LORD OXMANTOWN ON THE GREAT NEBULA IN ORION. 
nebulosity, whereas those nebulae which give a continuous spectrum appear to fade away 
tolerably uniformly on all sides from their nuclei, and although in some cases they have 
dark lanes running through them, the edges do not generally appear very well defined. 
Spectrum of 'Nebula in Orion. 
With regard to the spectrum of the nebula in Orion, three bright lines were seen 
several times, both with 8-feet and 6-feet instruments, but no attempt was made to 
identify them except on one occasion, when the spectrum of the nebula was compared 
with that of the electric spark in a capillary vacuum-tube containing a trace of hydrogen, 
similar to those used by M. Plucker in his researches on the spectra of gases, and one 
glimpse of the coincidence of the green line in the latter with the most refrangible of 
the three lines in the former was obtained. The least refrangible line was the brightest, 
the most refrangible was next in brightness, and the middle line the faintest. Both 
Mr. Ball* and I were almost certain that there was, in addition to the three bright lines, 
a faint continuous spectrum ; to me there appeared to be a dark space on the less re- 
frangible side of the least refrangible line, of breadth about equal to the distance from 
the same line to the second, and beyond this a faint light dying gradually away towards 
the red extremity. A continuous spectrum would probably explain this appearance, as 
the intensity of the bluish-green and green on the more refrangible side of (b) in a con- 
tinuous spectrum is much less than that of the yellowish-green and yellow. We also 
once suspected a very feeble light at the other side of the three bright lines. 
It might at first sight appear that these observations and those of Mr. Huggins on 
the spectrum of this nebula lead us to results which are completely at variance with 
those derived from our numerous observations, and those of Mr. Bond on the resolvable 
appearance of the Huygenian region and other parts of the nebula ; but when we con- 
sider the subject carefully we shall see that this is far from being the case. 
Beason why no continuous Spectrum was seen . 
It is evident that when a spectroscope whose collimator and telescope have object- 
glasses of equal focal length is placed with its slit in the plane of the image of a nebula 
giving out perfectly homogeneous light, a line of light of length and breadth equal to 
the length and breadth of the slit will be found at the focus of the spectroscope-tele- 
scope, and of brightness equal to the brightness of the part of the original image on the 
slit at the time, multiplied by a constant quantity (E) less than unity, depending on the 
number of reflecting surfaces in the apparatus, &c. ; and if the nebula give out light of 
three different refrangibilities, the mean brightness of the lines will be one-third of this 
quantity ; whereas if the light emitted be of all refrangibilities the mean brightness will 
be less in the ratio of the length of the spectrum to once or three times the breadth of 
the slit, according as we compare it with a spectrum of one or three lines. This is fully 
confirmed in practice ; for in the case of those nebulae whose spectra are continuous, the 
* The present Assistant. 
