On the Chief Line in the Spectrum of the Nelulce. 183 



their shadows on the slit plate. The slit was made as narrow as the 

 luminosity of the spark would allow. 



It was also noticed during the observations that errors may be in- 

 troduced by the insensible motions of the eye in front of the slit. 

 With a spectroscope having one flint glass prism of 60 and a tele- 

 scope magnifying about fifteen times, the displacement of the 

 lines due to this cause as referred to the cross wires was found to 

 amount to as much as forty units, or twice the distance between the 

 magnesium fluting and the less refrangible nitrogen line. With the 

 Cooke spectroscope having eight prisms the displacement wns not 

 more than twenty units. Pinholes of various sizes were placed in 

 front of the eyepiece, but the displacement was not at all diminished 

 by this. The motion of the lines over the pointer was found to be 

 quite rhythmical and to keep time with the beating of the heart. 



No doubt this displacement could be abolished by perfect focussing, 

 but the construction of instruments generally does not admit of the 

 focussing of the cross wires, and even if there be an adjustment, as 

 there is in the instrument used by me, one condition is only good for 

 one observer. 



These experiments, therefore, show that many precautions have to 

 be taken before the coincidence or non-coincidence of one line with 

 another can be determined with absolute certainty even when large 

 dispersion and stable laboratory conditions are employed. 



The general results of the laboratory comparisons may be briefly 

 stated thus : 



It must be remembered that ordinary observatory conditions are 

 not nearly so favourable for accurate measurements of the positions 

 of lines in spectra as laboratory ones. In the first place, the apparatus 

 is not so stable, and must of necessity be in motion, and again, the 

 collimator of the spectroscope with its slit exactly central must be 

 demonstrated to be absolutely in the optic axis of the telescope before 

 a measurement can be taken as final. 



Two series of observations should therefore be made, one with the 

 spectroscope in one position, and the other when it has been turned 



VOL. XLVIII. 



