Line in the Spectrum of the Nebula in Orion. 



203 



carrying electrodes or vacuum-tubes. The final adjustment was 

 tested by the comparison of the bright lines of magnesium and the 

 double line of sodium with the Fraunhofer lines 6 and D in the 

 spectrum of the Moon."* 



I have since adopted an arrangement in which, when once adjusted, 

 any sensible parallactic effect from a change of position of the source 

 of light seems to be impossible, for eveti a minute motion of the spark 

 or other source of light for comparison has the effect of throwing the 

 light to one side, without the slit; so that, as long as the comparison 

 spectrum is seen, there can be no doubt that the direction of the light 

 for comparison, as it fell upon the slit, had remained invariably the 

 same, relatively to the optical axis of the telescope, and consequently 

 to the celestial spectrum under observation. 



In the diagram, abed represents a section of the telescope- tube 

 near the middle of its length ; within this is firmly screwed a thin 



steel arm, e, carrying a minute mirror, /. This mirror is about a 

 quarter of an inch in width, and of about the same apparent length, 

 when seen fore -shortened from the slit. The mirror is fixed at a 

 distance of 6 feet 6 inches within the principal focus, where the slit 

 is placed. In the side of the tube opposite the face of the mirror is 

 a small hole, through which the light from the collimator rj passes on 

 to the mirror. At the other end of the collimator, which has a length 

 of about 7 inches, is a diaphragm with a small hole, h, before which 

 the source of light, whether an induction spark, a vacuum-tube, or 



Koy. Soc. Proc.,' vol. 20, p. 382. 



