190 . Mr. J. N. Lockyer. 



(3.) Magnesium burning at the centre of the siderostat. Result, 

 again perfect coincidence. 



It was found that magnesium burning at the centre of the sidero- 

 stat was coincident with magnesium burning behind the secondary 

 slit, thereby showing the accuracy of the adjustments. 



In all these observations the nebula line was seen to be sharper on 

 the right-hand edge than on the left, and it was irregularly bright 

 along its length, as in the Westgate observations. 



The observations have left 110 doubt in my mind as to the coin- 

 cidence of the chief nebula line with the magnesium fluting, under 

 such conditions that at the same time the coincidence of the F line of 

 hydrogen with another nebula line was demonstrated. 



Lieutenant Bacon and my assistants concur in this view. Even 

 with four prisms the observations are by no means easy, and are 

 very delicate, but it is important to note that in no observation 

 was the nebula line found more refrangible than the magnesium 

 fluting, and if the optical conditions were imperfect it seems hardly 

 likely that an error in the same direction would be reproduced on 

 four different nights, the apparatus being set up afresh each time. 



Arrangements are in progress for repeating the observations with 

 apparatus furnished with screw adjustments. 



There is one more test of the accuracy of the adjustments which 

 might have been applied had I thought of it in time. So far the test 

 relied on has been to see that the lines seen in the spectrum of mag- 

 nesium burning at the centre of the mirror were coincident with those 

 seen when the magnesium was burned behind the secondary slit. In 

 one case the light passed through the object-glass only, while in the 

 other it passed through the object-glass and secondary collimator. 

 When the observations are repeated, it is intended to further test 

 the adjustments by forming an image of the flame at the mirror 

 upon the secondary slit, so that in both cases the light will pass 

 through the collimator and object-glass. If the lines are still coin- 

 cident, the accuracy of the adjustments will be still further demon- 

 strated. 



IV. FLUTED CHARACTER OF THE CHIEF NEBULA LINE. 



Dr. and Mrs. Huggins state that the chief nebula line is perfectly 

 sharp and well-defined. This necessitates my giving in reply a com- 

 plete account of those recorded observations, which, coupled with 

 my own, have led me to the opposite conclusion, namely, that the 

 line is often noticed ill- defined at the edges, chiefly on the blue side, 

 and in some parts of the nebula in Orion presents even more unmis- 

 takable indications that it is the remnant of a fluting. 



Certain references in the paper suggest that it may be well that I 



