1891.] 



Chief Line in the Spectrum of the Nebula'. 



401 



Observatory,' as the first comparisons were made under exception- 

 ally favourable conditions. Some small improvements in the 

 apparatus make it probable, however, that it can be readied in the 

 iuture. 



Examination of the individual results for each night's work shows 

 that the errors are purely accidental ; hence, the mean of the results 

 for the third line will be used to determine a correction to the mean 

 )f the results for the first line. 



A displacement of the third line toward the red of 0'28 tenth- 

 letre corresponds to a displacement of the principal line, in the 

 same direction, of 0'29 tenth-metre, which is the amount by which 

 the principal line is seen to be too near the red end of the spectrum, 

 account of the recession of the nebula from the sun. 

 Hence the wave-length of the principal line, if determined by an 

 jbserver at rest relatively to the nebula, would be X 5005'93, and this, 

 therefore, is the normal position of the chief nebular line, according to 

 11 the observations of the nebula of Orion which have been made, up 

 to the present time, at the Lick Observatory. The probable error of this 

 jsult is, by the theory of least squares, 0'03 tenth-metre. The posi- 

 tion of the MgO fluting, on the same scale, is X 5006'36 or O43 tenth- 

 ictre below the normal position of the nebular line. An interval 

 )f this magnitude is not only measurable with my apparatus, but 

 loticeable at a glance in the telescope. 



An incident which occurred during the course of the work 

 lay be mentioned here, as showing how much greater the above- 

 ited interval is than any error which could be made under good 

 mditions of observation. The measures of January 26, 1891, on 

 )eing reduced the next morning, made the interval between the 

 icbular and lead lines 0'15 tenth-metre greater than it should have 

 en according to previous measures. This difference led me at 

 ice to infer that something was wrong with the apparatus, and on 

 examining the instrument I found that the observing telescope was 

 3t to a reading 5 different from the usual one, in such a direction 

 lat a higher dispersion than usual had been employed. On deter- 

 ling the value of the micrometer for this position of the grating, 

 id re-reducing the observations, the discrepancy was then but a 

 few hundredths of a tenth-metre. 



In the ' Journal of the British Astronomical Association,' Mr. 

 taunder says, in reference to the possibility of my having over- 

 leasured the interval between the chief nebular line and the edge of 

 the magnesium fluting, " Further, some allowance must be made for 

 the difficulty of comparing a line with a fluting ; we ought certainly 

 lot to measure from the centre of the nebular line to the extreme 

 of the fluting. This will apply a small, but a further, correction 

 the same direction." Mr. Maunder's criticism does not, however, 



