1879.] 



attending the Reversal of Lines. 



431 



study. The lines which reverse themselves most readily in the arc are 

 generally those, the absorption of which is most developed in the 

 flame ; thus the manganese triplet in the violet is magnificently 

 reversed in the flame, and the blue calcium line is thus often seen 

 widened, H and K being not only not absorbed, but entirely invisible. 



LIST OF PHOTOGRAPHS EXHIBITED. 



t 



Photographs showing passage from truncation to parallelism. 



r. Spectrum of Sr, showing two reversed lines (wave-lengths 4078"5 and 



4215"3) gradually broadening towards one end. 

 II. Spectrum of Ca, showing reversal of the blue hue, and of H and K. 

 While the blue line presents the appearance of a cone, through the 

 centre of which is the absorption line bounded by parallel sides, the 

 H and K lines are almost normal in their appearance, showing, however, 

 a slight widening at one extremity. 

 III. Spectrum of Mn, showing blue Ca line tapering to a point at one ex- 

 tremity and enlarging spindle-shape towards the other end ; the reversal 

 of this line does not extend through its whole length, but merely 

 through the bulging portion, tapering gradually to a point. 

 TV. Spectrum of Sr, showing the two fines (4078 - 5 and 4215"3) which this 

 time present an appearance very similar to the blue Ca fine in the last 

 photograph. In the more refrangible line, however, the reversal retain- 

 ing its tapering form extends through the whole length of the line. 

 Two photographs of the spectrum of Ca, in which not only the blue line but also 



the H and K lines present the appearance of truncated cones. 

 Spectrum of Mn, showing the absorption of its triplet (at wave-length about 4030) 



without its radiation. 

 Spectrum of Mn, in which the triplet is again reversed. Here the triplet, together 

 with its two included bright lines, looks exactly like a group of eight 

 radiation lines, each reversed fine giving the appearance of two bright 

 lines. 



Photographs showing non-symmetrical fines. 



I. Spectrum showing two Ag lines at about wave-lengths 4054*3 and 4210'0. 

 Both lines are fluffy and reversed ; the less refrangible line is much 

 more strongly expanded on its more refrangible side, and is carried up 

 to a much greater height as a radiation fine than its other side. The 

 more refrangible line is more symmetrical, but presents the same phe- 

 nomena to some extent, only in the opposite direction, its less refran- 

 gible side being the most developed. 

 II. Spectrum of Bb, showing fine at wave-length 4202. Here the two 

 ends of the line are produced by radiation alone, the central portion 

 showing absorption on its more refrangible side with fluffy shading on 

 its less refrangible side. 

 Spectra of Sr and Cs, showing the absorption of light due to the poles. 

 Photographs showing the trumpeting of lines. 



I. Spectrum of Ca, in which the reversal is seen to widen as we approach 

 the faint end produced by the cooler external region of the arc, thus 

 showing absorption increasing with reduction of temperature. 

 II. Another spectrum of Ca, showing the same thing again. 

 III. Spectrum of Pb, showing that the Pb line at wave-length 4058 also 

 trumpets. 



