202 



REPOET OF SPECTROSCOPIC OBSERVATION OF 

 THE TWILIGHT GLOWS DURING FEBRUARY 

 AND MARCH, 1884. 



By a. B. Biggs. 

 [Read April 8, 1884.] 



In venturing to submit the following notes to the Fellows 

 of the Royal Society, I would observe that I commenced the 

 observations only in the beginning of February. I much 

 regret that I had not the opportunity of conducting observa- 

 tions from an earlier period when the " glow " was in all its 

 grandeur. 



Referring to the diagram, the regular solar lines are dis- 

 tinguished as usual — A, B, C, etc. The features to which I 

 wish to draw particular attention are distinguished by 

 numerals — 1, 2, 3, etc. — 1 and 2 being the most remarkable. 

 I have adopted Roscoe's frontispiece scale — A being at 20, B 

 at 28, C at 34, and D at 50, etc. 



The diagram gives, as nearly as I can show it, the appear- 

 ance of the spectrum near the horizon when the " glow " is 

 moderately strong. By far the most prominent feature in 

 this spectrum is the line or band (2) at scale 41. I have 

 noticed that the deeper the glow the broader and deeper does 

 this band become. The line (1) at 37 also comes into great 

 prominence at such times, fully equalling, and sometimes 

 exceeding C in intensity. This line (1) is, however, very per- 

 sistent, continuing more or less conspicuous throughout the 

 day. (3) At about 44 is a faint line, which is scarcely per- 

 ceptible in the twilight. (4) Is a well-known vapour-band, 

 always more or less conspicuous about the horizon. (5) On 

 the edge of the green is a very broad band, shading off a 

 good way into the green and somewhat resembling a shadow. 



I noted, on the evening of 26th March, when the air was 

 filled with smoke from bush fires that C and 1 were very 

 intense ; 2 was as usual at times of pretty deep glow. The 

 sunset sky was very red on that occasion, evidently from 

 smoke. 



On 3Ist March (evening), after rain, I noted "C and I 

 very strong ; 2 much lighter than usual, being only a little 

 stronger than D." As an instance of the variability of these 

 lines, I noted on 6th inst., at 3 p.m. — " Glow lines all indis- 

 tinguishable except 1, which was very distinct — Bar., 30*25 ; 

 fine." 



On the evening of 31st March, by means of the gas flame 

 turned down to blueness, I got the spectrum of calcium into 

 the spectroscope with the " glow " spectrum, and found the 

 line 2 nearly, if not quite, coincident with the edge of the 

 principal calcium band towards the red end. 



