On the Chief Line in the Spectrum of the Nebulce. 187. 



the centre. The image of this was focussed on the slit of the 

 Steinheil, and when the magnesium was burning the spectrum was well 

 seen. 



N&oember 29th. During the day the siderostat was put in order. 



The position of the collimator of the Steinheil was tested by open- 

 ing the slit very wide, and burning magnesium at the centre of the 

 siderostat mirror (the mirror being temporarily removed for this 

 purpose). The image of the slit fell exactly in the centre of the 

 collimating lens, so that no alteration was necessary. 



An observation was also made of the displacement of the mag- 

 nesium fluting brought about by moving the spirit lamp in which the 

 magnesium was burned away from the centre of the mirror. It was 

 found that if the lamp were moved more than two inches on either 

 side the spectrum ceased to be visible. Between the two extreme 

 positions (i.e., 2 inches on each side of the centre), the displacement 

 produced was about one-third of the distance between the first and 

 second maxima of the magnesium fluting. 



The rough collimator that had been previously used was replaced 

 by the collimator of a student's spectroscope, the slit of which was 

 adjustable both for length and breadth. This was supported on a 

 light iron tripod, so that, when in position, ifc would prevent very 

 little light from the siderostat passing through the object-glass. By 

 this arrangement the spectra of magnesium and the nebula could be 

 superposed, it being intended to obtain the magnesium fluting in this 

 case from a quantity spark between magnesium poles. An enlarged 

 image of the secondary slit is, of course, formed on the slit of the 

 Steinheil spectroscope. 



A complete plan of the apparatus, drawn to scale, and showing 

 some of the principal dimensions, is given in fig. 3, The Steinheil 

 spectroscope employed has a circular table, 28 inches in diameter, 

 supported on a tripod stand weighing about eighty pounds. The col- 

 limator and observing telescope are each about 20 inches long, and 

 have object-glasses 1 inches in diameter. The prisms have faces 

 2" x 2", three of them having an angle of 45, and one of 60, each 

 one being supported on a stand provided with levelling screws. With 

 the four prisms the dispersion from A to H is 10. 



The reflecting eyepiece which was used to act as a finder was the 

 ordinary one used with the 10-inch equatorial, and could be lifted out 

 of its supports and put back again at pleasure. 



No work could be done in the observatory this evening on account 

 of fog. 



November SQth. Commenced work by adjusting the object-glass 

 and the collimator in front of it. 



The accuracy of this adjustment was checked by a comparison of b 



the spectrum of the Moon with b in the spectrum of magnesium 



