416 Mr. W. Huggins and Dr. W. A. Miller on the 



tude of Jupiter being small (about 22° above the horizon) at the 

 time of observation, it was of great importance to have satisfac- 

 tory evidence that this band was not due to absorption by our 

 atmosphere. 



On June 16, 1864, the moon and Jupiter being near each 

 other in the sky, the opportunity was seized to compare directly 

 the moon's light with that of Jupiter under precisely similar 

 conditions of atmosphere. The observations of this evening were 

 decisive in showing that this band in the spectrum of Jupiter 

 was due to a modification suffered by the solar light before 

 reaching our atmosphere, and therefore due probably to absorp- 

 tion by the atmosphere of Jupiter. 



On June 20, and on July J 2 and 14, an observation still more 

 crucial was obtained. The length of the opening of the slit is 

 much greater than the diameter of the telescopic image of Ju- 

 piter, even after elongation by the cylindrical lens. If, there- 

 fore, at the time of observation the light from the sky is suffi- 

 ciently intense to form a visible spectrum, the spectrum of the 

 sky is seen in the instrument together with the spectrum of 

 Jupiter, and much exceeding it in breadth. When the period is 

 so chosen that the degree of illumination of the sky is suitable 

 in proportion to the intensity of the light of Jupiter, the solar 

 lines and those due to our atmosphere are well seen in close con- 

 tiguity with the lines in the spectrum of Jupiter, and occupying 

 exactly similar relative positions. The sky-spectrum is seen 

 uuder precisely similar conditions of altitude and of state of 

 atmosphere. To the light of Jupiter under these circumstances 

 of observation is added the light reflected from the small area of 

 sky immediately between the observer and the planet. This 

 light, however, is too faint in proportion to that of Jupiter to 

 become a source of error. In the diagram, fig. 3, Plate V., the 

 position of this band is shown relatively to the spectrum of the 

 sky. The band at 914 of the scale appears to be coincident with, 

 but much stronger than, a faint band in the sky-spectrum. This 

 increase in the strength of the band is probably due to an ab- 

 sorptive action exerted by the atmosphere of the planet. 



The bands at 882 and 1033 of the scale are less intense in the 

 spectrum of Jupiter than in the spectrum of the light of the sky. 

 This variation of intensity is probably due to the circumstance 

 that the light from the southern sky, before it is reflected to the 

 observer, on account of the position of the sun, which is then 

 near the horizon, has had to traverse a very much larger amount, 

 and a more dense portion, of our atmosphere than that traversed 

 by the light received from Jupiter. It is in accordance with 

 this explanation that these bands are also less intense in the 

 spectrum of the moon when similarly compared with those of 

 the sky. 



