542 
MR. W. HUGGINS ON THE SPECTRA 
was reflected into the instrument, as in my former series of observations, by means of a 
mirror and a small prism. The precaution was taken to verify the accuracy of the 
position of the spectrum of comparison relatively to that of the nebula, by placing a 
small lamp before the object-glass in the way already described. 
The coincidence of the line in the nebula with the brightest of the lines of nitrogen, 
though now subjected to a much more severe trial, appeared as perfect as it did in my 
former observations. I expected that I might discover a duplicity in the line in the 
nebula corresponding to the two component lines of the line of nitrogen, but I was not 
able, after long and careful scrutiny, to see the line double. The line in the nebula was 
narrower than the double line of nitrogen ; this latter may have appeared broader in 
consequence of irradiation, as it was much brighter than the line in the nebula. 
The following observations are suggestive in connexion with the point under consi- 
deration. Electrodes of platinum were placed before the object-glass in the direction of 
a diameter, so that the spark was as nearly as possible before the centre of the lens. The 
spark was taken in air. I expected to find the spectrum faint, for the reasons which 
have been stated in a previous paragraph, but I was surprised to find that only one line 
was visible in the large spectroscope when adapted to the eye-end of the telescope. 
This line was the one which agrees in position with the line in the nebula, so that under 
these circumstances the spectrum of nitrogen appeared precisely similar to the spectra 
of those nebulse, of which the light is apparently, monochromatic. This resemblance 
was made more complete by the faintness of the line ; from which cause it appeared 
much narrower, and the separate existence of its two components could no longer be 
detected. When this line was observed simultaneously with that in the nebula, it was 
found to appear but a very little broader than that line. When the battery circuit was 
completed, the line from the spark coincided so accurately in position with the nebular 
line, that the effect to the eye was as if a sudden increase of brightness in the line of 
the nebula had taken place. In order to make this observation, and to compare the 
relative appearance of the lines, the telescope was moved so that the light from the 
nebula occupied the lower half only of the slit. The line of the spark was now seen to 
be a very little broader than the line of the nebula, and appeared as a continuation of 
it in an unbroken straight line. These observations were repeated many times on 
several nights. 
An apparent want of coincidence, which would be represented by 002 division of 
the head of the micrometer-screw, would be about the smallest difference that could be 
observed under the circumstances under which these observations were made. At the 
part of the spectrum where this line of nitrogen occurs, the angular interval measured 
by -02 division of the micrometer corresponds to a difference of wave-length of -0460 
millionth of a millimetre. 
At the time the comparisons were made the earth was receding from the part of the 
heavens in which the nebula is situated by about half its orbital velocity. If the velo- 
city of light be taken at 185,000 miles per second, and the wave-length of the nitrogen 
