130 DR L. BECKER ON THE SOLAR SPECTRUM. 



the former it may be supposed that they are solar lines on which fainter telluric lines are 

 superposed. Also, the great change in intensity of the one line may be accounted for by 

 the table. By comparing the intensities of many close double lines with the intensities 

 observed when they were not separated, we deduced that two close faint lines combined 

 appeared hardly one class of intensity darker than the components, while two close dark 

 lines produced the effect of a line two classes darker in intensity than its components. In 

 this particular case the line " 3 " ought to consist at least of 4 lines of intensity " 2," in order 

 to present at low sun the appearance of a line of intensity 11. If the probability of this 

 explanation is not conceded, one is obliged to assume that the absorption produced by 

 one and the same medium in the same part of the spectrum need not necessarily obey the 

 same law. Treating all the other classes of intensity in the same way, we find amongst 

 376 telluric lines, 23 superposed on solar lines and 12 close multiple lines. These 

 numbers will increase if the range of each class be diminished. 



We have to draw attention to one particular telluric line, which, invisible at medium 

 altitudes of the sun, comes into existence as one of the broadest lines of the spectrum, on 

 the less refrangible side of D 2 , when the sun is near the horizon. At an altitude of 

 about 6° it appears just as broad and of the same intensity as its great solar companion. 

 Besides the observations given below, we found the D 2 -\me double on 1 2 occasions from 

 June 14, 1887, to August 15, 1888, the sun being at an average altitude of 6 degrees. 

 On July 19, 1888, this appearance was confirmed in the third spectrum under a higher 

 magnifying power of the viewing telescope. Further to the less refrangible side, another 

 line of the intensity " 4 " was observed close to it on three of these days. When the sun 

 approached the horizon both lines broadened and formed one dark band with D 2 . Now 

 D 2 is one of those broad solar lines whose intensity of blackness is below that of a telluric 

 line of the same breadth. If, therefore, the telluric companion at a certain altitude showed 

 the same intensity and breadth as D 2 , we might explain its appearance by supposing the 

 telluric line produced by a set of very close atmospheric lines of less intensity. 



In the next region, A = 5840 to 5780, we meet with faint telluric lines of different 

 behaviour. In a high sun they are hardly perceptible, while near the horizon their 

 intensity is increased only two or three classes of the scale. The changes in their intensity 

 may be explained by Table VI. in the same manner as before. This band appears 

 to be continued in the following region, A = 5780 to 5720, although there are 

 some lines to be found which suffer a much greater absorption. On account of the 

 small changes in the intensity of most of the lines it seems improbable that the lines 

 which alter up to nine classes of intensity are produced by bands of lines. We rather 

 believe them to be due to the action of another medium. Under this supposition we 

 represent the changes of intensity, as exhibited by Table V., by two sets of intensities 

 in Table VI. 



Both classes of lines are continued within the region A. = 5720 to 5660; the 

 number of the faint telluric lines has however decreased, while the darker ones are 

 still as numerous as before. 



