18S4.I Observations on Twilight. 453 
intense ; beneath this green, along the horizon, is an orange- 
yellow stratum of about 3", which on its lower side begins 
to change into rose, brownish red to purple-violet, and ex- 
ends from south to north. In the east the green becomes 
more intense, and rises up to 6°, whilst immediately on the 
honzon there appear faint rose-colour and yellow of about 
2 in altitude. 
When the sun has gone down the green in the eastern sky 
extends up to 9 0 , the yellow to 6°, and the full rose to 4°. 
ine lattei colour is more intense on its lower side, and fades 
away into an ill-defined stratum of deep steel-blue of V in 
altitude the beginning of the earth’s shadow. 
In the western sky the rose-colour has decreased; the 
yellow takes a more orange tint; the green increases in 
intensity, whilst the pale blue descends to about 45 0 . 
Important changes now take place in the east. When 
the sun is about f° below the horizon the deep steel-blue 
appears as a segment of about i° in altitude and 75 0 in 
azimuth. The superimposed rose has become deeper, and 
extencis to 10 , whilst the yellow and green have generally 
entirely disappeared. 
As the sun sinks deeper the rose colour takes a more 
purple tone, and the margin of the dark segment turns violet 
whilst this segment itself becomes lighter, and changes from 
a steel-blue to an ash-grey. These colourations of the 
eastern heavens above the earth’s shadow are known as the 
“ counter-dawn,” and vary in their altitude with the moisture 
of the atmosphere, extending higher as it increases. On an 
average the boundary arc of the earth’s shadow can be ob- 
served up to an altitude of 15 0 ; the segment then extends 
in azimuth foi 150 , when the sun is 4*6 below the horizon. 
The superincumbent rose disappears generally at an altitude 
of 25 , whilst the sun has descended to 4^9 below the hori- 
zon. I he so-called “ civil twilight ” — as distinguished from 
the astronomical twilight— is then at an end. 
In the western heavens the orange-yellow band of 3 0 in 
altitude remains unaltered ; the brownish red below it dis- 
appears entirely, and the green extends only to 8°. When 
the sun is 3‘8 J below the horizon the sky takes a rosy cast 
up to 23 , this coloui becomes more decided and extends 
so that a losy led segment of about 40° is superimposed 
upon the lower strata, and attains its greatest intensity of 
colour when the sun has sunk to 4-3°, at the spot where the 
first appearance of colour was observed. This first rosy 
light sinks rapidly downwards, whilst the lower strata vary 
little, and when the sun has sunk to 6" it has in general 
quite disappeared. 
