162 Mr. J. N. Lockyer. On the Classification [Jan. 10, 



approach the sun, and vice versa, as well as the diminution of period 

 presented by some of them, can be explained on the probable supposi- 

 tion that a comet is a mere cloud of small masses such as stones and 

 fragments of meteoric iron, shining by reflected light alone, except 

 where these masses impinge on one another, or on other matter 

 circulating round the sun, and thus produce luminous gases, along 

 with considerable modifications of their relative motions. Thus the 

 gaseous spectrum of the nucleus was assigned to the same impacts 

 which throw out from the ranks those masses which form the tail."* 



It is not too much to say that at the present time it is generally 

 accepted that the heads of comets are meteor-swarms, possibly the 

 densest portion of each swarm, or portions with the same orbit in the 

 case of multiple comets. 



I propose now to set forth the spectroscopic evidence which I have 

 obtained bearing upon the nature of, and the changes which take place 

 in, these meteoric swarms which have become entangled in our 



II. COMETS AT APHELION. LOWEST TEMPERATURE. 

 Magnesium Radiation, \ 500. 



When a tube such as I have already described is used in experiments 

 to determine the spectrum of meteoric dust at the lowest temperature, 

 we find that the dust in many cases gives a spectrum containing the 

 magnesium fluting at 500, which is characteristic of the nebulae, and 

 is often seen alone in them. If the difference between nebulae and 

 comets is merely of cosmographical position, one being out of the 

 solar system, and one being in it, and further, if the conditions as 

 regards rest are the same, the spectrum should be the same, and we 

 ought to find this line in the spectrum of comets, when the swarm 

 most approaches the undisturbed nebulous condition, the number of 

 collisions being at or near a minimum, i.e., when the comet is near 

 aphelion, the fluting should be visible alone. 



As a matter of fact in comets of 1866 and 1867, when they were 

 observed away from the sun, the only line seen was the one at 500.f 



It is probable also that the fourth band mentioned by Konkoly in 



* Tait, ' Edinb. Roy. Soc. Proc.,' vol. 6, p. 553 (1869). 



f " In January, 1866, I communicated to the Royal Society the result of an 

 examination of a small comet visible in the beginning of that year ('Roy. Soc. 

 Proc.,' vol. 15, p. 5). I examined the spectrum of another small and faint comet in 

 May, 1867. The spectra of these objects, so far as their feeble light permitted 

 them to be observed, appeared to be very similar. In the case of each of these 

 comets the spectrum of the minute nucleus appeared to consist of a bright line 

 between b and F, about the position of the double line of the spectrum of 

 nitrogen, while the nebulosity surrounding the nucleus and forming the coma 

 gave a spectram which was apparently continuous " (Huggins, ' Roy. Soc. Proc.,' 

 vol. 16, p. 381). 



