1889.] of the various Species of Heavenly Bodies. 177 



approach to the sun, puts on the appearance of a central nucleus 

 (or nuclei), with surrounding envelopes, or jets, or both. Because 

 the former now falls upon one part of the slit of the spectroscope, and 

 the latter upon another, the difference between the nucleus and the 

 envelopes is best made out when the comet is nearest to the sun and 

 earth. 



When a comet approaches very near to the sun, we get the bright 

 lines, especially in the spectrum of the nucleus, so that in addition to the 

 long flutings of carbon (if they be then visible), we have short lines 

 added along the nucleus in the red, yellow, green, and so on. 



The lines characteristic of the more volatile substances extend some 

 distance from the nucleus. 



It does not always happen, however, that a comet gives a bright 

 line spectrum while near or at perihelion, for the perihelion passage 

 may occur at some distance from the sun, and then the spectrum 

 will be simpler. 



In Comets &, 1881 (perihelion passage June 16), and d, 1882 

 (perihelion passage September 17), the only lines recorded were 

 magnesium ~b but the apparent absence of the other lines might be 

 due to continuous spectrum. 



It should be noted that the greatest brilliancy and maximum of 

 action is observed after perihelion, hence the temperature must be 

 highest after perihelion. 



Magnesium Radiation. 



In cometary spectra we have already seen that magnesium is first 

 indicated by the fluting at 500, and at a more advanced stage by the 

 fluting at 521. There is evidence to show that magnesium is repre- 

 sented by b at perihelion. This was the case in the Great Comet of 

 1882 as observed by Copeland on September 18th, the day after 

 perihelion passage b was probably also seen in Comet III, 1881, by 

 Copeland* (perihelion passage, June 16th). It is described as a well- 

 defined bright line standing at the edge of the bright-green band. 



Carbon Radiation. 



The disappearance of the flutings of carbon in comets which have 

 short perihelion distances when near perihelion, taken in conjunction 

 with laboratory experiments, at once suggests that the disappearance 

 of the flutings ought to be accompanied by the appearance of carbon 

 lines. 



The principal line in the spectrum of carbon is at wave-length 

 426. This has only been recorded on two occasions, in cometary 

 spectra, namely in Comet Wells. On May 28th (perihelion passage, 



* ' Copernicus,' vol. 2, p. 229, 



