278 Mr. L. Bell on the Absolute 



The probable error appears to be rather less than in the 

 measurements of grating I. As, however, the angular deter- 

 minations made with I. are ihe better, so far as probable 

 errors of observation are concerned, the results from the two 

 gratings are about equal in value. 



Computing now the wave-lengths corresponding to the given 

 values of <f> and A for each grating, we have finally for the 

 wave-length of D x at 20° C. and 760 millim. pressure : — 



From grating I. uncorrected, 5896*11 tenth metres. 

 From grating II. „ 5895*95 „ 



The difference in the above results is by no means large 

 compared with the results obtained from different gratings by 

 other investigators, but it certainly is enormously great com- 

 pared with the experimental errors alone. 



As nearly as can be judged, these ought not in either 

 grating to exceed one part in two hundred thousand, while 

 the above discrepancy is about one part in thirty- five thousand. 



Its cause must be sought in the individual peculiarities of 

 the gratings, rather than in the method of using them. 



All gratings are subject to irregularities of ruling, and the 

 effects of these are various, according to the nature and 

 magnitude of the defects. Linear or periodic errors in 

 ruling, unless very small, will make themselves apparent by 

 changing the focus of the spectra or producing ghosts, re- 

 spectively ; and if such errors are large, render the grating 

 totally unfit for exact measurement. Accidental errors, such 

 as a flaw or break in the ruling, are also serious, but are 

 easily detected and may be approximately corrected, as was 



o 



done by Angstrom in the case of one of his gratings. Any 

 marked and extensive irregularities of spacing will produce 

 bad definition or false lines, and in most cases both. If, then, 

 a grating on microscopical examination is free from flaws 

 and on the spectrometer gives sharply defined spectra, alike 

 in focus and free from ghosts, it is safe to conclude that it is 

 tolerably free from the errors above mentioned ; but, unfor- 

 tunately, there is one fault that does not at once become 

 visible, while it introduces a very serious error in the measure- 

 ments : this is a rather sudden change in the grating-space 

 through a portion of the grating, usually at one end. {Such 

 an error is usually due to abnormal running of the screw 

 when the dividing-engine is first started, and may in this 

 case be avoided by letting the engine run for some time 

 before beginning to rule. Thus grating I., ruled with this 

 precaution, is nearly free from this error. Sometimes, how- 

 ever, it is the terminal or an intermediate portion of the 



