OF SOME OF THE STABS AND NEBULAS. 
543 
line at 500‘80 millionths of a millimetre, the effect of half the orbital motion would be 
to degrade the refrangibility of the line by 0'023, an alteration of wave-length which 
would correspond to about 0-01 of the large micrometer-head, an interval too small to 
be detected. 
We learn from these observations, that if the line be emitted by nitrogen, the nebula 
is not receding from us with a velocity greater than ten miles per second ; for this motion, 
added to that of the earth’s orbital velocity, would have caused a want of coincidence 
that could be observed. Further, that if the nebula be approaching our system, its 
velocity may be as much as twenty miles or twenty-five miles per second ; for part of its 
motion of approach would be masked by the effect of the motion of the earth in the 
contrary direction. 
The double line in the nitrogen-spectrum does not consist of sharply defined lines, 
but each component is nebulous, and remains of a greater width than the image of the 
slit* * * § . The breadth of these lines appears to be connected with the conditions of tension 
and of temperature of the gas. PLUCKERf states that when an induction-spark of great 
lieating-power is employed, the lines expand so as to unite and form an undivided band. 
Even when the duplicity exists, the eye ceases to have the power to distinguish the 
component lines, if the intensity of the light be greatly diminished. 
Though I have been unable to detect duplicity in the corresponding line in the 
nebula, it might possibly be found to be double if seen under more favourable condi- 
tions ; I incline to the belief that it is not double J. 
In my Tables of the lines of the air § I estimated the brightness of each of the com- 
ponents of the double line in the spectrum of nitrogen at 10, and the components of the 
double line next in brightness in the orange at 7 and 5, and those of a third double line 
on the less refrangible side of D at 6 and 4. It was with reference to these two double 
lines next in apparent brilliancy that I wrote || , in speaking of the line in the nebula, 
“ If, however, this line were due to nitrogen, we ought to see other lines as well ; for 
there are specially two strong double lines in the spectrum of nitrogen, one at least of 
which, if they existed in the light of the nebulse, would be easily visible.” 
As the disappearance of the whole spectrum of nitrogen, with the exception of the one 
double line, was unexpected, though, indeed, in accordance with my previous estimations, 
I examined the spectrum of nitrogen with a spectroscope furnished with one prism with 
a refracting angle of 60°, in which the whole of the spectrum from C to G is included 
in the field of view. I then moved between the eye and the little telescope of the spec- 
troscope a wedge of neutral-tint glass corrected for refraction by an inverted similar 
* Secchi states that with his direct spectroscope this line in the annular nebula in Lyra appears double. As 
the image of the nebula is viewed directly, after elongation by the 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. 
f Philosophical Transactions, 1863, p. 13. 
i “On the Spectra of the Chemical Elements,” Philosophical Transactions, 1864, p. 141. 
§ Ibid. || Ibid. p. 443. 
4 F 
MDCCCLXVm. 
