OF ARTS AND SCIENCES : JANUARY 9, 1866. 61 



by Kirchoff in his chart of the solar spectrum between the two D lines, 

 and it is referred by him to the Nickel vapor, — as the D lines them- 

 selves are to the Sodium vapor, in the sun's atmosphere. It is an un- 

 doubted 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 ap- 

 pears within the D lines, as shown in Fig. 1. With a slightly greater 

 amount of vapor the line /S makes its appearance. As the amount of 

 vapor continues to increase, this line becomes more and more prominent, 

 until at last, as shown in Fig. 4, it is even more intense than the fine a. 

 A careful comparison of these two lines might indeed serve as an approx- 

 imate measure of the amount of vapor in the atmosphere ; and a series 

 of comparisons made under the same conditions at different heights 

 would give data for determining the law according to which the amount 

 of vapor decreases with the elevation above the sea level. 



All the aqueous fines change in intensity like the line /3. They are 

 first seen very faintly when the amount of vapor in the air reaches a defi- 

 nite point, varying for the different lines, and gradually gain in inten- 

 sity as the amount of vapor increases. Thus the group of three lines 

 8 e f do not appear in Fig. 2, are barely visible in Fig. 3, but become 

 very marked in Fig. 4.* The lines 77 and 6 and the nebulous band < do 

 not appear until the air is very moist ; and even when it contains 6.57 

 grains of vapor per cubic foot, they are still very faint. Under yet 

 more unusual atmospheric conditions they will undoubtedly become 

 more intense, and we shall then probably be able to completely resolve 

 the nebulous band and count the lines of which it consists. 



It is hardly necessary to repeat, that the examples here given are 

 selected from a large number of observations. During the cold dry 

 weather of winter the appearance of the D line is uniformly as shown 

 in Fig. 1, the line /3 only occasionally appearing when the atmosphere 

 becomes more moist. During the warm weather of summer, when the 

 absolute amount of moisture in the air is in almost all cases greater 

 than in winter, the appearance of the D line is as uniformly that 

 shown in Fig. 3. It is only very rarely in the dry climate of New 

 England, even during the summer, that all the lines shown in Fig. 4 



* With an increasing quantity of vapor in the atmosphere the line y of Fig. 3 

 is seen before the group of lines fit f, and an intermediate figure between 2 and 

 o might he given showing only the lines Di a y & D s . 



