1889.] On the Spectra of Meteor-swarms. 391 



VII. Tests. 



We have an important test of the accuracy of the preceding observa- 

 tions in tracing the continuity of the lines in passing from the earlier 

 to the later species of the group. In the map which accompanies this 

 paper, the stars have been arranged in order of temperatures by 

 reference to the thickness of F, it being universally agreed that those 

 stars in which the hydrogen lines are thickest are the hottest. With 

 the stars in this order we ought to find that if a line be visible in any 

 two of the stars, it is also visible in any other star of the group in 

 which "P is of an intermediate thickness. On first arranging the 

 stars in this way, it was found that there were here and there breaks 

 in the continuity of the lines, but further observations, made with 

 special reference to the breaks, showed that the discontinuity was due 

 to the incompleteness of the first sets of observations. The only 

 break now shown on the map is the apparent absence of Mg 5527 in 

 X Ophiuchi, and this was not discovered before the star had got too far 

 to the west to be re- observed. 



We have another test in tracing the variations in the intensities of 

 the various lines in passing through the series. Assuming that a 

 sufficient number of stars have been taken, there ought to be no 

 abrupt change in the thickness of a line in passing from star to star. 

 The temperature at which a line is at its maximum thickness will 

 depend on the volatility of the substance which produces it, so that 

 all the lines need not necessarily have their greatesb thicknesses in the 

 same star. The continuity as regards the intensities of the lines is 

 quite as perfect as could be expected from a preliminary survey. 

 Thus D gradually thins from Ceti to a Lyras; b thickens from 

 a Ceti to e Pegasi, and then thins gradually to a Lyrse. This differ- 

 ence in the behaviour of b and D is obviously due to the fact that all 

 the sodium would be distilled out of the meteorites before all the 

 magnesium was driven out. E (5268), 5327, 540, and 499 gradually 

 thicken to (3 Ophiuchi and then thin out. The line at 579 is almost 

 equally thick in (3 Ophiuchi, e Pegasi, a Aquarii, and 7 Aquilee. The 

 line at 568 has a decided maximum in x Ophiuchi. The lines near 

 546 and 558 have their greatest thickness in the earliest stage of the 

 group, gradually thinning out towards the last. The remnant of the 

 iron fluting (615) is seen to gradually disappear between a Ceti and 

 Tauri ; no trace of this fluting was seen with the dispersion em- 

 ployed in any of the stars of a higher temperature than Tauri. As 

 the fluting disappears it is replaced by iron lines of gradually increas- 

 ing intensities. The hydrogen line at Gr was not seen in any of the 

 stars below a Cygni, but it does not follow that it was absent, because 

 the lower stars being generally fainter, the attention of the observers 

 was not directed so far into the blue. 



