Spectra of some of the Fixed Stars. 415 



about Jupiter. The same, though in a much less marked degree, 

 is probably true of Saturn and Mars. In addition, the dimi- 

 nished brightness of the disk of Jupiter near the periphery sup- 

 ports the inference that an atmosphere exists about that planet. 



The planet Jupiter was observed on April 12, 1862, and 

 April 14 and May 1, 1863. The solar lines B, C, D, E, b, F, 

 and G were seen, with numerous fine intermediate lines, and D, 

 E, b, and F were measured; but no marked lines other than 

 those usually present in the solar spectrum were detected. 



[Since these observations were made, we have had a spectrum- 

 apparatus constructed by Mr. Browning, optician, of theMinories, 

 which is similar in general arrangement to that already described, 

 but possesses much less dispersive power. In this apparatus the 

 cylindrical lens, the collimating lens, and the object-glass of the 

 small telescope correspond exactly in diameter and in focal length 

 with those of which a description has been given ; but the eye- 

 piece of the telescope is of less power, and has a magnifying- 

 power of about three diameters. A second eyepiece was occa- 

 sionally used, magnifying nine diameters. Two prisms are em- 

 ployed ; one has a refracting angle of 35°, the other a refracting 

 angle of 45°. 



With this apparatus, in the spectrum of Jupiter a strong line 

 in the red is seen which is scarcely distinguishable with the more 

 powerful instrument, and was from this cause overlooked in our 

 earlier observations. The remarkable increase of visibility of this 

 line is due to the much greater brilliancy of the spectrum in this 

 apparatus ; and this is much more than inversely proportional 

 to the diminution of the dispersion, since, on account of the 

 greatly reduced obliquity of incidence, the loss of light at the 

 surfaces of the prisms by reflexion is much less. This saving of 

 light in the spectrum-apparatus is of very great importance in 

 observations of the planetary spectra. The image of a planet in 

 the telescope is not a point, but forms a disk of considerable 

 magnitude relatively to the image of a star. Of this image, en- 

 larged in one direction by the cylindrical lens, a very narrow 

 section only, corresponding to the breadth of the slit, passes on 

 through the collimating lens to the prisms; and this portion 

 only of the total light collected by the object-glass becomes 

 available to form the spectrum. On this account we have found 

 the observations of the planets much more difficult than would 

 be observations of stars possessing an equal apparent brilliancy. 



This band of which we are now speaking in the spectrum of 

 Jupiter occurs in a rather obscure part of the spectrum ; more- 

 over, by the instrument of greater dispersive power, it appears 

 to be resolved into two or more lines, which are severally very 

 faint, and are less visible than a single stronger line. The alti- 



