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 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 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 /i, 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 y 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'" 29**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 g 1 ' 55 m and io h 45™, New York meantime, 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 u-nderlying 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 " On Photographing the Spectra of the Stars and Planets," 

 read before the National Academy of Sciences, Oct. 28, 1879, an d pub- 

 lished in this Journal, Dec, 1879, and in Nature, Nov. 27, 1879. 



