On the Chief Line in the Spectrum of the Nebulce. 189 



burning behind the secondary collimator. There was perfect coin- 

 cidence between the lines. Four prisms were used for the comparison, 

 and the centre of the Moon's disc was thrown on the slit. 



Everything being now in perfect adjustment, the nebula was 

 turned to, and three good comparisons made of the chief line and the 

 magnesium fluting, the magnesium being burned behind the secondary 

 collimator. The pointer was displaced and readjusted in each obser- 

 vation. In each case the coincidence with the least refrangible 

 maximum appeared perfect. These observations were made by 

 Mr. Fowler and Lieutenant Bacon. In order to further test the 

 result obtained, Mr. Fowler put the pointer of the micrometer exactly 

 on the nebula line, and left it for Lieutenant Bacon to say how its 

 position was with respect to magnesium ; again it was perfectly 

 coincident. 



Lieutenant Bacon made three independent comparisons, the position 

 of the pointer being changed each time, and twice found coincidences 

 with the least refrangible maximum, whilst once the nebula line 

 appeared to be on the right-hand edge of the same maximum. Mr. 

 Gregory also made one comparison which confirmed the above 

 results. 



December \st. The object-glass having been adjusted by means of 

 Aldebaran as before, the secondary collimator was put in position, 

 and the magnesium spark from a quantity coil put behind the slit. 

 With this arrangement a comparison of the nebula line with the lines 

 of nitrogen could also be made. The collimator was adjusted by 

 means of 6, as seen in the Moon and in the magnesium spark. 



The pointer of the micrometer was then put upon the F line in the 

 Moon, and the nebula was afterwards brought upon the slit. The 

 two lines were coincident, showing the probable accuracy of the 

 adjustments. 



Three comparisons were made of the nebula line with magnesium 

 by Mr. Fowler, the pointer being displaced each time ; in each case 

 the coincidence was perfect. 



Comparisons with the nitrogen lines showed the nebula line to be a 

 little less refrangible. 



I made the comparison twice, and in each case the coincidence 

 with the magnesium fluting was perfect. In one case I saw the 

 pointer of the micrometer, the nebula line, and the nitrogen lines at 

 the same time ; the pointer was on the nebula line, but both appeared 

 to the right of the nitrogen double. 



Finally, Mr. Fowler put the pointer on the nebula line, Lieutenant 

 Bacon agreeing with him as to the setting. Then I made the follow- 

 ing comparisons : 



(1.) With the nitrogen lines. Result, pointer to the right. 



(2.) Magnesium burning behind slit. Result, coincidence perfect. 



