DR L. BECKER ON THE SOLAR SPECTRUM. 131 



About the two isolated bands, X = 5538 to 5386 and X = 51 1 1 to 4981 , we could 

 not arrive at a satisfactory conclusion. Certainly, most of the lines can be brought into 

 agreement with the values given in the table on p. 93, but some would still remain 

 liable to a far greater absorption. Perhaps the values given in that table for the intensity 

 at low sun may have to be increased, and many further coincidences of telluric and solar 

 lines admitted. 



From the foregoing examination we come to the conclusion that the telluric lines 

 from X = 6020 to 5660 can be arranged in three bands, and that all the lines of the 

 same band are probably due to the same absorptive medium. 



There is no doubt left by our observations that most of the lines of the first band, 

 X = 6020 to 5840, originate in a variable constituent of our atmosphere. That this 

 constituent is water- vapour was established long ago by M. Janssen. The band is univer- 

 sally known as the rain-band. Our observations ascribe also the darker lines of the third 

 band, X = 5780 to 5660, to the absorption of a variable element, whereas the origin of the 

 group of faint lines which form the second band and overlap the third cannot be deduced 

 from our observations alone. 



On the refrangible side of the rain-band Brewster's 1 map contains a very dark band, 

 which he calls 3, and which is very probably identical with our third band. He gives the 



following description of it : — " it is one of the most characteristic features of the 



prismatic image of the light that has passed through a long space of air. It is discernible 

 in the diffuse light of a dull day at any hour ; it is that which Professor W. A. Miller 

 observed manifesting itself on the occasion of a thunder shower, 2 and it becomes evident in 



the direct solar rays when the luminary is several degrees above the horizon : and 



when the sun is just setting, it becomes a broad space of almost total darkness. It appears 



to cover a larger amount of the image in the direction of E, as it deepens in shade 



There seems to be a difference in the visibility of these bands at different times, 



thus on October 29, 1837, at Allerly, near Melrose, at the instant of sunset the luminous 

 sky gave a spectrum in which C6 ( = a), though distinctly seen, was not black, nor was D, 



nor 8, while the line B was very broad and deep and until the twilight had gone, 



the forementioned bands, usually so prominent, did not appear either black or white. On 

 October 31, again, the atmospheric lines were not so dark as usual, while the rays beyond 



ClO ( = rain-band) had evidently suffered a considerable absorption, but that the 



phenomena did not depend on either the absence or presence of humidity in the atmosphere, 

 is evident from the fact that on the earlier date there was a keen frost, while on the later 



day the weather was wet, the thermometer being 38° F That moisture has some 



influence in the production of these bands, is shown by the effect of a fog on the solar 

 radiations; thus on November 20, 1858, at 10 o'clock a.m., at London, the sun loomed red 

 through a mist, and a prismatic analysis of its light showed a and B with extreme 

 distinctness, and the characteristic C (6), S, and >?." 



1 Phil. Trans., vol. 150, 1860, p. 154, London, 1861. 



2 Phil. Mag., August 1845, p. 85. 



VOL. XXXVI. PART I. (NO. 6). Y 



