160 REPORT—1868. 
my paper “On the Spectra of the Chemical Elements” *. The least refran- 
gible of the bands occurred in the yellow portion of the spectrum at about 
the distance from E of one-third of the interval which separates E from D. 
In Brorsen’s comet, at the time of the observations, the whole of the bright 
middle part of the nebulosity forming the coma was self-luminous, and it 
was only the extreme very faint portions of the object which gave a conti- 
nuous spectrum. 
Comet IT., 1868. 
On June 18 a comet was discovered by Dr. Winnecke at Carlsruhe, and 
also independently the same night by M. Becquet, Assistant-Astronomer at 
the observatory of Marseilles. 
On June 22 I examined this comet, which consisted of a nearly circular 
coma, which became rather suddenly brighter towards the centre, where 
there was a nearly round spot of light ; from this a tail was traced for nearly 
a degree (see fig. 1. Pl. V.). 
When a spectroscope, furnished with two prisms of 60°, was applied to 
the telescope, the light of the comet was resolved into three very broad bands, 
which, as will be seen in the diagram, do not correspond exactly to the bright 
bands of Brorsen’s comet. 
In the two more refrangible bands the light was brightest at the less re- 
frangible limit, and gradually diminished towards the other side of the bands. 
In the middle and brightest band the gradation of light was not uniform, 
but continued of nearly equal brilliancy for a distance from the less refran- 
gible side of about one-third of the breadth of the band. This band appeared 
to be commenced at its brightest side by a distinct bright line. 
The least refrangible of the bands did not exhibit a similar gradation of 
brightness, but was rather brighter about the middle of its breadth. 
It will be seen, by a reference to the diagram (fig. 2. Pl. V.), that the two 
more refrangible bands are longest at their brightest limits, and become shorter 
in the same proportion as they become fainter. This appearance does not show 
any difference between the light of the coma and of the bright central spot, 
but arises from the circumstance that the light of the comet becomes gra- 
dually fainter from the centre towards the circumference; and consequently 
the light which the eye perceives in the spectrum, though similar through- 
out, is too feeble to be perceived at the ends, as the bands become fainter. 
This obvious explanation is shown to be the right one by the appearance of 
the least refrangible band, in which a gradation of light did not take place. 
In this band the increase of light towards the centre of the coma showed 
itself as a bright axial line gradually fading off in both directions. 
When the marginal positions of the coma were brought upon the slit, the 
three bright bands could still be seen; but when the light became very 
faint, the spectrum appeared to me to be continuous, but it was too faint to 
be traced beyond the positions of the bands, towards the violet, or the red. 
The tail was brought upon the slit, but I was unable, from its excessive faint- 
ness, to determine anything as to the character of its spectrum. 
The bands are laid down in the diagram from careful measures obtained 
with the micrometer-screw attached to the small telescope of the spectrum- 
apparatus. When I compared the spectrum of the comet obtained in this 
way with some diagrams of the spectrum of carbon which I had prepared in 
1864, I was much interested to find that the three bands of the comet’s 
spectrum agreed exactly, not only in position, but also in their general cha- 
* Phil. Trans. 1864, p. 151. 
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