90 H. J. Grayson: 



With respect to lines suited for diffraction gratings they must be 

 clean and sharply defined and for normal light distribution in the 

 different spectra the groove should be symmetrical or isosceles with 

 respect to the ruled surface. The length of service of which a 

 diamond is capable depends very greatly upon the treatment to 

 which it is subjected. The shock which a diamond gets due to its 

 fall at the conuuencement of each fresh line has a more detrimental 

 effect upon its cutting edge than the wear due to the act of cutting. 

 A ruling diamond in the case of grating ruling should have its fall 

 restricted to less than .02 in., and the "blow from even that height 

 must be carefully moderated with damping devices or the character 

 of the line will change as the ruling progresses and the resulting 

 grating rendered valueless. 



In order to illustrate several of the most common defects resulting 

 from the improper adjustment of a ruling diamond, a few photo- 

 graphs of rulings have been prepared and are shown in Plate XVII. 

 Two of these photographs (Figs. 1 and 2) were taken to show the 

 character of lines resulting from a correct adjustment of the ruling 

 diamond. Only the starting and terminal ends of the lines were photo- 

 graphed as these show most clearly that the diamond has been 

 correctly adjusted. These lines were ruled at the rate of 1000 to 

 the inch by liand which accounts for any slight irregularity. The 

 diamond with which these lines were ruled had been in use for 

 some weeks and had ruled several hundred thousand lines with 

 apparently no change in its condition. Fig. 3, PI. XVI., shows 

 a group of lines ruled upon speculum metal with a slight displace- 

 ment of the diamond edge from a position parallel with the line 

 ruled and also slightly tilted from the perpendicular. The effect 

 produced is sufficiently striking. The diamond, in cutting these 

 lines, appears to have acted somewhat after tli^ manner of a plough- 

 share, the material removed coming aAvay in a thread-like form 

 from certain areas of the metal surface. Upon readjustment the 

 line appeared normal and without a break. The angle made by the 

 knife edge with tlie ruled surface was then increased and the effect 

 which Avas produced appears in Fig. 4. Here the lines ruled are 

 fairly symmetrical but somewhat ragged on both edges ; some of the 

 material removed from the lines appearing as minute spirals. 

 Fig. 5, PI. XVII., is a photograph of lines rided with a carefully 

 adjusted diamond at the rate of 9000 to the inch. Lines of this 

 quality would be. well suited for grating work up to 20,000 lines 

 per inch. Two examples of finer ruling are seen in Fig. 6, the 



