182 Mr. J. N. Lockyer. 



Four prisms and an observing telescope magnifying about thirty 

 times were first used with the new conditions, and it was then found 

 that the magnesium fluting was a little less refrangible than the 

 nitrogen line. In this case the magnesium spectrum was obtained by 

 burning magnesium in front of the bulb, and the non- coincidence 

 with the nitrogen line was put beyond doubt by observing the two 

 spectra simultaneously. This was further confirmed with the 

 Rowland grating 1 , the magnifying power employed being about fifty- 

 five times. 



Comparisons with the solar spectrum were then made, with the 

 result shown in fig. 1. 



The solar spectrum shown in the drawing is from an enlargement 

 of Rowland's map of the region in question, and the positions of the 

 fluting of magnesium and the lines of nitrogen and lead are as deter- 

 mined with four prisms and confirmed with the Rowland, which differs 

 from Angstrom's scale by one division ; e.g., 5007*5 on Rowland's scale 

 is equivalent to 5006'5 on Angstrom's. (This difference was deter- 

 mined by a comparison of twelve lines of iron mapped by Thalen with 

 the corresponding solar lines shown in Rowland's map.) Two 

 spectra of nitrogen are shown in the map, the first one, that seen 

 when the small jar was used and the slit was rather wide ; and the 

 second, that seen with the jar spark in rarefied air and the slit as 

 narrow as possible. In the first case the lines are very broad and 

 begin to fade away rather suddenly on both sides. Two spectra of 

 magnesium are also shown, one with the slit wide and the other with 

 it narrow. 



It will be seen that when the slit is rather wide the haziness of the 

 less refrangible nitrogen lines overlaps the first maximum of the 

 magnesium fluting, when seen with the same slit. This is the same 

 whether magnesium or platinum poles were used for the air spark, 

 and this shows that the apparent coincidence is not due to the 

 remnant of the magnesium fluting being superposed on the air lines. 

 The importance of using a narrow slit and a spark with large jar, 

 preferably in air at a reduced pressure, for comparison with the 

 nebulae is, therefore, obvious. 



The results given are not to be absolutely relied upon, as there may 

 be slight errors, due to the various light sources not being perfectly 

 in the line of collimation. It has been found, for example, that a 

 change of 111 minutes of arc in the direction of the beam from the 

 siderostat displaces the lines about two divisions of Rowland's scale, 

 or more than the difference between the positions of the chief nebula 

 line and the fluting of magnesium as determined by Dr. Huggins. 

 Every precaution was taken, however, to ensure the accuracy of the 

 observations. The beam from the siderostat was first directed on the 

 slit, and the spark and lens placed in the same direction by adjusting 



