of the Lengths of Waves of Light. 493 



The values of the wave-lengths contained in the series (y) 

 depend on measurements of the first interference-spectrum of 

 a glass grating which was considerably finer than the one I em- 

 ployed. According to Fraunhofer' s statement, in fact, 



e= 0-0001223 of a Par. inch. 



Since, however, the number of marks in this grating of Fraun- 

 hofer's amounted only to 3601, the breadth reduces itself to 



5-2833 Par. lines; 



and consequently it must have been considerably less luminous 

 than that of Nobert. In another respect, too, Fraunhofer's 

 grating, although an excellent one, appears to me to have been 

 inferior to that of Nobert ; for the line B could not be mea- 

 sured even in the first spectrum, and the lines from C to G were 

 not visible in any of the spectra beyond the second. 



Nevertheless, since almost all the differences {ct-y) have the 

 same value, a constant error appears to be indicated, either in 

 my measurements or in those of Fraunhofer. That an error of 

 this character cannot have affected the value of ® in my mea- 

 surements, is evident from the fact that the value of this angle 

 was obtained from mutually agreeing observations on four dif- 

 ferent spectra. The introduction of such an error into Fraun- 

 hofer's measurements is equally inadmissible, since on calculating 

 the wave-lengths of the lines from C to G (which Fraunhofer 

 also observed in the second interference-spectrum, though he did 

 not introduce them into his calculation), the following mutu~ 

 ally according values are obtained from the two spectra: — 



C. D. E. F. G. 



First spectrum . 2422-00, 2174-58, 1944-81, 1793-98, 1586-89; 

 Second spectrum. 2421*54, 2174*36, 1944-63, 1793*92, 1588-07. 



It is only for the line G that the difference is somewhat 

 greater. 



The reason of the differences (a-7), therefore, must arise 

 from an erroneous determination of the value of e; which latter 

 may have been caused either by a wrong enumeration of the 

 lines in one of the two gratings, or by an incorrect estimation of 

 their breadth. In order to make the two values of the wave- 

 lengths for the line D agree, in the series (a) and (7), by alter- 

 ing the value of e } the breadth of Nobert's grating would have 

 to be diminished by 



0-0123 of a Par. line =0-001025 of a Par. inch, 



or the number of lines in the grating increased by 6. 



The same object would be attained by increasing the breadth 



