320 Royal Society : — Mr. W. Huggins on the Spectrum 



that it is suitable for observation as a crucial test of the correctness 

 of the usually received opinion that the resolution of a nebula into 

 bright stellar points is a certain and trustworthy indication that the 

 nebula consists of discrete stars after the order of those which are 

 bright to us. Would the brighter portions of the nebula adjacent to 

 the trapezium, which have been resolved into stars, present the same 

 spectrum as the fainter and outlying portions ? In the brighter parts, 

 would the existence of closely aggregated stars be revealed to us 

 by a continuous spectrum, in addition to that of the true gaseous 

 matter ? 



The telescope and spectrum apparatus employed were those of 

 which a description was given in my paper already referred to. 



The light from the brightest parts of the nebula near the trapezium 

 was resolved by the prisms into three bright lines, in all respects similar 

 to those of the gaseous nebulae, and which are described in my former 

 paper. 



These three lines, indicative of gaseity, appeared (when the slit 

 of the apparatus was made narrow) very sharply defined and free 

 from nebulosity ; the intervals between the lines were quite dark. 



When either of the four bright stars, a, j3, y, d Trapezii was 

 brought upon the slit, a continuous spectrum of considerable bright- 

 ness, and nearly linear (the cylindrical lens of the apparatus having 

 been removed), was seen, together with the bright lines of the nebula, 

 which were of considerable length, corresponding to the length of 

 the opening of the slit. The fifth star y and the sixth a! are seen 

 in the telescope, but the spectra of these are too faint for observation. 



The positions in the spectra of a, (3, y, 3 Trapezii, which corre- 

 spond to the positions in the spectrum of the three bright lines of 

 the nebula, were carefully examined, but in no one of them were 

 dark lines of absorption detected. 



The part of the continuous spectra of the stars a, j3, y, near the 

 position in the spectrum of the brightest of the bright lines of the 

 nebula, appeared on a simultaneous comparison to be more brilliant 

 than the line of the nebula, but in the case of y the difference in 

 brightness was not great. The corresponding part of d was perhaps 

 fainter. In consequence of this small difference of brilliancy, the 

 bright lines of the adjacent nebula appeared to cross the continuous 

 spectra of y and 3 Trapezii. 



Other portions of the nebula were then brought successively 

 upon the slit ; but throughout the whole of those portions of the 

 nebula which are sufficiently bright for this method of observation 

 the spectrum remained unchanged, and consisted of the three bright 



indicates, by a sort of granular texture, its consisting of stars, and when examined 

 under the great light of Lord Eosse's reflector, or the exquisite defining power of 

 the great achromatic at Cambridge, U. S., is evidently perceived to consist of 

 clustering stars. There can therefore be little doubt as to the whole consisting 

 of stars too minute to be discerned individually even with these powerful aids, 

 but which become visible as points of light when closely adjacent in the more 

 crowded parts . . . ." — Sir John Herschel, ' Outlines of Astronomy,' 7th edi- 

 tion, pp. 651, 652. 



