1889. J of the various Species of Heavenly Bodies. 189 



carbon give the second and last members of the ultra-violet group, 

 at wave-lengths 3873 and 3850, or lines coincident with them, when 

 the other three are entirely absent. I have, however, found no con- 

 dition under which the first two members of the group, at wave- 

 lengths 3883 and 3870, are as much brighter than the remaining 

 ones, as they are shown in the lithograph which accompanies 

 Dr. Huggins's paper. As shown in the lithograph, the distance 

 between the two brightest members of the group is considerably greater 

 than the distance between the first two members of the ultra-violet 

 carbon group, and if this fairly represents the photograph, the sugges- 

 tion is that we have to deal with the two lines at 3850 and 3873 to 

 which I have referred. Under the conditions at which these are 

 produced, however, I have never obtained at the same time the group 

 in the blue beginning at 4*215, and we should therefore not expect to 

 find them associated with each other in comets. It is also worth 

 noting that nearly all the lines of this group approximate very closely 

 to lines in the flame spectrum of iron. We know that bright iron 

 lines do occur in comets, as, for instance, in Comet Wells and the 

 Great Comet of 1882, and it is nearly certain that the four faint lines 

 between h and H are flame lines of iron and manganese ; it is quite 

 possible, therefore, that the blue-group is not due to carbon at all. 

 The group of four faint lines is certainly not due to carbon under 

 conditions which we are able to reproduce. 



VII. SEQUENCE OF PHENOMENA IN COMETARY SPECTRA. 



The first stage in the spectrum of a comet is, we have seen, that in 

 which there is only the radiation of the magnesium. The next is 

 that in which Mg 500 is replaced wholly or partially by the spectrum 

 of cool carbon. Mg 5201 is then added, and cool carbon is replaced 

 by hot carbon. The radiation of manganese 558 and sometimes 

 lead 546, is then added. Absorption phenomena next appears, 

 manganese 558 and lead 546 being indicated by their masking effect 

 upon the citron band of carbon. The absorption band of iron is also 

 sometimes present at this stage. At this stage also the group of 

 carbon flutings which I have called carbon B* probably also makes its 

 appearance. As the temperature increases still further, magnesium 

 is represented by 6, and lines of iron appear. This takes place when 

 the comet is at or near perihelion. At this stage the repellant action 

 of the sun upon the comet is most effective, and if the vapours are 

 driven off in the line of sight with sufficient velocity, the bright lines 

 will suffer displacement. A double set of phenomena would thus be 

 presented ; there would be radiation lines of one wave-length from 



* Bakerian Lecture, p. 53. 

 VOL. XLV. 



