OF SOME OF THE STARS AND NEBULAE. 
545 
found to coincide with the line of hydrogen, which corresponds to Fraunhofer’s F. The 
substance in the nebulae which is indicated by this line appears to be subject to much 
greater variation in relative brilliancy, or to be more affected by the conditions under 
which it emits light ; for while the brightest line is always present, the line of which I 
am speaking seems to be wholly wanting in some nebulae, and to be of different degrees 
of relative brightness in some other nebulae 
In the nebula of Orion this line is relatively stronger than in 37 H. IV. Draconis, and 
some other nebulae. I have suspected that the relative brightness of this line varies 
slightly in different parts of this nebula. It may be estimated perhaps in the nebula of 
Orion at about the brightness of the second line. The second line suffers in apparent 
brilliancy from its nearness to the brightest line, and may, without due regard to this 
circumstance, be estimated as brighter than the third line. 
In order to compare the position of the line with that of the corresponding line in the 
spectrum of hydrogen, I employed a vacuum-tube containing hydrogen at a very small 
tension, which was placed before the object-glass of the telescope. Under these condi- 
tions the line appears narrow, when the slit is narrow, without any sensible nebulosity at 
the edges. The character of the line is altered, as has been shown by Plucker, when 
hydrogen at the atmospheric pressure is employed ; the line then expands into a nebu- 
lous band of considerable width, even with a very narrow slit. Such a condition of the 
line is obviously unsuitable for the delicate comparisons which it was proposed to 
attempt. 
The narrow, sharply-defined line of hydrogen, when the vacuum-tube was before the 
slit, was observed to coincide perfectly in position with the third line of the nebula. 
This observation, which shows the coincidence of these lines with an accuracy three or 
four times as great as my former observations, increases in the same ratio the probability 
that the line in the nebula is really due to luminous hydrogen. 
I suspect that although the third line in this nebula may impress the eye as strongly 
as the second line, yet it is not so narrow and well defined as that line. If this 
suspicion be correct, this condition of the line might indicate that the hydrogen exists at 
a rather greater tension than that in the so-called vacuum-tubes, but that it is not nearly 
so dense as would correspond to the atmospheric pressure at the surface of the earth. 
As, however, the character of the lines of hydrogen is also greatly modified by tempera- 
ture it is not possible to reason with any certainty as to the state of things in this distant 
object, the light of which we have now under examination. 
I am still unable to find any terrestrial line which corresponds to the middle line. I 
have made the additional observation that the line in the nebula is in a very slight 
degree less refrangible than the line of oxygen at 2060 of the scale of my map. It is in 
a rather larger degree less refrangible than the strong line of barium at 2075 of my scale; 
Several other nebulae have been observed with the large spectroscope, I prefer, how- 
ever, to re-examine these objects before I publish any observations of them. 
4 f 2 
