On 7] ArgiXs and Jupiter's Spectrum. 27 



widely unequal tliicknesses of atmospliere, or, perhaps with 

 greater reason, that the least thickness was sufficient to 

 produce a maximum absorption on light of the refrangibUity 

 of 914. 



According to the generally received opinion, in P Q R and 

 similar dark belts, we obtain views of the body of the planet; 

 the other phenomena are also fairly accounted for on the sup- 

 position that P Q is a dense stratum of cloud ; that the light 

 from other parts of the disc is also reflected mainly from 

 cloud, less dense, however, and more interrupted, affording 

 partial views of the body of the planet ; hence the duskiness 

 of Q R, the yellowness of this zone, and the same but less 

 marked colour of the polar segments being due to the general 

 absorption of the violet end of the spectrum* by aqueous or 

 other vapours. 



We have yet no certain evidence as to what substance or 

 substances absorb light of the refrangibilities covered by the 

 group 914, but it would seem that aqueous vapour cannot 

 be largely concerned in the production of this group, for its 

 darkness on the zone Q R accuses a very considerable thick- 

 ness of the particular absorbents above that cloud band, and 

 if aqueous vapour existed there to any great extent, we 

 might expect the zone to be less distinctly white. 



The observations of Mr. Huggins are very strongly in 

 favour of the supposition that another line (838) has its 

 origin in Jupiter's atmosphere, the evidence adduced here 

 by repeated comparison of the spectra of Jupiter and the 

 Moon at considerable and nearly equal altitudes, leave little 

 doubt of the truth of this supposition. 



This line is near G, and in low sun spectra is much more 

 conspicuous than 914 ; as the reverse holds on Jupiter, it 

 would seem that these two lines are due to different 

 absorbents. 



From the researches of Messrs. Angstrom and Jansen it 

 appears that 838 belongs to aqueous vapour. On this sup- 

 position, if the foregoing explanations of the phenomena 

 observed on Jupiter is true, we might expect to find the line 

 less distinctly marked on the white zone than on the other 

 parts of the surface ; the line, however, was so little con- 

 spicuous on any part that no certain evidence could be 

 elicited. 



* Combined -with the selective absorption at the less refrangible end, 

 evidenced by the dark Hnes, and the apparent obliteration of the extreme 

 red (not seen perhaps merely on account of faintness). 



