312 On a New Critical Standard 3Ieasure [^C'vX1un"T^io! 



The lines ai32)eared the most distinctly and sharply defined when 

 the corrections were so managed with direct light that each groove 

 appeared like a convex cylindrical glass thread. 



When the diffraction lines were separated from the real in a 

 particular manner, then they obliterated the true lines altogether. 



In otJier cases the finer lines were transformed into irregular 

 wavy coarser lines, and too few in number ; or the band appeared 

 streaked with one or two lines just at points where diffraction lines 

 coincided like a vernier with the true, rendering all the rest in- 

 visible. 



Fig. 3. 



1 III ill iji in III II 111 



1 i II i i il 



Vernier Diffraction Lines (clotted) seen in Nobert's I'late more or less close to the tnie. 



With oblique light the spectral lines appear, as it were, to float 

 away from the true according to the focussing, corrections, and 

 obliquity. The spurious diffraction lines, no doubt, form the 

 greatest obstacle to the full and fair resolution of these celebrated 

 lines. Now, however, that they have been fairly photographed, we 

 may smile at that peculiar prejudice (in favour of a theory) which 

 represented the resolution impossible because of the interpretation 

 of the undulatory theory of light. Even Herr Nobert himself for 

 a long time believed in this impossibility, and has, as far as we are 

 informed, never himself seen the XlXth or even the XYIIIth band. 

 Yet their invisibility depended upon the fault of the glasses, not 

 upon the precision of his truly wonderful ruHng machine. In some 

 cases I have observed the ruled lines broken down, as it were, and 

 a fine displaced thread left after the ruling; but the edges of the 

 lines are a perfect marvel of smoothness and truth. 



Having tried several experiments upon ruling glass with a 

 diamond point, I have been surprised to find that there was one 

 position into which the axis of the holding tool must be rotated, 

 and one only, in which true shavings and curls of glass could be 

 obtained : all other positions causing fracture, chipping, and split- 

 ting of the ruled edge of the intended groove. But these beautiful 

 lines are as uniform in general as glass threads, and apparently 

 formed of a semi-circular groove; to which shape probably the 

 diamond is cut by grinding with diamond powder on a soft iron 

 disk to the semi-circular curve, so as to cut and plough a clean and 

 perfect groove at one stroke, and several points are experimented 

 with till the most efficient cutting tool can be selected. 



