264 Professors Liveing and Deiuar, On the oircumstances [Oct. 30, 



lines H and K is peculiar. These lines are often absent altogether, 

 when the line wave-length 4226 and the two near M are well 

 seen, and when the two aluminium lines between them and many 

 of the iron lines are sharply reversed. Even the introduction of a 

 small quantity of metallic calcium or calcium chloride into the 

 tube did not bring them out reversed. They were only seen as 

 bright lines, not very strong, when the small rod was removed. 

 The lithium lines at 4603 and 4131 are often bright when many 

 other lines in the neighbourhood are reversed, and must therefore 

 be regarded as relatively difficult of reversal. As a rule the lines 

 less refrangible than 4226 are balanced as to their emissive and 

 absorptive power and, therefore, disappear, while the more refrangi- 

 ble are reversed. 



Both the indium lines 4101 and 4509 are persistently reversed, 

 and so are several lead lines. Tin gives lines, of which some are re- 

 versed, in highly refrangible regions besides a channelled spectrum, 

 and silver gives a fine fluted-looking spectrum in the blue. Chlo- 

 ride of calcium gives a striking set of six or seven bands between 

 L and M, which may be seen both bright and reversed. 



VI. Occasionally a double self-reversal of lines occurs, i.e. in 

 the middle of the reversed or absorption line a bright line appears. 

 We have many times seen this but not often photographed it, 

 We have a plate which shews it in the case of the magnesium 

 triplet between K and L. The photograph is that of the arc 

 itself projected on to the slit of the spectroscope by a lens, and 

 was taken at the moment when apiece of magnesium was dropped* 

 into the crucible. This has caused such an expansion of the 

 bright lines that they have extended beyond the nearest two 

 cyanogen bands and caused them to be reversed. On this bright 

 background the magnesium lines themselves are reversed as a 

 dark band in the centre of the field, and on this dark band are 

 three narrow bright lines forming the second reversal. It is not 

 difficult to understand how this occurs. The magnesium is very 

 quickly evaporated and part of the vapour is driven by the expansion 

 up the tube of the crucible before it has got very hot, and it drives 

 before it some hotter vapour previously in the hottest part of the 

 crucible. The cool vapour stops the radiation of the arc and the 

 hotter vapour in front of it of course appears bright. The effect 

 is little more than momentary. In this case the cool vapour was 

 only just around the arc, for at top and bottom the same narrow 

 lines which are bright in the middle of the field (black in the 

 negative photograph) are reversed in the top and bottom. 



It is worthy of remark that the facility of reversal of lines 

 appears, speaking generally, to increase as the wave-length 

 diminishes. This is very notable in the case of iron. It seems 



