422 
ME. W. HUGGINS AND DE. W. A. MILLEE ON 
scopic image of Jupiter, even after elongation by the cylindrical lens. If, therefore, at 
the time of observation the light from the sky is sufficiently 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 under precisely similar conditions of altitude and 
of state of atmosphere. To the light of Jupiter under these circumstances of observa- 
tion is added the light reflected from the small area of sky immediately between the 
observer and the planet. This light is, however, too faint in proportion to that of 
Jupiter to become a source of error. In the diagram, fig. 3, Plate X., 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 absorptive 
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. 
Other lines less refrangible were perceived in the spectrum of Jupiter, but were not 
sufficiently distinct to be measured. The bands in the orange and the red to which we 
have referred, when examined in the spectrum apparatus of greater dispersive power, 
and with a much stronger illumination by directing the apparatus to the sun when near 
the horizon, are resolved into groups of lines. The stronger of these lines are repre- 
sented in the upper spectrum of the diagram. The relative position of the band in the 
red due to lines of oxygen and nitrogen when the induction spark is taken in air, is 
shown below the spectrum of Jupiter. This band is in a small degree more refrangible 
than the strong band due to Jupiter. 
If this band, at 914 of the scale, in Jupiter’s spectrum consists of lines severally coin- 
cident with the lines composing the faint atmospheric band with which it appears to 
correspond in position, it would seem entitled to be regarded as an evidence of the 
similarity of J upiter’s atmosphere with our own, with respect at least to one of its con- 
stituents, or to one of the vapours diffused through it. The smaller intensity of the 
bands 882 and 1033 would appear to oppose the supposition that Jupiter’s atmosphere 
is identical with our own. This negative evidence, however, cannot be regarded as of 
much weight, since telescopic observations show that the light which we receive from 
