ME. W. HUGGINS ON THE SPECTRA OE SOME OF THE NEBULAE. 
443 
be in great measure, if not wholly, absorbed by the portion of gas through which it 
would have to pass, and for this reason there would be presented to us a luminous sur- 
face only*. 
Sir John Herschel further remarks f, “Whatever idea we may form of the real 
nature of the planetary nebulae, which all agree in the absence of central condensa- 
tion, it is evident that the intrinsic splendour of their surfaces, if continuous, must 
be almost infinitely less than that of the sun. A circular portion of the sun’s disk, 
subtending an angle of 1', would give a light equal to that of 780 full moons, while 
among all the objects in question there is not one which can be seen with the naked 
eye.” The small brilliancy of these nebulae is in accordance with the conclusions sug- 
gested by the observations of this paper; for, reasoning by analogy from terrestrial 
physics, glowing or luminous gas would be very inferior in splendour to incandescent 
solid or liquid matter. 
Such gaseous masses would be doubtless, from many causes, unequally dense in 
different portions ; and if matter condensed into the liquid or solid state were also 
present, it would, from its superior splendour, be visible as a bright point or points 
within the disk of the nebula. These suggestions are in close accordance with the 
observations of Lord Rosse. 
Another consideration which opposes the notion that these nebulae are clusters of 
stars is found in the extreme simplicity of constitution which the three bright lines 
suggest, whether or not we regard these lines as indicating the presence of nitrogen, 
hydrogen, and a substance unknown. 
It is perhaps of importance to state that, except nitrogen, no one of thirty of the 
chemical elements the spectra of which I have measured has a strong line very near the 
bright line of the nebulae. 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 nebulae, would be 
easily visible J. In my experiments on the spectrum of nitrogen, I found that the 
* Sir 'William Heeschel in 1811 pointed ont the necessity of supposing the matter of the planetary nebulae 
to have the power of intercepting light. He wrote : — “ Admitting that these nebulae are globular collections 
of nebulous matter, they could not appear equally bright if the nebulosity of which they are composed consisted 
only of a luminous substance perfectly penetrable to light Is it not rather to he supposed that a certain 
high degree of condensation has already brought on a sufficient consolidation to prevent the penetration of light, 
which by this means is reduced to a superficial planetary appearance ?” 
“ Their planetary appearance shows that we only see a superficial lustre such as opaque bodies exhibit, and 
which could not happen if the nebulous matter had no other quality than that of shining, or had so little 
solidity as to be perfectly transparent.” — Philosophical Transactions, 1811, pp. 314, 315. 
t Outlines of Astronomy, 7th edit. p. 646. 
+ Philosophical Transactions, 1864, p. 154 and Plate I. 
For the purpose of ascertaining whether the absence of the other bright lines of nitrogen might he con- 
nected with the presence of hydrogen, I arranged an apparatus in which, while the spectrum of the induction- 
spark in a current of nitrogen was being observed, a current of hydrogen could be introduced, and the propor- 
