SCIENCE. 
83 
ON A PHOTOGRAPH OF JUPITER’S SPECTRUM, 
SHOWING EVIDENCE OF INTRINSIC LIGHT 
FROM THAT PLANET. 
By Professor Henry Draper, M. D.* * 
There has been for some years a discussion as to whether 
the planet Jupiter shone to any perceptible extent by his 
own intrinsic light, or whether the illumination was alto- 
gether derived from the sun. Some facts seem to point to 
the conclusion that it is not improbable that Jupiter is still 
hot enough to give out light, though perhaps only in a peri- 
odic or eruptive manner. 
It is obvious that spectroscopic investigations may be use- 
fully employed in the examination of this question, and I 
have incidentally, in the progress of an allied inquiry, 1 made 
a photograph which has sufficient interest to be submitted 
to the inspection of the Astronomical Society. 
If the light of J upiter be in large part the result of his own 
incandescence, it is certain that the spectrum must differ 
from that of the sun, unless the improbable hypothesis be 
advanced that the same elements, in the same proportions 
and under the same physical conditions, are present in both 
bodies. Most of the photographs I have made of the spec- 
trum of Jupiter answer this question decidedly, and from 
their close resemblance to the spectrum of the sun indicate 
that, under the average circumstances of observation, 
almost all the light coming to the earth from Jupiter must 
be merely reflected light originating in the sun. For 
this reason I have used the spectrum of Jupiter as a refer- 
ence on many of my stellar spectrum photographs. 
But on one occasion, viz. : on September 27, 1879, a 
spectrum of Jupiter with a comparison spectrum of the 
moon was obtained which shows a different state of things. 
Fortunately, owing to the assiduous assistance of my wife, 
I have a good record of the circumstances under which this 
photograph was taken, and this will make it possible to 
connect the aspect of Jupiter at the time, with the spectrum 
photograph, though I did not examine Jupiter with any care 
through the telescope that night, and indeed did not have 
my attention attracted to this photograph till some time 
afterwards. 
I send herewith to the Astronomical Society for examina- 
tion the original negative which is just as it was produced, 
except that it has been cemented with Canada balsam to 
another piece of glass for protection. Attached to the 
photograph is an explanatory diagram, intended to point out 
the peculiarities which are of interest. It will be noticed at 
once that the main difference is not due to a change in the 
number or arrangement of the Fraunhofer lines, but rather 
to a variation in the strength of the background. In the case 
of the moon the background is uniform across the width of 
the spectrum in any region, but in the case of Jupiter the 
background is fainter in the middle of the width of the 
spectrum in the region above the line h, and stronger in the 
middle in the region below h, especially towards F. The 
observer must not be confused by the dark portion where 
the two spectra overlap along the middle of the combined 
photograph. 
In order to interpret this photograph it must be under- 
stood that the spectrum of Jupiter was produced from an 
image of the planet thrown through the slit of the spectro- 
scope, by a telescope of 183 inches focal length, the slit being 
placed approximately in the direction of a line joining the 
poles of the planet. The spectroscope did not, therefore, 
integrate the light of the whole disk, but analyzed a band at 
right angles to the equator and extending across the disk. 
If either absorption or production of light were taking place 
on that portion of Jupiter’s surface there might be a modifi- 
cation in the intensity of the general background of the 
photographed spectrum. 
A casual inspection will satisfy any one that such modifi- 
cations in the intensity of the background are readily per- 
ceptible in the original negative. They seem to me to point 
out two things that are occurring : first, an absorption of 
solar light in the equatorial regions of the planet ; and 
second, a production of intrinsic light at the same place. 
We can reconcile these apparently opposing statements by 
the hypothesis that the temperature of the incandescent sub- 
stances producing light at the equatorial regions of Jupiter 
did not suffice for the emission of the more refrangible rays, 
and that there were present materials which absorbed those 
rays from the sunlight falling on the planet. 
If the spectrum photograph exhibited only the absorption 
phenomenon above h , the interest attached to it would not 
be great because a physicist will readily admit from theo- 
retical considerations that such might be the case owing to 
the colored belts of the planet. But the strengthening of 
the spectrum between h and F in the portions answering to 
the vicinity of the equatorial regions of Jupiter bears so 
directly on the problem of the physical condition of the 
planet as to incandescence that its importance cannot be 
overrated. 
The circumstances under which this photograph was taken 
were as follows : Longitude of observatory 4 11 65"' 2g 9 '7 west 
of Greenwich. Night not very steady. Jupiter and the 
moon differed but little in altitude. Jupiter’s spectrum was 
exposed to the photographic plate for fifty minutes, the moon 
was exposed for ten minutes. Jupiter was near the mer- 
idian. The photograph of Jupiter’s spectrum was taken 
between 9 11 55'" and io' 1 45'", New York mean time, Septem- 
ber 27, 1879. 
I have suspected that perhaps there may have been an in- 
fluence produced by the great colored patch on Jupiter 
which has made itself felt in this photograph. It may be 
that eruptions of heated gases and vapors of various com- 
position, color, and intensity of incandescence are taking 
place on the great planet, and a spot which would not be 
especially conspicuous from its tint to the eye might readily 
modify the spectrum in the manner spoken of above. 
SECULAR CHANGES IN THE EARTH’S FIGURE. 
An interesting hypothesis has been promulgated before 
the French Academy by M. Faye. It has long been 
known from geodetic surveys and pendulum experi- 
ments that contingents and mountain ranges do not 
exert that attraction on the pendulum which might 
be expected of them, judging from the observed at- 
traction of such isolated masses as Mount Schehallion, 
in Scotland, or the great pyramid. In fact, the deficiency of 
mountains in this respect is so striking that in order to ac- 
count for it geologists and astronomers have imagined that 
there are vast cavities underlying continents and mountain 
chains. A somewhat different explanation of the feeble 
action of Himalayas on the pendulum has been offered by 
Sir George B. Airy, who supposes that the attraction of the 
mountains is counteracted by still fluid lakes of rock below 
them. But this suggestion does not meet the fact, elicited 
by M. Saigey, that the attraction on islands of the sea is 
greater than it ought to be. It appears to be clear, however, 
that there is a relative lack of matter under continents, and 
an excess of it under oceans. The hypothesis of M. Faye 
would seem to solve the problem in a very simple and 
reasonable manner. He holds that under the sea the earth’s 
crust has cooled much more quickly than under dry land, 
and hence the solid sea-bed is denser and thicker than the 
sub-continental mass. Water is a good conductor of heat as 
compared with rock, and being liquid it is also able to con- 
vey heat from its underlying basin. Geodesy shows that the 
present figure of the earth is an ellipsoid of revolution ; but 
if M. Faye’s hypothesis be correct, it has not always been so. 
At first it was an ellipsoid, but the unequal cooling of the 
earth, due to the liquid mantle covering it, led to unequal 
stress and the elevation of continents where the crust was 
thinner. These continents, according to M. Faye, sur- 
rounded the north pole, and the level of the ocean over 
our hemisphere was raised, thus bringing the earth to a 
more spheroidal form. Finally, as the cooling continued, 
the austral continents attracted the oceans, and the figure 
became once more elipsoidal, as it is to-day. If this in- 
genious speculation were the true one, it would unquestion- 
ably help geologists to explain the origin of the glacial 
period. — Engineering. 
* Read before the Royal Astronomical Society, May 14, 1880. 
*See paper kl On Photographing the Spectra of the Stars and Planets ” 
read before the National Academy of Sciences, Oct. 28, 1879, and pub- 
lished in this Journal, Dec., 1879, and in Nature , Nov. 27, 1879, 
