lord 
1577.) ‘ 5 { Draper, 
gases in Plicker’s tubes and also in an apparatus in which the pressure 
could be varied,but for the present illustration, the open air spark was, 
all things considered, best. By other arrangements the Nitrogen lines can 
readily be made as sharp as the Oxygen are here, and the Iron lines may 
be increased in number and distinctness. For the metals the electric arc 
gives the best photographic results, as Lockyer has so well shown, but as 
my object was only to prove by the Iron lines that the spectra had not 
shifted laterally past one another, those that are here shown at 4325. 4307. 
4271. 4063. 4045. suffice. In the original collodion negative many more can 
be seen. Below the lower spectrum are the symbols for Oxygen, Nitrogen, 
Tron and Aluminium, 
No close observation is needed to demonstrate to even the most casual 
observer that the Oxygen lines are found in the Sun as bright lines, while 
the Iron lines have dark representatives. The bright Iron line at G (4307), 
on account of the intentional overlapping of the two spectra, can be seen 
passing up into the dark absorption line in theSun. Atthe same time the 
quadruple Oxygen line between 4845 and 4550 coincides exactly with the 
bright group in the Salar Spectrum above. This Oxygen group alone is 
almost sufficient to prove the presence of Oxygen in the Sun, for not only 
does each of the four components have a representative in the Solar spec- 
trum, but the relative strength and the general aspect of the lines in each 
case is similar. I do not think that in comparisons of the spectra of the 
elements and Sun, enough stress has been laid on the general appearance 
of lines apart from their mere position ; in photographic representations 
this point is very prominent. The fine double line at 4319. 4317. is plainly 
represented in the Sun. Again there is a remarkable coincidence in the 
double line at 4190. 4184. The line at 4133 is very distinctly marked. 
The strongest Oxygen line is the triple one at 4076. 4072. 4069., and here 
again a fine coincidence is seen though the air spectrum seems proportion- 
ately stronger than the solar. But it must be remembered that the Solar 
spectrum has suffered from the transmission through our atmosphere, and 
this effect is plainest in the absorption at the ultra-violet and violet regions 
of the spectrum. From some experiments I made in the Summer of 1873, 
it appeared that this local absorption is so great, when a maximum thick- 
ness of air intervenes, that the exposure necessary to obtain the ultra-violet 
spectrum at sunset was two hundred times as long as at mid-day. I was 
at that time seeking for atmospheric lines above H like those at the red 
end of the spectrum, but it turned out that the absorptive action at the 
more refrangible end is a progressive enfeebling as if a wedge of neutral 
tinted glass were being drawn lengthwise along the spectrum towards the 
less refrangible end. 
I shall not attempt at this time to give a complete list of the Oxygen 
lines with their wave lengths accurately determined, and it will be 
noticed that some lines in the air spectrum which have bright analogues 
in the sun are not marked with the symbol of Oxygen. This is because 
there has not yet been an opportunity to make the necessary detailed com- 
