1882.] producing the reversal of spectral lines of metals. 259 



image is cut off sharply by the shadow of the shutter. Strong- 

 lines extend into the shadow more or less, and if there is a real 

 reversal the extension of the reversed part into the shadow is 

 trumpet-shaped (a), whereas if it is only a pseudo-reversal it is 

 closed (b). 



We would call especial attention to the enormous expansibility 

 of the magnesium line w. 1. 2852. The limits are quite indefinable 

 either up or down the spectrum and the absorption (i.e. the reversed 

 line) is a broad band, sometimes reaching up to the solar line U 

 as seen in Plate I. fig. 2, which is a negative photograph of the 

 same part of the arc as fig. 1, but had a fragment of magnesium 

 dropped into it at the moment it was taken. You might fancy 

 that magnesium were the substance which produces the absorption 

 of the sun's rays above U, if it were not that the light reappears 

 in the case of magnesium above the dark band as well as below it 

 while the sun's light seems to be quite cut off above U. The 

 cessation of the solar spectrum above the magnesium line may be 

 due to the general absorption of the lines of iron and other metals 

 which are far more numerous in that region than in the visible 

 part of the spectrum, and it is just possible that a very long 

 exposure of sensitive plates may yet reveal some extension of the 

 solar spectrum beyond U. 



In these cases of self-reversals the light of the line is only 

 partially absorbed but there are other cases of 



II. Complete reversal, where the lines are seen only as dark 

 lines. To this class belong the reversals before described when 

 the lines of volatile metals were seen dark against the bright 

 background of the hot bottom and sides of the tubes heated in a 

 furnace. But the arc itself does not generally give such a back- 

 ground. It gives generally a discontinuous spectrum consisting of 

 bright lines with only a very faint continuous spectrum. There 

 are however some parts of the arc, as we used it, which are so full 

 of fine closely set lines that it serves as a good background against 

 which a dark reversed line can be seen. The tail of fine lines 

 formed by the ultra-violet Cyanogen bands is such a region. An- 

 other is in the neighbourhood of Q where there are a great many 

 fine iron, manganese and chromium lines, and where also the fine 

 lines of the water spectrum add to the effect so as to give some- 

 thing like a continuous spectrum. 



One of our photographs shews the reversal of a Thallium line 

 near L. In the negative photograph it is a simple absorp- 

 tion line. Another photograph shews silver lines near Q reversed 

 as mere white lines on the general spectrum. In some cases 

 we have succeeded in getting a good background by bringing 

 up the hot carbon pole into the line of view. In that case the 

 larger part of the bright metallic lines disappear and the con- 



