1890.] Rayefs Bright-Line Stars in Cygnus. '67 



In our observations of this and the other stars we did riot attempt 

 micrometric measures of the blue band, but we estimated their positions 

 by means of the intervals between the five flutings of the band of the 

 Bunsen flame. In the case of objects so faint in our instrument 

 when viewed under the dispersion of spectroscope B, we did not 

 consider there would be any real gain of accuracy by attempting to 

 take measures. 



Though the wave-lengths assigned to our positions must therefore 

 be regarded as not more than approximately correct, we have no hesi- 

 tation in considering them fully accurate enough for the purpose of 

 our investigation. 



The star band is not split up into well- separated maxima, as is the 

 Bnnsen flame band, but we have little doubt that the brightest part 

 of the band, from \468 to X469, which is much, and rather suddenly, 

 brighter than its beginning and termination, consists of bright lines. 

 Lines appear to flash out at moments, but in our instruments they 

 cannot be seen with sufficient steadiness for us to be sure of their 

 number and position. 



Under certain conditions of the electric discharge, the normal rela- 

 tive brightness of the component flutings of the blue hydrocarbon band 

 has been observed to be so far changed that the position of maximum 

 intensity is moved from the less refrangible end of the band towards 

 the blue end; but the five flutings remain without any change of 

 their position in the spectrum.* 



Dr. Hasselberg, by means of feeble disruptive discharges from tin- 

 foil terminals placed outside an exhausted tube containing vapour of 

 benzole, obtained a nearly pure spectrum of the order of that in a 

 hydrocarbon flame mixed only with faint lines of hydrogen. He 

 says: " Es war aber hier die violette Gruppe sehr schwach. Dagegen 

 schien mir die blaue Gruppe relativ heller als im Flammenspectrum, 

 und sie hatte ausserdem entschieden ihre grosste Intensitat nicht an 

 der weniger brechbaren Kante, sondern inehr nach dem Violetten 

 hin. Dasselbe schien mir auch mit der gelben Gruppe der Fall zu 

 sein. In Bezug auf die grime Gruppe konnte ich aber keine Vcr- 

 schiebung des Intensitatsmaximums bemerken." 



Dr. Hasselberg gives curves to show the amount of this change of 

 intensity in the blue group and in the orange group. In the blue 

 group the maximum is moved from the first to the third line, that 



* "It is necessary to state that the maximum luminosity of the blue band, under 

 some conditions, is about 468. . . . The conditions under which this band hns 

 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 has a very decided maximum at about 468 " (Appendix to the Bakerian 

 Lecture for 1888, 'Koy. Soc. Proc.,' vol. 45, pp. 167, 168). 



