466 
NATURE 
[ Adrel 17, 1873 
ON THE SPECTROSCOPE AND ITS 
APPLICATIONS 
VII. 
TA SOTRER point was also very obvious to those who 
are familiar with these inquiries, namely, that if these 
prominences really consisted of gas, by the use ofa powerful 
spectroscope it was perfectly unnecessary to wait for 
eclipses at all. The reason for this will be clear on a little 
consideration ; if we take a continuous or unbroken spec- 
trum and apply successively a number of prisms, the 
spectrum will become proportionately lengthened, and 
therefore more and more feeble, and in fact we can thus 
reduce} the light to any degree required ; if now, on the 
a 
1 
hydrogen hytroge h hydrogen 
ULNA A 
other hand, we take a spectrum which consists only of 
bright lines, say of one line in the red and another 
in the blue, and as before apply successively a number 
of prisms, we shall, it is true, increase the length of 
the spectrum, that is the distance between the two 
lines, but this will be all; the additional prisms have no 
power to alter the width of the lines themselves, for we 
have seen that these are simply the images of the slit, 
Their light, therefore, will only be slightly enfeebled. 
owing to reflection merely. Thus if we have a mixed 
light to analyse, part of which comes from a source giving 
out a continuous spectrum, and the rest that of a glowing 
gas, although when working with a single prism no lines 
may be visible on account of the brightness of the con- 
| 
| | | 
hydrogen 
p 
raagnesium sod tum 
Fic. 40.—Spectrum of the Sun’s Photosphere (below) and Chromosphere (above). 
tinuous spectrum, yet by using say five or seven prisms 
we can so dilute the continuous spectrum as to render the 
bright lines of the glowing gas clearly visible. The case 
of the red flames round the sun is a case in point. They 
jare invisible to the naked eye and in telescopes on account 
of the intensely illuminated atmosphere which also pre- 
vents anything like bright lines being observed from these 
red flames, until the bright continuous spectrum has been 
much reduced, when this has been done the bright lines 
of the spectrum, should there be any, will appear on a 
ara 
Cc 
& Fic. 41 —C line bright in chromosphere, dark in sun. 
comparatively dark background. M. Janssen, who was 
sent out by the French Government to observe the 
eclipse which was visible in India in 1868, Major Ten- 
nant, and others, had no difficulty in recognizing in a 
moment, when the sun was eclipsed, that these things 
really did consist of gases or vapours, and M. Janssen, a 
very careful observer, had no difficulty in determining that 
the gas in question was really hydrogen gas. M., Janssen 
and myself were also enabled to determine this by obser- 
vations on the uneclipsed sun, by means of the new 
method I have just sketched out. The accompanying 
woodcut (Fig. 40) shows the spectrum which is observed 
from these solar prominences. The spectrum of the pro- 
minences is shown in the upper, and that of the sun in 
the lower half of the engraving. This method is very 
easy to understand if you bear in mind the engraving of 
the spectroscope for solar work, and recollect that when 
we wish to examine the regions round the sun, the light 
of the sun is allowed to fall on the slit in such a way that 
uP 
tl 
B 
Fic. 42.—F liae in chromosphere, showing widening near the sun. 
one half of the slit at the focus of the object glass of the 
large telescope is occupied by the brilliant image of the 
sun, and the other half is fishing, so to speak, around the 
limb or edge of the sun, so that if there is anything 
at all around the limb, the spectroscope, in the—to 
the eye—unoccupied part outside the image, picks up 
this something, and gives us its light sorted out into 
its proper bright lines in the spectrum. This spectrum 
shows that there is first a bright line, Fig. 41, in 
