772 



Prof. R. W. Wood on the 



for tho half-tone process, was gold-plated and polished. 

 The plates were found sufficiently flat for the purpose and 

 had a much better optical surface than anything that I was 

 enabled to produce on a copper plate by grinding and polish- 

 ing, for the final polishing always produced irregular undu- 

 lations (possibly owing to variations in the hardness), and I 

 was unable to get any suggestions from professional opticians 

 accustomed to the polishing of glass and speculum metal. 

 If any one has worked out a method of getting a flat optical 

 surface on such metals as copper or gold, I shall be very glad 

 to hear of it, as it will doubtless improve the quality of the 

 gratings. 



A carborundum crystal was used for the ruling point, and 

 the ruling, in the ease of tho gratings of very large constant, 

 was done on a small laboratory dividing-engine by hand. 

 Tin- machine had bad periodic errors, and the best, gratings 

 were made with Rowland's first mac-bine, with a 7-tooth and 

 a 15-tooth cam, which gave 20G2 and [h\2 lines to the inch 

 respectively. 



The hexagonal carborundum crystals were selected by 

 breaking up a mass of the substance as it comes from the 

 furnace. Specimens of these iridescent crystalline masses are 

 to be found in most chemical or mineralogical museums. 

 The crystals have the form shown in figure 1, and are 



Grat/'nj Plate 



mounted as shown in the figure. The natural edges are so 

 straight that they rule a groove with optically perfect sides. 

 Everything depends upon the nature of the edge and the 

 angle at w hich it is set with respect to the direct/on of the line, 

 i. e. the tilt forwards or back. Some edges will not rule 

 properly at any angle, "chattering " over the surface and 

 tearing off a thread of metal. No metal is removed when the 

 ruling is going on properly, the groove being formed by 

 compression of the metal. Jf the edge is properly chosen, 

 mounted at the proper angle and correctly weighted, a 

 beautiful groove is made with a very little elevation of the 



