IN CONNEXION WITH THE SPECTRUM OP THE SUN. 
481 
I proposed to myself to attempt to clear away all difficulties touching coincidences, and 
to give below it complete maps of all the solar elements with the long and short lines, 
showing at a glance, therefore, those which are and are not reversed at the present 
epoch. I am glad to say that the map is making good progress. 
I have also commenced a preliminary search for other elements in the sun ; and this 
has necessitated an inquiry into the absorption-spectra of the metalloids by means of 
photography, which is not yet complete. 
Of the above-named branches of the research I propose on the present occasion to refer 
more particularly to the following : — 
I. The experiments made on a possible quantitative spectrum-analysis. 
II. The method of photographing spectra. 
III. The coincidences of spectral lines. 
IV. The preliminary inquiry into the existence of elements in the sun not previously 
traced. 
I. THE EXPERIMENTS MADE ON A POSSIBLE QUANTITATIVE SPECTRUM-ANALYSIS. 
Some time ago I gave an account of some experiments with some tin and magnesium 
amalgams, and I remarked*: — “It is possible to begin with an alloy which shall 
only give us the longest line or lines in the spectrum of the smallest constituent, and 
by increasing the quantity of this constituent the other lines can be introduced in the 
order of their length. This reaction is so delicate that I learnt from it a thing I had 
not before observed, that the least refrangible line of b, the triple line of magnesium, 
is really a little longer than its more refrangible companion ; for the spectrum of 
magnesium was reduced to this one line in an alloy in which special precautions had 
been taken to introduce the minimum of magnesium. 
“ It follows from this statement that not only is the spectrum-analysis almost 
infinitely more delicate than it has hitherto been supposed to be in the case of the 
elements in which the difference between the longest and the shortest lines is least f, 
but that in time it may become quantitative.” On a second occasion J I went further and 
laid before the Society for their inspection maps of the spectra of certain alloys, pointing 
out that the lines of any constituent of a mechanical mixture disappeared from the 
spectrum as its percentage was reduced. 
After the second paper was sent in to the Eoyal Society I commenced a series of 
observations, the object of which was to study not only the disappearance of the lines, 
but other general changes which might supervene ; and for this purpose I mounted a 
micrometer eyepiece on the observing-telescope of the spectroscope. This enabled me 
* Philosophical Transactions, 1873, p. 261. 
t “ The great lengths of the lines of sodium, lithium, &c. at once account for the delicacy of their spectrum 
reactions.” 
+ Philosophical Transactions, 1873, p. 655. 
