THE SPECTRA OE SOME OF THE FIXED STARS. 
421 
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 pre- 
sent in the solar spectrum were detected. 
[Since these observations were made, we have had a spectrum apparatus constructed 
by Mr. Browning, optician, of the Minories, 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 corre- 
spond exactly in diameter and in focal length with those of which a description has 
been given ; but the eyepiece of the telescope is of less power, and has a magnifying 
power of about three diameters. A second eyepiece was occasionally used, magnifying 
nine diameters. Two prisms are employed ; 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 rela- 
tively to the image of a star. Of this image, enlarged 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 obser- 
vations 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 ; moreover, 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 altitude of Jupiter being small (about 22° 
above the horizon) at the time of observation, it was of great importance to have satis- 
factory evidence that this band was not due to absorption by our atmosphere. 
On June 16, 1864, the moon and Jupiter being near each other in the sky, the oppor- 
tunity was seized to compare directly the moon’s light with that of Jupiter under precisely 
similar conditions of atmosphere. The observations of this evening were decisive in 
showing that this band in the spectrum of Jupiter was due to a modification suffered by 
the solar light before reaching our atmosphere, and therefore due probably to absorp- 
tion by the atmosphere of Jupiter. 
On June 20, and on July 12 and 14, an observation still more crucial was obtained. 
The length of the opening of the slit is much greater than the diameter of the tele- 
