H. Draper—Photograph of Jupiter's Spectrum. 119 
graph which has sufficient interest to be submitted to the 
inspection of the Astronomical Society. 
If the light of Jupiter 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 spectrum of Jupiter, 
answer this question decidedly, and from their close resem- 
blance to the spectrum of the sun indicate that, under the 
average circumstances of observation, almost all the light com- 
ing to the earth from Jupiter must be merely reflected light 
originating in the sun. For this reason I have used the spec- 
trum of Jupiter as a reference spectrum on many of my stellar 
spectrum photographs. 
ut 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 
J apa 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. 4 
__ I send herewith to the Astronomical Society for examination, 
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 A, especiall 
toward F. The observer must not be confused by the dar 
portion where the two spectra overlap along the middle of the 
combined photograph 
In order to interpret this photograph it must be understood 
that the spectrum of Jupiter was produced from an image of the 
planet thrown upon the slit of the spectroscope, 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. | 
spectroscope did not, therefore, integrate the light of the whole 
. 
