260 Professors Liveing and Dewar, On the circumstances [Oct. 30, 



tinuous spectrum becomes much stronger, so that the reversed lines 

 are more readily seen. We have various plates shewing complete 

 reversals of the lines of several metals in one or other of these 

 ways. The ultra violet lines of potassium form a series extending 

 at decreasing intervals to about the solar line U. They are all 

 reversed, appearing only as white lines in the (negative) plates. 

 The least refrangible is a double line just below O. Probably they 

 are all double in reality, and harmonically related to the violet 

 pair. Plate II. shews most of these potassium lines reversed. 



In other plates are seen complete reversals of copper and 

 silver, and lead lines. Some of our plates shew complete re- 

 versals of a great many iron lines. In two of these plates the re- 

 versals were obtained by putting an iron wire into the arc through 

 one of the carbons, which was perforated for the purpose. Some 

 of the lines are greatly expanded, but they appear only as absorp- 

 tion lines ; they have no bright wings. In another case the iron was 

 introduced into the arc as ferrocyanide of potassium, and the lines 

 are sharp clean fine lines just like the Fraunhofer lines in the 

 solar spectrum, the continuous background being heightened by 

 the potassium, which seems to give some continuous spectrum in 

 the ultra-violet region. Plate III. shews a great many iron lines 

 so reversed. 



III. Another class of reversals are produced by the vapour 

 of one metal seen against a bright background given by the ex- 

 pansion of a bright line of some other metal. For this purpose 

 the expanded lines of magnesium are most effective from their 

 great breadth. We have several plates shewing sharp clean re- 

 versals of iron lines against the continuous light of the expanded 

 magnesium line, w. 1. 2852 ; one shews lines reversed in this 

 way against the bright expansions on both sides of the absorption 

 band of magnesium. One of the plates shews some iron lines near 

 M and chromium lines reversed in this way. Plate I. fig. 2 sheAvs 

 the principal iron lines between S and U reversed against the 

 expanded magnesium line w. 1. 2852. 



IV. A fourth mode of obtaining reversals we cannot so easily 

 explain. It is by allowing a very gentle stream of hydrogen, coal 

 gas, or ammonia, to pass into the arc either through one of the 

 carbons perforated or through a separate opening into the crucible. 

 The effect of such a current is generally to sweep away most of the 

 metallic lines, make those which remain fainter, and increase, 

 relatively at least, the continuous spectrum, and reverse many lines. 

 The character of the effect is fairly seen in a photograph, Plate IV., 

 of the part of the spectrum near U, in which the magnesium line 

 (2852) appears about as strong as usual in a magnesia crucible, 

 but wholly as an absorption, while the ether magnesium lines are 

 still bright. Many of the iron lines are reversed in this photo- 



