SPECTRUM OF THE GREAT NEBULA IN ORION. 75 
hydrogen at H,, H;, H., and H,, and the strong line in the ultra-violet near \ 373. 
H, is by far the strongest line in the spectrum. The wave-length of the least 
refrangible line on the photograph was taken as 5006°5, as determined at Ken- 
sington, and this, together with the hydrogen lines, the line at 44471, and the 
ultra-violet magnesium triplet in the comparison spectrum, formed the basis of the 
curve for determining the positions of the fainter lines. The photograph was 
measured with a micrometer reading to 0°00001 inch. 
All the lines are shown in the table which follows. In all, fifty-four lines have 
been recorded, and, of these, about twenty are seen without difficulty. The remainder 
require a favourable light, but no line has been inserted in the table which has not 
been measured several times by two observers. The spectrum extends from the 
ultra-violet to the green, and the intensities of the lines on the photographs 
naturally do not correspond to the visual ones; the F line, for instance, appears 
stronger than the brightest line in the visible spectrum at 15006. The photographic 
intensities are recorded in the table, six representing the strongest and one the 
feeblest line. 
Some of the wave-lengths referred to in the preliminary paper have been slightly 
changed by the new reduction. 
