544 
ME. W. HUGGINS ON THE SPECTRA 
wedge of crown glass, and which I had found to be sensibly equal in absorbing power 
on the different parts of the visible spectrum. As the darker part of the wedge was 
brought before the eye, the two groups in the orange were quite extinguished, while 
the lines in the green still remained of considerable brightness. The line which under 
these circumstances remained longest visible next to the brightest line, was one more 
refrangible at 2669 of the scale of my map. This observation was made with a narrow 
slit. When the induction-spark was looked at from a distance of some feet with a 
direct-vision prism held close to the eye, I was surprised to observe that the double line 
in the orange appeared to me to be the brightest in the spectrum, and when the neutral- 
tint wedge was interposed, this line in the orange remained alone visible, all the other 
lines being extinguished. 
When, however, in place of the simple prism a small direct-vision spectroscope pro- 
vided with a slit was employed, I found it to be possible, by receding from the spark, to 
find a position in which the double line in the green, with which the line in the nebula 
coincides, was alone visible, and the spectrum of the spark in nitrogen resembled that 
of a monochromatic nebula. 
It is obvious that if the spectrum of hydrogen were reduced in intensity, the line in 
the blue, which corresponds to that in the nebula, would remain visible after the line 
in the red and the lines more refrangible than F had become too feeble to affect the eye. 
It therefore becomes a question of much interest whether the one, two, three, or four 
lines seen in the spectra of these nebulae represent the whole of the light emitted by 
these bodies, or whether these lines are the strongest lines only of their spectra which, 
by reason of their greater intensity, have succeeded in reaching the earth. Since these 
nebulae are bodies which have a sensible diameter, and in all probability present a con- 
tinuous luminous surface, or nearly so, we cannot suppose that any lines have been 
extinguished by the effect of the distance of these objects from us. 
If we had evidence that the other lines which present themselves in the spectra of 
nitrogen and hydrogen were quenched on their way to us we should have to consider 
their disappearance as an indication of a power of extinction residing in cosmical space, 
similar to that which was suggested from theoretical considerations by Cheseaux, and 
was afterwards supported on other grounds by Olbers and the elder Struve. Further, 
as the lines which we see in the nebulae are precisely those which experiment shows 
would longest resist extinction, at least so far as respects their power of producing an 
impression on our visual organs, we might conclude that this absorptive property of 
space is not elective in its action on light, but is of the character of a general absorp- 
tion acting equally, or nearly so, on light of every degree of refrangibility. Whatever 
may be the true state of the case, the result of this reexamination of the spectrum of 
this nebula appears to give increased probability to the suggestion that followed from 
my former observations, namely, that the substances hydrogen and nitrogen are the prin- 
cipal constituents of the nebulae of the class under consideration. 
I now pass to observations of the third line of the nebular spectrum, the one which I 
