192 Mr. J. N. Lockyer. On the Classification [Jan. TO, 



the vapours thus driven off, and absorption lines of a different wave- 

 length from the vapours surrounding the stones in the head. 



As the comet recedes from perihelion, these changes take place in 

 inverse order. 



The map, fig. 12, represents the sequence which the discussion has 

 shown to be the most probable. 



The following is a list of the comets which most nearly approach 

 the conditions represented, the numbers referring to those placed 

 opposite the various horizons in the map : 



13. Great Comet, 1882 Copeland. 



19 



LLJm 5) 5) M ,j 



11. Comet 6, 1881 Huggins. 



10. I, 1882 Vogel. 



9. I, 1868 Huggins. 



8. Coggia's Comet, 1874 Vogel. 



7. Comet III, 1868 Huggins. 



6. III, 1881 Copeland. 



5. Great Comet, 1882 ri 



4. Comet d, 1880 Christie. 



3. 111,1881 Copeland. 



2. Winnecke's Comet, 1868 . . . Wolf. 



1. 'Comet I, 1866 Huggins. 



This complete sequence has never been observed in any single 

 comet, but it has been continued in some comets where it has been 

 left off in others. Many comets have never been observed beyond 

 the hot carbon stage, whilst others, like Wells's Comet, have not been 

 observed below that stage. Again, this sequence is what we should 

 expect from laboratory observations. The table on p. 191 shows the 

 sequence of the different spectra in a few cases, and it will be seen 

 that in each case, as far as the observations go, the different bands 

 appear in the foregoing order. 



In the case of Encke's Comet, 1871 (p.p. December 28th), as the 

 comet approached perihelion, hot carbon radiation was succeeded by 

 the integrated radiations of hot carbon and manganese, and this again 

 by the integration of hot carbon radiation and manganese absorption 

 as shown in fig. 13. 



The slight variations shown in the positions of the green band 

 (517) are assumed to be due to errors of observations. As I have 

 already explained, the apparent position of the blue band depends 

 upon temperature, the point of maximum luminosity varying between 

 468 and 474. 



The case of Comet III, 1881 (fig. 14), is a little more complicated, 

 but the general result is the same, namely, that radiation phenomena 

 succeed absorption as the comet recedes from perihelion. Twelve 



