180 J. P. Cooke on Aqueous Lines of the Solar Spectrum. 
solvable into lines of still lower magnitude.’ It is impossible to 
represent this band accurately with a wood-cut, and the shaded 
broad band marked *, on the right hand side of fig. 4, only 
Serves to indicate its position and approximate breadth. 
The 26th of December was also a warm day for the season, 
with a brilliant sun. At 1 P.M. the dry bulb thermometer 
marked 55°, the wet bulb 50°, and hence the amount of moist- 
ure in the atmosphere was 3°76 grains per cubic foot.. The ap- 
pearance of the D line at this time is shown in fig. 3. Two of 
the lines, 7 and %, and the nebulous band «, seen on the 17th of 
November were invisible, and moreover, the group of three 
lines 5 ¢¢, on the left hand side of the figure, were only just 
within the limits of visibility. 
noon the dry bulb thermometer marked 10°, the wet bulb 9”, 
and hence the amount of moisture in the atmosphere was only 
ae 
appeared as in fig. 1, on the 8th of January, 1866, when the 
thermometer at noon stood at 10° below zero Farenheit, and 
when the barometer attained the unusual height of 31 inches. 
The above figures have been drawn so as to show as neal! 
as possible the relative intensity of the different lines under du 
ferent atmospheric conditions. As no accurate means of making 
comparison are yet known, I was obliged to depend upo? 
my eye alone, and small differences at different times of observ- 
ation may easily have escaped my notice. Indeed I should 
have been liable to great error were it not for the fact that one 
of the lines within the D lines, marked @ in all the figures does 
not vary in intensity, and served as a constant standard in 
ing the observations. "This is the only line which is given y 
Kirchoff in his chart of the solar spectrum between the two 
lines, and it is referred by him to the nickel vapor as the 
lines themselves are to the sodium vapor in the sun’s atmos 
phere. It is an-undoubted solar line and has been drawn with 
the same strength in all the figures in order to show that it is 1¥_ 
variable. e 
? We use this word in the same sense in which it is used by astronomers in ref- 
erence to the fixed stars. a 
Rome 
