IN CONNEXION WITH THE SPECTRUM OF THE SUN. 
48.5 
maimer. This method was found to be perfectly successful. The central portion of 
the spectrum due to the dense core of the arc is found to consist of all the lines which 
this part of the arc alone can give, the longer lines given by the outer less dense strata 
extending beyond the spectrum of the core to the various distances from the centre to 
which the vapour capable of giving them extends. (See Plate XL.) 
The lines thus photographed are pointed at either end, and disappear from the centre 
in the order of their length, so that a double or duplicate determination exquisitely 
symmetrical of their lengths is thus obtained. 
Although the lengths, thicknesses, and intensities of the lines are thus readily recorded, 
we have so far no scale by which to fix their positions. In order to obviate this objec- 
tion, I resolved to photograph the solar spectrum on each plate immediately above or 
below the metallic spectrum under examination. To accomplish this an extension of 
the ordinary method of working was introduced, depending upon the following con- 
siderations. 
It is obvious that when we observe a spectrum its breadth will depend upon the 
length of the slit. If we could at the same time illuminate different portions of the 
slit with rays proceeding from different vapours, the spectra of the different light-sources 
could be seen at once. But we cannot do this. What we can do when we introduce 
photography is to illuminate successively different portions of the slit, the effect being 
that on different portions of the photographic plate the various spectra will successively 
record themselves. 
Acting on this principle I first covered up the upper half of the slit, allowing the 
image of the horizontal arc to fall centrally on the slit, so that in this way I got impressed 
an image of half the thickness of the horizontal arc. After this was accomplished I 
covered up the half of the slit first used, opening that which was previously closed, and 
through this newly opened portion I admitted sunlight. But to do this effectively 
certain precautions were necessary. A description of the method used will show how 
these have been taken. 
The laboratory in which the work has been carried out has two windows, one nearly 
due (magnetic) south, the other nearly (magnetic) west. Outside each window level 
slate slabs have been erected as supports for a heliostat. Either window can be used 
at pleasure. The spectroscope is supported on a platform on rollers, the height of the 
platform being such that the horizontal beam from the heliostat is coincident with the 
axis of the collimator. In addition to the lens placed between the lamp and the slit 
to throw an image of the arc on the latter, another lens is now introduced between 
the heliostat and the lamp — heliostat, lenses, lamp, and collimator being of course in 
the same straight line. The action of the newly interpolated lens is to throw an 
image of the sun between the poles of the lamp, so that when the spectrum of the arc 
is properly focused by the camera lens on to the photographic plate, the solar spectrum, 
when subsequently thrown in, is also in focus. 
MDCCCLXXIV. 3 T 
