278 EEPoiiT — 1880. 



the general rule. It would be interesting to determine the behaviour of 

 the ultra-violet lines of hydrogen which have recently been discovered. 



In the case of oxygen, Plucker and Hittorf have remarked that the 

 less refrangible lines widen most easily. But Plucker and Hittorf did 

 not separate the two line-spectra of oxygen. (See Report on Spectra of 

 Metalloids.) The lines belonging to the lower temperature widen more 

 easily perhaps than any other lines, with the exception of the blue line, 

 which always remains sharp, and presents a striking contrast to the other 

 lines. The two green lines belonging to this spectrum widen more easily 

 than the red line ; so that the lines which do expand follow the rule. 

 The lines of the other line spectrum do not expand very much, though 

 their edges lose their sharpness at high pressures. Plucker and Hittorf 

 remark that the blue group widens more easily than the violet groups. 



The more refrangible of the two double sodium lines (D2) widens 

 more than the less refrangible component. According to Ciamician's 

 experiments, mercury follows the rule, aiid widens the most refrangible 

 lines most easily. It has often been remarked that all lines lose the 

 ehai'pness of their edges when the pressure is increased,' but there is a 

 great difference between the cases we have just mentioned and the lines, 

 for instance, of chlorine, bromine, and iodine, or nitrogen, which may 

 become fuzzy, but never spread over an appreciable part of the spectrum. 

 Though the difference is one of degree onl}-, it is very marked. We may 

 eay that, as a general rule, if a system of lines widens much or easily, 

 the more refrangible lines of the system will be the first to widen, 

 ■while if a system of lines shows the broadening to a small degree only, 

 no general rule can be given. 



When a line widens, it may do so either symmetrically on both sides, 

 or the widening may be greater on one side. It is a remarkable fact that 

 when a line widens chiefly towards one side, that side is in nearly all, if 

 not in all cases, the less refrangible one. 



In the case of the hydrogen lines, it is of some importance to deter- 

 mine whether they widen symmetrically, because the small displacements 

 of the line in stars which are referred to star motions, may be in part 

 due to a one-sided widening. The Greenwich observers * have, therefore, 

 made a very careful series of measurements, in order to find out whether 

 the centre of the F line shifts as it broadens. No shift was detected in 

 a range of pressure from o'O mm. to 600 mm., the width of the line 

 altering considerably within that range. J. J. Miiller's experiments, 

 giving an apparently different result, will be presently referred to. 



The lines of sodium seem also to widen nearly equally on both sides. 

 ZoUner ^ has examined these lines with his reversion-spectroscope — an 

 instrument which is pre-eminently fitted for such an investigation. He 

 gives the results in the following words : ' In the more refrangible line, 

 which, with increase of the vapour-density, was the most widened, no 

 displacement was perceptible ; meanwliile there appeared to take place in 

 the other line, as the brightness increased, an extremely slight displace- 

 ment in the direction of a diminution of the refrangibility.' 



But Zollner does not seem himself to attach very much value to this 

 observation, the displacement being very .slight. Dr. J. J. Miiller * has 



' c.ff. CaiUetet, C. R. Ixxiv. p. 1282 (1872). 



• Besults of the Antiwioiiiirti! OhttcrrHfiniif. made at the lioi/t/l Ohiervatory, 

 tjr.feiimich, 1876, p. 118. 



» Phil. Mag. xli. p. 204 (1871). 



* Lr.^7Jg. Ber. (1871) and I>o,j<j. Ann. el. \>. I'.U (1873). 



