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XVII. Further Observations on the Spectra of some of the Nehulce, with a Mode of deter- 

 mining the Brightness of these Bodies. By William Huggins, F.B.S. 



Received January 30, — Eead February 15, 1866. 



§ I. Introduction. 



Its my former papers, " On the Spectra of some of the Nebulse" *, and " On the Spec- 

 trum of the Great Nebula in Orion " f , I described the results of a prismatic examina- 

 tion of the light of some of the objects in the heavens which have been classed together 

 under the common denomination of Nebulae. The present paper contains the results 

 of the application of the same method of research, with the same apparatus, to the 

 light of others of the same class of bodies. To these observations with the prism are 

 appended the results of an attempt to determine the intrinsic intensity of the light 

 emitted by some of the nebulae which give a spectrum indicating gaseity. 



On account of the great faintness of the light of most of the objects described in this 

 paper, I have not found it possible to determine more than the general characters of 

 the spectra which they form in the instrument. The present observations confirm the 

 results which I have already presented to the Royal Society, namely, that with my 

 instrument clusters and nebulae give either a spectrum which is apparently continuous, 

 or a spectrum consisting of one, two, or three bright lines. 



The description " continuous spectrum " in this paper and in my former papers must 

 not be understood to mean more than that when the slit was made as narrow as the 

 feeble light of the object permitted, the spectrum was not resolved into bright lines. 

 Whether the continuous spectrum was in any case interrupted by dark lines in a manner 

 similar to the spectra of the sun and fixed stars, I was not able to ascertain ; for when 

 the feeble light of a nebula is dispersed by the prism into a spectrum consisting of 

 light of all refrangibilities, the spectrum is extremely faint and difficult of examina- 

 tion. Before the slit is made sufficiently narrow for the detection of dark or bright 

 lines, the spectrum becomes, in the case of nearly all the objects examined, too faint to 

 be visible. When, however, a nebula is observed the light of which is monochromatic, 

 or nearly so, the one, two, or three bright lines in which the light remains concentrated 

 can usually be seen when the slit is made narrow. Some of these nebulae have been 

 examined with a slit not exceeding 3^ of an inch in width. 



[A conclusion of some importance to our interpretation of the phenomena of these 

 bodies, and especially of value in reference to the theoretical views we may form of the 

 relation of the gaseous nebulae to the other nebulae and clusters, presents itself in con- 



* Philosophical Transactions, 18C4, p. 437. f Proceedings of the Eoyal Society, vol. xiv. p. 39. 



MDCCCLXVI. 3 G 



