

Line in the Spectrum of the Nelula in Orion. 211 



the results of the lead comparisons, which were, however, more 

 easily made, as it is difficult to see the exact position of the short 

 nebular line when it is upon the bright fluting.* 



2. Addendum on the Character of the Line. 



I am permitted by Dr. Copeland, Professor Young, and Mr. Keeler, 

 of the Lick Observatory, to quote the following observations, which 

 they have been so kind as to make at my request, of the character of 

 the principal line in the spectrum of the Great Nebula in Orion. 



Dr. Copeland writes, dated March 26, 1890: " I find it difficult to 

 make anything satisfactory of nebular spectra with my present 



apparatus, working in the smoke of Edinburgh On the 14th 



I saw the three lines as well as I am likely to see them until we get 

 to work at the new observatory. All the lines were just as broad as 

 the slit ; when the slit was wide open they were broad, and when 

 the slit was closed slowly they gradually became narrower and 

 narrower." 



Professor Young, writing on March 21, 1890, says: "I have not 

 been able this winter to try the observations for wave-length, having 

 no convenience for the comparison spectrum, but I have carefully 

 examined the spectrum of the nebula of Orion, both with a heavy 

 glass prism, and with a grating of 14,000 to the inch, and a collimator 

 of 16 inches focus. With the prism the brightest nebular line seemed 

 absolutely sharp, and cleanly defined on both sides; with the grating 

 the line was fainter, and 1 could not use so narrow a slit, the dispersion 

 was much higher also ; the line therefore was a little hazy, but equally 

 so on both sides." 



At the Lick Observatory there was a continuance of bad weather 

 during the early months of the year, but Mr. Keeler, with Dr. 

 Holden's kind permission, observed the nebula on two nights. He 

 observed successively with one prism, a powerful compound prism, 

 aud then with a Rowland grating, 14,000 + lines to the inch. With 

 this grating, the collimator was 20 inches in length, the observing 

 telescope 10^ inches, with an eye-piece magnifying 13'3 times. The 

 slit was narrow, 0'0025 inch. The spectra up to the fourth order 

 were employed. 



Mr. Keeler says : " One thing that struck rue particularly, and 

 that there could be no doubt of, was the perfect sharpness and 



* [On one night, as had been frequently done when we were observing the nebula 

 in Orion, after the comparison of the nebula with lead and magnesium had been 

 made, the spectroscope was left attached to the telescope in order that we might 

 verify the correctness of position of the comparison spectrum by means of the light 

 of sky on the following day, without any change whatever having taken place in the 

 adjustments of the instrument. The result of this verification was, as always when 

 working on Orion, absolutely satisfactory. July 4.] 



