ON THE BLUE MOUNTAINS. 119 
twenty lines between C.A. and D 1, where only three lines had 
been seen at noon; their positions are shown in diagram 3, as 
well as the D lines seen at the same time. 
the Ds; the definition was wonderfully fine and stea Just 
he sun tting I counted twenty-six lines instead of the 
twenty above measured ; in fa ines 8 oming 
Another fact was now clearly made out. In Sydney, owing to 
unsteady definition near the horizon, I had been led to suppose 
that the D 2 became twice as thick, and that D 3 disappeared. 
Now I learn that the thickening was due to D 3, and at sunset 
it seemed another D 2, and only the faintest line of light sepa- 
rated these lines. See diagrams 2 an : : 
While observing the spectrum to-day, I noticed a bright line 
near D line 6. I have seen the same less distinctly in Sydney ; 
it was no subjective effect of contrast. Mr. Hirst also saw it 
_ very distinctly ; it is not a defined line, but a bright streak, and I 
find its wave length is, taking a mean of three measures, 5893: 
very faintly five lines between the Ds, bat they are almost in- 
-visible—little better than shading ; and between Ca and D 1 are 
P.m., count twenty-six lines between Ca and D 1, and the three 
next D 1 are very dark and thick (intensity 2). 
