SLIPHER— NEBULA. 



405 



The iron-vanadium spark comparison spectrum is exposed a 

 number of times during the nebular exposure in order to insure that 

 the comparison lines are subjected to the same influences as the 

 nebular lines. The spectrograph is electrically maintained at a con- 

 stant temperature which avoids the ill effects of the usual fall of 

 the night temperature. 



The equivalent slit-width is usually about .06 mm. 

 The linear dispersion of the spectra is about 140 tenth-meters 

 per millimeter in the violet of the spectrum which is sufficient to 

 detect and measure the velocities of the spiral nebulae. As the 

 objects yet to be observed are fainter than those already observed 

 the prospects of increasing the accuracy by employing greater 

 dispersion are not now promising. 



The plates are measured under the Hartmann spectrocomparator 

 in which one optically superposes the nebular plate of unknown 

 velocity upon one of a like dark-line spectrum of known velocity, 

 used as standard. A micrometer screw, which shifts one plate 

 relatively to the other, is read when the dark lines of the nebula and 

 the standard spectrum coincide ; and again when the comparison 

 lines of the two plates coincide. The dift'erence of the two screw 

 readings with the known dispersion of the spectrum gives the veloc- 

 ity of the nebula. By this method weak lines and groups of lines 

 can be utiHzed that otherwise would not be available because of 

 faintness or uncertainty of wave-length. 



TABLE I. 

 Radial Velocities of Twenty-five Spiral Nebula. 



