340 



they are merely the deepening of bands seen at any time, in other 

 cases they are bauds which appear for the first time when the sun 

 is close to the horizon. Professor Piazzi Smyth has represented some 

 of these lines in his delineations of the spectrum as observed from 

 the Peak of Teneriffe, whence he had the advantage of seeing the 

 sun at an altitude of 1*1. The most remarkable of the atmo- 

 spheric lines are situated in the orange and yellow spaces, and one 

 band just beyond D is discernible in the diffused light of a dull day 

 at any hour, though it covers what is about the brightest part of the 

 prismatic image obtained from direct sunshine. The western sky 

 after sunset exhibits these phenomena in a striking manner, and with 

 some variations that do not appear to depend altogether on the 

 absence or presence of moisture, although when the sun looms red 

 through a fog these lines also make their appearance. They are in 

 no respect due to the mere reduction in the quantity of light. 



The dispersion and absorption of the more refrangible rays by the 

 atmosphere, and by fogs, smoke, and such media as dilute milk and 

 water, is a quite independent phenomenon. 



8th and 9th. Enlarged views of A and B, when the light is acted 

 upon by a long passage through the atmosphere. 



10th. A map of the spectrum, exhibiting on a large scale the dark 

 lines and bands which were seen by Sir David Brewster when nitrous 

 acid gas is interposed between the prism and the source of light. 

 Their position is identified by the insertion of the principal lines of 

 the solar spectrum. They differ considerably from a smaller drawing 

 of the same by Professor W. A. Miller, who employed a deeper 

 stratum of the red gas. 



The light of the moon, which is only that of the sun reflected from 

 her surface, exhibits all the principal lines from about B to H, and 

 no fresh ones ; and when the luminary was near the horizon, the more 

 prominent atmospheric lines were detected. The green colour was 

 observed to extend a little beyond E- in the spectrum of moonlight, 

 and the space between G and H appea'red lavender or lavender-grey 

 instead of violet. 



In respect to the origin of these lines, it is conceivable that the light 

 when emitted from the photosphere itself is deficient in these rays, 

 or that they are due to absorption by the atmosphere of the sun, or 

 by that of the earth. The first of these suppositions scarcely admits 



