214 Professor Dewar [June 6, 



refrangible of the two lines in the extreme red (wave-length 7,800) was 

 seen to have a decided dark line in the middle, and it continued so for 

 some time. The addition of more rubidium failed to cause any reversal 

 of the extreme red line, or of any but the three lines already mentioned. 



On putting lithium carbonate into the crucible, the violet line 

 of lithium appeared as a nebulous band, and on adding some alu- 

 minium this violet band became enormously expanded, but showed no 

 reversal. The blue lithium line (wave-length 4,604) was well reversed, 

 as was also the red line, while a fine dark line passed through the 

 middle of the orange line. On adding a mixture of aluminium filings 

 and the carbonates of lithium and potassium, the red line became a 

 broad black band, and the orange line was well reversed. The green 

 line was exceedingly bright, but not nebulous or reversed, and the violet 

 line still remained much expanded, but unreversed. 



Metallic indium placed in the crucible gave the lines with wave- 

 lengths 4,101 and 4,509, and both were seen strongly reversed. No 

 other absorption line of indium could be detected. 



In some cases a current of hydrogen or of coal-gas was intro- 

 duced into the crucibles by means of a small lateral opening, or by 

 a perforation through one of the carbon electrodes, as is shown in 

 Plate II. Fig. 4 ; sometimes the perforated carbon was placed 

 vertically, and we examined the light through the perforations. 

 "When no such current of gas is introduced, there is frequently a 

 flame of carbonic oxide burning at the mouth of the tube. The 

 current of hydrogen produces very marked effects. As a rule, it 

 increases the brilliance of the continuous spectrum, and diminishes 

 relatively the apparent intensity of the bright lines, or makes them 

 altogether disappear with the exception of the carbon lines. When 

 this last is the case, the reversed lines are seen simply as black 

 lines on a continuous background. The calcium line with wave- 

 length 4,226 is always seen under these circumstances as a more 

 or less broad black band on a continuous background, and when 

 the temperature of the crucible has risen sufficiently, the lines 

 with wave-lengths 4,434 and 4,454, and next that wdth wave-length 

 4,425, appear as simple black lines. So, too, do the blue and red lines 

 of lithium, and the barium line of wave-length 5,535, appear steadily as 

 sharp black lines, when no trace of the other lines of these metals, 

 either dark or bright, can be detected. Dark bands also frequently 

 appear, with ill-defined edges, in the positions of the well-known bright 

 green and orange bands of lime. 



With sodium chloride, the pair of lines (5,687, 5,681) next more 

 refrangible than the D group were repeatedly reversed. In every 

 case the less refrangible of the two was the first to be seen reversed, 

 and was the more strongly reversed, as has also been observed by 

 Mr. Lockyer. But our observations on this pair of lines differ from 

 his in so far as he says that " the double green line of sodium shows 

 scarcely any trace of absorption when the lines arc visible," while we 

 have repeatedly seen the reversal as dark lines appearing on the 



