384 



Dr. W. Huggins on the 



[June 13, 



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 1 5 inches of mercury, 

 the line-spectrum fades out to give place to Pliicker's spectrum of the 

 first order. During this process a state of things occurs when, for reasons 

 already stated, the spectrum becomes sensibly monochromatic 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. 

 Y/hen, however, the pressure becomes exceedingly small, from O'l inch to 

 0*05 inch, or less, of mercury, there is a condition of the discharge in 

 which the line again appears, while the other lines remain very faint. 

 Under these conditions I have always been able, though with some diffi- 

 culty, on account of the faint light when the necessary dispersive power 

 (spectroscope B with second or third eyepiece) and a narrow slit are 

 used, to see the line to be double, but it is narrower than when the gas is 

 more dense, and may be easily mistaken for a single line. I have not yet 

 been able to find a condition of luminous nitrogen in which the line has 

 the same characters as those presented by the line in the nebula, where it 

 is single and of the width of the slit. 



Upon the whole I am still inclined to regard the line in the nebula as 

 probably due to nitrogen. 



If this should be found to be the case, and that the nebular line has 

 originally the refrangibility of the middle of the double line of nitrogen, 

 then we should have evidence that the nebula is moving from the earth. 

 The amount of displacement of the nebular line from the middle of the 

 double line of nitrogen corresponds to a velocity of 55 miles per second from 

 the earth. At the time of observation the part of the earth's orbital 

 motion, which was from the nebula, was 14*9 miles per second. From 

 the remaining 40 miles per second would have to be deducted the probable 

 motion from the nebula due to the motion of the solar system in space. 

 This estimation of the possible motion of the nebula can be regarded as only 

 approximate. 



If the want of accordance of the line in the nebula with the middle of the 

 double line of nitrogen be due to a recession of the nebula in the line of 

 sight, there should be a corresponding displacement of the third line as 

 compared with that of hydrogen. For reasons which will be found in a 

 subsequent paragraph, I have not been able to make this comparison with 

 the necessary accuracy. 



In my former paper* I gave reasons against supposing so large a motion 

 in the nebula ; these were based on the circumstance that the nebular 

 line falls upon the double nitrogen line, which the present observations 

 * Phil. Trans. 1868, pp. 542, 543. 



