1894.] Photographic Spectrum of the Great Nebula in Orion. 285 



If the brightness of the lines in reality remains constant, they will 

 appear relatively brightest at the two minima, owing to the reduction 

 of continuous spectrum which is associated with the increased bright- 

 ness of the star at maximum, and for the same reason they should 

 appear brighter at principal than at secondary minimum. Estimates 

 of the brightness of the lines in relation to the continuous spectrum 

 have been made independently by four of my assistants, and, although 

 estimates of this kind are liable to error, the general agreement is 

 sufficient to indicate that when all allowance is made for the varying 

 continuous spectrum, there is a maximum of brightness of the bright 

 lines about half a day after secondary minimum. The apparent 

 increase of brightness near principal minimum seems to be due 

 solely to the reduced intensity of the continuous spectrum. 



I have to express my obligations to Messrs. Fowler, Baxandall, 

 Shackleton, Butler, Wardale, Crabtree, and North, who, at different 

 times, have assisted in taking the photographs. 



XIII. " On the Photographic Spectrum of the Great Nebula in 

 Orion." By J. NORMAN LOCKYER, C.B., F.R.S. Received 

 June 13, 1894. 



(Abstract.) 



The paper consists of a description and discussion of photographs 

 of the spectrum of the Orion Nebula, taken with the 30-inch reflector 

 at Westgate-on-Sea in February, 1890, of which a preliminary 

 account was communicated to the Royal Society at the time. Fifty - 

 four lines are tabulated as belonging to the spectrum of the nebula, 

 nine of them being due to hydrogen. Tables are given showing : 



1. The wave-lengths, intensities, and probable origins of the lines 



photographed in the spectrum of the nebula. 



2. A comparison of the lines in the spectrum of the nebula with 



lines in the spectra of (a) P. Cygni, (&) bright line stars 

 and planetary nebulae, and (c) stars in Groups II, III, 

 and IV, of the classification according to the meteoritic 

 hypothesis. 



The complete discussion has led to the following general con- 

 clusions : 



1. The spectrum of the nebula of Orion is a compound one con- 

 sisting of hydrogen lines, low temperature metallic lines and flutings, 

 and high temperature lines. The mean temperature, however, is 

 relatively low.* 



* ' Roy. Soc. Proc.,' vol. 43, p. 152, 1887. 



