158 REPORT—1868. 
T was not able, even with this great dispersive power, and after long and 
careful scrutiny directed to this point, to discover any duplicity in the line 
of the nebula corresponding to that which characterizes the line of nitrogen. 
The line of the nebula appeared to me narrower, under the same circum- 
stances of slit, than the double line of nitrogen; but the latter line may 
have appeared broader in consequence of irradiation, as it was brighter 
than the line of the nebula. It is worthy of remark that when the induc- 
tion spark was placed before the object-glass, the line of nitrogen was so 
much fainter that it ceased to appear double, and resembled the nebular 
line *. 
The third line of this nebula was also compared, with equal care, with the 
narrow line of hydrogen when the spark is taken in rarefied hydrogen. The 
coincidence of the line of the nebula with that of hydrogen appeared to be 
perfect. ; ‘ 
Now these coincidences with hydrogen and nitrogen were made with an 
apparatus in which a difference in wave-length of 0-0460 millionth of a . 
millimetre would have been detected. The great probability that these lines 
are emitted by hydrogen and nitrogen existing in the nebula, appears by 
these observations to be strengthened almost to certainty. We learn also 
that the nebula was not receding from us with a velocity greater than 10 
miles per second; for this motion, in addition to the earth’s motion in the 
same direction at the time, would have caused a want of coincidence that 
could have been observed. If, however, the nebula were approaching us it 
might have a velocity as great as 20 miles per second ; for part of this motion 
would be annulled by the earth’s motion in a contrary direction. 
It was found that when the intensity of the spectrum of nitrogen was 
greatly diminished by removing the induction-spark in nitrogen to a dis- 
tance from the slit, the whole spectrum disappeared with the exception of 
the double line, which agrees in position with the line in the nebula, so 
that under these circumstances the spectrum of nitrogen resembled the mono- 
chromatic spectrum of some nebule. It is obvious that if the spectrum of 
hydrogen were similarly reduced in intensity, the strong line in the blue, 
which corresponds to the third line of the nebular spectrum, would remain 
visible after the line in the red and the lines more refrangible than F had 
become too feeble to affect the eye. 
These observations suggest the interesting question, whether the few lines 
of the spectra of these objects represent the whole of the light emitted by 
them, or whether these lines are the strongest lines only of their spectra 
which have succeeded in reaching the earth. At present we have no posi- 
tive evidence on this point. Since these bodies have a sensible diameter, 
and in all probability present a continuous luminous surface, we can hardly 
suppose that any lines have been extinguished by the effect of the distance 
of the objects from us. If we should ever have reason to believe that the 
other lines which are present in the spectra of nitrogen and hydrogen are 
quenched on their way to us, we should, it seems, have to regard their dis- 
appearance as an indication of a power of extinction residing in cosmical 
space, similar to that which was suggested from theoretical considerations 
by Chéseaux, and afterwards supported on other grounds by Olbers and the 
elder Struve. 
* Secchi, indeed, states that with his direct spectroscope this line in the annular nebula 
in Lyva was double. As, however, the image of the nebula was viewed directly after 
elongation by a cylindrical lens, and without a slit, it is probable that the two lines may 
correspond to the two sides of the elongated annulus of the nebula, 
_— ee 
