438 
ME. W. HUGGINS ON THE SPECTEA OF SOME OF THE NEBULAE. 
diameters. For the greater part of the following observations on the nebulae, the cylin- 
drical lens is not necessary, and was removed from the instrument. The numbers and 
descriptions of the nebulae, and their places for the epoch 1860, January 0, included 
within brackets, are taken from the last Catalogue of Sir John Herschel*. 
[No. 4373. 37 H. IV. R.A. 17 h 58 m 20 s . N.P.D. 23° 22' 9"-5. A planetary nebula ; 
very bright; pretty small; suddenly brighter in the middle, very small nucleus.] 
In Draco. 
On August 29, 1864, I directed the telescope armed with the spectrum apparatus to 
this nebula. At first I suspected some derangement of the instrument had taken place ; 
for no spectrum was seen, but only a short line of light perpendicular to the direction 
of dispersion. I then found that the light of this nebula, unlike any other ex-terrestrial 
light which had yet been subjected by me to prismatic analysis, was not composed of 
light of different refrangibilities, and therefore could not form a spectrum. A great 
part of the light from this nebula is monochromatic, and after passing through the 
prisms remains concentrated in a bright line occupying in the instrument the position 
of that part of the spectrum to which its light corresponds in refrangibility. A more 
careful examination with a narrower slit, however, showed that, a little more refrangible 
than the bright line, and separated from it by a dark interval, a narrower and much 
fainter line occurs. Beyond this, again, at about three times the distance of the second 
line, a third, exceedingly faint line was seen. The positions of these lines in the spec- 
trum were determined by a simultaneous comparison of them in the instrument with 
the spectrum of the induction spark taken between electrodes of magnesium. The 
strongest line coincides in position with the brightest of the air lines. This line is due 
to nitrogen, and occurs in the spectrum about midway between b and F of the solar 
spectrum. Its position is seen in Plate Xl.f 
The faintest of the lines of the nebula agrees in position with the line of hydrogen 
corresponding to Fraunhofer’s F. The other bright line was compared with the strong 
line of barium 2075 J: this line is a little more refrangible than that belonging to the 
nebula. 
Besides these lines, an exceedingly faint spectrum was just perceived for a short 
distance on both sides of the group of bright lines. I suspect this is not uniform, but 
is crossed with dark spaces. Subsequent observations on other nebulae induce me to 
regard this faint spectrum as due to the solid or liquid matter of the nucleus, and as 
quite distinct from the bright lines into which nearly the whole of the light from the 
nebula is concentrated. 
In the diagram (fig. 5, Plate X.) the three principal lines only are inserted, for it would 
be scarcely possible to represent the faint spectrum without greatly exaggerating its in- 
tensity. 
The colour of this nebula is greenish blue. 
* Philosophical Transactions, Part. I. 1864, pp. 1-138. 
t See also Philosophical Transactions, 1864, p. 156, and Plate I. ± Ibid. p. 156. 
