On T] Argils and Jupiter's SpectruTn. 25 



With the Melbourne Reflector we have command of con- 

 ditions more favorable than those under which Mr. Huggins' 

 worked, but the conditions may be considerably varied at 

 pleasure, and when by such variation the light was reduced 

 to an intensity probably much less than that at Mr. Huggins' 

 disposal, the line in question (914 of Mr. Huggins' diagram) 

 was still conspicuous ; so unexpectedly conspicuous, indeed, 

 that until its position had been accurately determined, the 

 line was mistaken for an atmospheric one strongly marked 

 in low sun spectra, but, as was afterwards found, not readily 

 visible on Jupiter when at considerable altitudes.* 



Considering therefore that the line or rather group, escaped 

 Mr. Huggins' notice with his earlier apparatus, there is good 

 reason for supposing that the absorption by Jupiter's atmo- 

 sphere of that particular kind of light varies considerably. If 

 this should prove to be the case, it will be interesting to note 

 the degree of absorption in connection with the character of 

 Jupiter's visible disc. 



With reference to this point, I may remark that the 

 appearance of Jupiter last year was somewhat unusual, the 

 principal peculiarity being a change in the colour of the 

 central band from white to yellow, and I believe a greater 

 yellowness of the general surface. 



A sufficient reason for increase of visibility in the Jupiter 

 line may be found in a diminution or depression of cloud, 

 whereby the light would have to traverse greater thickness 

 of atmosphere ; the greater yellowness of the surface is also 

 fahly accounted for on the same supposition. 



Jupiter was taken in hand principally to note any pecu- 

 liarity in the light from different parts of the surface, for 

 which purpose the Melbourne Reflector, owing to its great 

 focal length, is specially suited. 



The method generally adopted was to place the slit of the 

 spectroscope perpendicular to Jupiter's equator; by this 

 arrangement a spectroscopic picture of the surface is pre- 

 sented to the view, and an admirable opportunity afforded 

 of comparing the spectra of the different zones, and of noting 



* 882 of Mr. Huggins' diagram (the numbers throughout refer to this 

 diagram accompanying Mr. H.'s paper on Jupiter.) 



This line was well seen together with 914 when Jupiter was near the 

 horizon. 



882 was nearly as dark as 914, which did not seem to have increased from 

 the additional absorption of the earth's atmosphere ; this was not unexpected, 

 for the correBponding group in low sun spectra is very faint. 



