320 Prof. F. L. 0. Wadsworth on the Resolving Power of 



If we take the higher limit we find that the limit of resolving 

 power of the best and largest gratings now in use (ruled 

 surface 5J inches) is for the middle part of the spectrum 

 (X,= *00055 mm.) about 375000 units, just sufficient to 

 " resolve " a double line whose components are about *016 

 tenth-metres apart. The view at one time held that higher 

 resolving powers than this were unnecessary because of the 

 discontinuities in a train of light-waves is now known to be 

 erroneous. Michelson's recent work has shown that some of 

 the spectral lines which appear single in the most powerful 

 spectroscopes yet constructed, are in reality very complex, 

 consisting of three, four, or even more components whose 

 distance apart in some cases is probably not much more than 

 0*006 tenth-metres. To resolve these by means of a grating, 

 we need, therefore, instruments having at least three times 

 the aperture of those now in use. Were the interferometer 

 or wave-comparer universally applicable in spectroscopic 

 analysis, there would be little occasion to attempt to rival its 

 performance by gratings, but it is unfortunately only applic- 

 able to the more intense of the bright lines of a spectrum. 

 For the more detailed study of faint lines, and absorption- 

 lines, gratings of larger resolving power than have yet been 

 constructed would seem to be the first essential. The 

 mechanical difficulties to be overcome are very formidable. 

 The chief difficulty does not seem to lie in the production of 

 a screw of sufficient accuracy, since by Rowlands method we 

 are enabled to produce a screw of the required length in 

 which the errors of run, periodicity, &c. are less than those 

 unavoidably introduced by eccentricities in the mountings 

 and divided head*; but in avoiding the errors of spacing 

 caused by unequal wear of the ways on which the ruling- 

 point carriage moves, and in maintaining sufficiently constant 

 temperature conditions during the ruling. 



How great these difficulties actually are may be better 

 appreciated when it is remembered that to rule a 15-inch 

 grating (of 20000 lines per inch) the ruling- engine would have 

 to run continuously for nearly two weeks (a 6-inch grating 

 requires five days and nights), that in such a grating a dis- 

 placement of one five-hundred-thousandth part of an inch in 

 the position of the lines in any part of the grating would 

 greatly impair the definition and resolution in any order 

 higher than the second, and that such a change would be 

 brought about by the smallest amount of unequal wear, or 

 even by a slight change in thickness of the film of oil on one 

 of the ways of the ruling-carriage, or by an unsymmetrical 

 * See Eowland's article on the Screw, JE?ic. Brit. vol. xxi. 



