168 



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



many photographs of the spectra of carbon compounds which show it. 

 The diagram is taken from a photograph of the spectrum of alcohol 

 vapour in a capillary tube with a 9-iiich spark. 



The conditions under which this band has its maximum luminosity 

 at 468 in Geissler tubes seem to be those of maximum conductivity. 

 If the pressure be high all the members of the group are sharp, and 

 the luminosity of the band is almost uniform throughout. This 

 always occurs when the pressure is very low. At intermediate stages 

 of pressure, however, the luminosity of the band has a very decided 

 maximum at about 468. 



This latter condition has been reproduced in many comets, though 

 generally the band has been stated to end at 474, or thereabouts, the 

 maximum possibly having been overlooked. 



It seems probable that a detailed study of this band in our labora- 

 tories will enable us in the future to determine the approximate 

 temperature of a comet by the appearance of this band in its 

 spectrum. 



In the spectrum of Comet 6, 1881 (Observation, June 28th, P.P. 

 June 16, * Copernicus,' vol. 2, p. 227), Copeland states that this band 

 has a fairly sharp edge at 474, and a maximum at 468. 



To measure a maximum in any band is at all times difficult and 

 extremely so in the cases of cometary spectra and Copeland says of 

 the above comet: "The spectrum seemed to change in intensity 

 from moment to moment like a dancing aurora borealis." 



The following table includes the above case, and gives also two 

 other comets in which the blue band had the same appearance : 



The Irregularities Observed in the Citron Fluting. 



It has long been known that the least refrangible band in cometary 

 spectra shows great variation in position from the edge of the true 

 citron carbon-band at 564, and many of these variations have been 



* Astr. Phys. Obs.,' vol. 2, p. 180. 



f * Copernicus,' vol. 2, p. 227. 



