Spectrum of the Great Nebula in Orion. 137 



First line. — With spectroscope B and eyepiece 1 and 2, the slit 

 oeing made very narrow, this line was seen to be very narrow, of a 

 width corresponding to the slit, and defined at both edges, and 

 undoubtedly not double. The line of nitrogen when compared with 

 it appeared double, and each component nebulous and broader than 

 the line of the nebula. This latter line was seen on several nights 

 to be apparently coincident with the middle of the less refrangible 

 line of the double line of nitrogen. This observation was on one 

 night confirmed by observation with the more powerful spectroscope C. 



The question suggests itself whether, under any conditions of pres- 

 sure and temperature, the double line of the nitrogen-spectrum be- 

 comes single ; and further, if this should be found to be the case, 

 whether the line becomes single by the fading out of its more re- 

 frangible component, or in what other way the single line of the 

 nebula comes to occupy in the spectrum; not the position of the 

 middle of the double line of nitrogen, but that of the less refrangible 

 of the lines. 



I stated in my former paper that when for any reason the light from 

 the luminous nitrogen is greatly reduced in intensity, the double line 

 under consideration is the last to disappear, and consequently a 

 state of things may be found in which the light of nitrogen is sen- 

 sibly monochromatic when examined with a narrow slit*. Under 

 these circumstances the line of nitrogen appears narrower, and the 

 separate components can be detected with difficulty, if at all. 



I stated also that the breadth of the component lines appears to 

 be connected with the conditions of density and temperature of the 

 gas. As was to be expected from theoretical considerations, the 

 lines become narrower and less nebulous as the pressure is diminished. 

 My observations of this change seemed to show that the diminution 

 of the breadth of the lines takes place chiefly at the outer sides of 

 the lines ; so that in the light from very rarefied gas the double line 

 is narrower, but the space of separation between the components is 

 not as much wider as would be the case if the lines had decreased 

 equally in width on the sides towards each other. 



When the pressure of the gas is reduced to about 15 inches of 

 mercury, the line-spectrum fades out to give place to Pliicker's spec- 

 trum of the first order. During this process a state of things occurs 

 when, for reasons already stated, the spectrum becomes sensibly mo- 

 nochromatic when viewed with a narrow slit and a spectroscope of 

 several prisms. The line is narrower but remains double, and has 

 the characters described in the preceding paragraph. 



As the pressure is diminished, the double line fades out entirely, 

 and the spectrum of the second order gives place to the spectrum of 

 the first order. When, however, the pressure becomes exceedingly 

 small, from 0*1 inch to 0"05 inch, or less, of mercury, there is a 

 condition of the discharge in which the line again appears, while 



* Phil. Trans. 1868, pp. 540-546. Observations on this point were subse- 

 quently made by Frankland and Lockyer (Proc. Roy. Soc. vol. xvii. p. 453). 

 It sbould be stated that tbe authors make no reference to this observation, 

 though they refer to a purely hypothetical suggestion contained in the same paper. 



