1869.] Lieut. HerscheFs Observations of Southern Nebulae. 307 



looking at No. 1185 I had appended a remark that "the principal nebula 

 shows a great deal of continuous light on this side," an observation which 

 seemed to require confirmation. The following extracts from my note- 

 book must speak for themselves : — 



No. 1179. The great Nebula of Orion. Oct. 25. " A fourth line, almost 

 beyond question : measured twice with reference to principal 

 line, 7-3 — 5-05 = 2-25, 7-6 — 5-06 = 2*54, mean 2*4 \ con- 

 tinuous spectrum suspected, but, owing to moonlight and low 

 altitude, there was no conviction." Nov. 7. " Previous ob- 

 * serration confirmed. The fourth line is a fact. The dif- 



fused light, which also is certainly visible, to the extent of 

 rendering the edges of the field visible beyond the immediate 

 neighbourhood of the lines, can only be a continuous spec- 

 trum." Nov. 9. "Fourth line, 7"9 — 5*1 = 2'8, very rough. 

 I am satisfied that there is a continuous spectrum, though I 

 am not certain it may not be dispersed stellar light." Ditto, 

 later : " I have no longer any hesitation as to the continuous 

 spectrum." Nov. 10. "Fourth line, 7'92, 7'88, 791, 

 — 5 09 = 2-81 *. Continuous spectrum distinctly ending 

 coincidently with the bright lines at the edge of the bright 

 part of the nebula." 

 There is nothing very remarkable in the presence either of an additional 

 line or of a continuous spectrum ; but as this nebula has been examined 

 very carefully in England without the detection of eitherf, it appeared neces- 

 sary to put both beyond question ; taken, too, in connexion with the very 

 different character of its near neighbour, 1185, and with others in which 

 the relative intensity of the two kinds of spectra varies in degree, it appears 

 to break down, to a considerable extent, the barrier between " gaseous " 

 and " solid " nebulous matter, and to lead towards the inference that con- 

 densation is in a more or less advanced stage in all nebulse, and in the 

 vast majority of cases, including all " clusters," has become complete. 



I am sorry to say that I shall be unable to prosecute these observations 

 for some months, as my survey duties require my presence elsewhere. In 

 the meanwhile I should be glad to learn whether the course I have been 

 pursuing appears a desirable one to continue, now that so large a number 

 of the southern nebulae have been tested, or whether a reexamination 

 would be preferred. I remain, dear Sir, yours truly, 



J. Herschel. 



* Adopting D-l-2-19 as the position of the principal line 

 gives D+5 - 00 „ „ fourth line. 



Kirchhoff ft 271j 1 — D +4 80 j ^ence ^ e f our th line is about midway between these. 

 28o-5=D-f 5-25 J 



t [A faint continuous spectrum appears to have been seen with Lord Eosse's great 

 telescope. See a paper by Lord Oxmantown " On the Great Nebula in Orion," Phi- 

 losophical Transactions for 1868, p. 72.— Gr. Gr. S.] 



