78 PROFESSOR J. N. LOCKYER ON THE PHOTOGRAPHIC 
the chief lines in the spectra of the elements concerned. The table shows that a large 
number of the lines appear to have no terrestrial equivalent, but they are present in the 
spectra of other celestial bodies. These coincidences are discussed in a subsequent part 
of the paper. 
IV. THe VARIATION OF THE SPECTRUM IN DIFFERENT REGIONS OF THE NEBULA. 
The earlier investigations of the photographic spectrum of the Orion Nebula 
seemed to indicate that the spectrum was different for different regions. 
In my own observations in 1891, with the 30-inch reflector at Westgate, the 
variations were very striking. 
I stated in a paper communicated to the Royal Society in December, 1889,* “TI 
obtained momentary glimpses of many bright lines between H, and H,, on October 31.” 
These were also seen by Mr. Fow Ler, and it was observed that, as the nebula was 
swept across the slit, in some parts the lines were seen together, while in other parts 
first one group and then another made their appearance. In the same paper I 
referred also to the variations in the same field of view of some of the lines. These 
observations were made with an enlarged form of pocket spectroscope, with a 
dispersion that does not split D. I found that in certain parts of the nebula, in the 
same field, certain lines were knotted, as often seen in prominences on and off the sun, 
and in other parts broken ; in the former case, whilst the F line thickened equally on 
both sides, the chief nebular line thickened only on the more refrangible side.t 
This result is shown in fig. 2. 
* ‘Roy. Soc. Proc.,’ vol. 48, p. 195. 
+ In another paper (‘ Phil. Trans., ’A, 18938, vol. 184, p. 714), I wrote as follows with regard to the 
chief line: “I have convinced myself of the fluted nature of the line by new observations made with 
instruments best fitted to show it, while the Lick telescope is, perhaps, the ideal telescope not to employ 
in such an inquiry. Hence, although the visibility of magnesium is not fundamental for my argument, 
I still hold that it is more probably the origin of the nebular line than an unknown form of nitrogen.” 
The recent remarks of Professor Krrier (‘ Ast. and Ast. Phys.,’ January, 1894, p. 61), and Mr. 
CampBeE.t (‘ Ast. and Ast. Phys.,’ May, 1894, p. 385), as to the relative efficiency of telescopes in regard 
to the observation of spectrum lines, seem to indicate that the matter has not been sufficiently thought 
out. I have not seen a statement as to the percentage of light utilised in the case of the Lick telescope, 
but I may say that at the time my observations were made, the mirrors of my telescope were newly- 
silvered, so that probably only a small percentage of light was lost. Neglecting the loss of hght due to 
absorption in the case of the refractor, and to reflection in the case of the reflector, the brightness of the 
image formed on the slit of the spectroscope by the Westgate telescope is about sixteen times that of 
the imaye formed by the Lick telescope, and it is scarcely necessary to add that having this great 
illuminating power, the collimator of the spectroscope has been designed to take full advantage of it. 
[Professor CAMPBELL, who has succeeded Professor KEgLEer at the Lick Observatory, is of the same 
opinion as myself. He writes (‘Astr. and Ast.-Phys.,’ 1893, p. 53): “The 36-inch telescope presents 
several positive disadvantages. . . . . The ratio of the focal length 19:1 is much larger than 
exists in small telescopes, and hence the latter would form much brighter images on the slit plate.” 
Note added 4.1.95. ] 
