of the Solar Spectrum. 339 



ture in the atmosphere was 3*76 grains per cubie foot. The ap- 

 pearance of the 1) line at this time is shown in fig. 3. Two of 

 the lines, rj and S, and the nebulous band k, seen on the 17th of 

 November, were invisible, and, moreover, the group of three lines, 

 8, e, f, on the left-hand side of the figure, were only just within 

 the limits of visibility. 



On the 25th of December only two lines were visible within 

 the D lines, marked « and (3 in fig. 2, and the last of these was 

 quite faint. The temperature at the time of observation was 

 46°; the wet-bulb thermometer indicated 40°, and the amount 

 of moisture in the air was 2*42 grains per cubic foot. The sky 

 was clear, and the sun brilliant. Lastly, on January 5, 1866, 

 one of the clear cold days which are so common in our climate 

 during the winter, only the single line « was visible within the 

 D line, as is shown in fig. 1. At the time of observation (near 

 noon) the dry-bulb thermometer marked 10°, the wet-bulb 9°; 

 and hence the amount of moisture in the atmosphere was only 

 0*81 of a grain per cubic foot. The sun, however, was as bril- 

 liant as in either of the previous cases. The D line also ap- 

 peared as in fig. 1 on the 8th of January, 1866, when the ther- 

 mometer at noon stood at 10° below zero Fahrenheit, and when 

 the barometer attained the unusual height of 31 inches. 



The above figures have been drawn so as to show, as nearly 

 as possible, the relative intensity of the different lines under dif- 

 ferent atmospheric conditions. As no accurate means of making 

 the comparison are yet known, 1 was obliged to depend upon 

 my eye alone ; and small differences at different times of obser- 

 vation 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 a in all the figures, does 

 not vary in intensity, and served as a constant standard in 

 making the observations. This is the only line which is given 

 by Kirchhoff in his chart of the solar spectrum between the two 

 D lines, and it is referred by him to the nickel-vapour, as the D 

 lines themselves are to the sodium-vapour, in the sun's atmo- 

 sphere. 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 

 invariable. 



With a very dry atmosphere the line a is the only one which 

 appears within the D lines, as shown in fig. 1. As the amount 

 of vapour increases, the line (3 makes its appearance. At first it 

 is barely visible ; but as the amount of vapour increases still 

 further it becomes more and more prominent, until at last, as 

 shown in fig. 4, it is even more intense than the line a. A 

 careful comparison of these two lines might indeed serve as an 

 approximate measure of the amount of vapour in the atmosphere; 



