1872.] Spectrum of the Great Nebula in Orion. 



383 



Spectroscope C is furnished with four similar prisms. 



The small telescopes of the three spectroscopes are of the same size : 

 diameter of object-glass 1 J inch ; each is furnished with three eyepieces 

 magnifying 5*5, 9*2, and 16*0 diameters. 



Spectrum of the Nebula of Orion. 



With spectroscopes A and B four* lines are seen ; they are represented 

 in the diagram which accompanies this note. The scale in the diagram 

 gives wave-lengths. 



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

 made very narrow, this line was seen to be very narrow, of a width cor- 

 responding 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 pressure 

 and temperature, the double line of the nitrogen-spectrum becomes single ; 

 and further, if this should be found to be the case, whether the line 

 becomes single by the fading out of its more refrangible component, or in 

 what other way the single line of the nebula comes to occupy the position 

 in the spectrum, not of the middle of the double line of nitrogen, but that 

 of the less refangible 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 sensibly monochromatic 

 when examined with a narrow slitf. 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 con- 

 nected 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 



* The fourth line was first seen in nebula 18 H. IV. (Phil. Trans. 1864, p. 441). 



t Phil. Trans. 18G8, pp. 540-546. Observations on this point were subsequently 

 made by Frankland and Lockyer (Proc. Roy. Soc. vol. xvii. p. 453). It should be 

 stated that the authors make no reference to this observation, though they refer to a 

 purely hypothetical suggestion contained in the same paper. 



