DR L. BECKER ON THE SOLAR SPECTRUM. 127 



In this table n is the number of lines in 100 that show an average difference Ai 

 between the means of the intensities in low and in high sun. The first part of the 

 table, in which there are no telluric lines, proves that the same scale of intensity applies 

 to both low and high sun. Only in the regions of blue-green have the lines been 

 estimated too faint near the horizon. In the regions of telluric lines, especially in the 

 less refrangible part, where many dark lines spring up in low altitudes, the solar lines 

 are estimated much too faint in a low sun. The reason for this may be traced to the 

 effect of contrast. This difference, however, simply emphasizes the lines produced by 

 atmospheric absorption. 



8. The Tellueic Lines. 



A great many of the telluric lines could be designated immediately, while others 

 presented much difficulty. The faint lines of the two lowest classes of intensity naturally 

 gave the most trouble; at high altitudes they were easily overlooked in the strong light 

 of the continuous spectrum, while near the horizon they might be easy objects under 

 favourable conditions of the sky. In these instances we were guided in our decision by 

 the behaviour of other faint lines of undoubted solar origin. 



In order to avoid mistakes this part of the work was repeated several times. Of course, 

 in a region where many telluric lines occur, there is a tendency to ascribe lines to 

 atmospheric absorption which in other places would pass as solar. For this reason the 

 sheets on which the observations were entered were taken at random, when being 

 examined respecting the origin of the lines. 



Due regard was also paid to the mass of air the light had to pass through. Throughout 

 we endeavoured to reduce the intensity of the telluric lines to a uniform depth of 

 atmosphere, at least in the same spectral region. In the yellow they correspond to about 

 89°, and in the green to 88° - 3 zenith-distance for an average amount of water- vapour. 



With few exceptions all the telluric lines thus picked out were found to be arranged in 

 three bands, the first with 678 lines stretching from \= 6020 to 5666 A.U., the second 

 from A = 5530 to 5386 A.U., with 106 lines, and the third from \ = 5111 to 4981 A.U., 

 with 116 lines. 



One would think that telluric lines which are of equal intensity in the same part of 

 the spectrum at medium altitudes of the sun would behave alike, if the absorption be 

 increased. This, however, does not happen as a rule. 



In the following table the horizontal rows show the number of telluric lines of a given 

 intensity in any part of the spectrum at medium altitudes, while the vertical column in 

 which the number stands indicates the intensity of the same lines, when seen near the 

 horizon. E.g., from the second row of the table we see that with high sun there are 14 lines 

 in \ — 602 to 584 of an intensity = 1, which assume an intensity of 6 near the horizon. 

 Lines that are not visible in a medium sun are ranged in the row of intensity = 0. 



