig. 3, which shows a corner of the grating. The scratch 

 across the corner was, as will be seen, almost entirely removed 

 by the rulings. We should expect with such a surface that 

 the apparent width of the ruling would depend on the obli- 

 quity of the illumination. Such variation from line to line 

 in the width of the rulings is noticeable in some of the 

 photographs. Fig. 4 shows a blemish on the grating. The 

 diamond appears to have got fouled for a number of strokes 

 and has lacerated the surface in some rulings and failed to 

 rule at all elsewhere. 



I wish to thank Mr. Gifford for his permission to include 

 in this paper the photographs and measurements of his 

 grating, and I am indebted to Dr. Glazebrook for his interest 

 in the work. 



The National Physical Laboratory, 

 Teddingrton. 



