IX CONNEXION WITH THE SPECTRUM OF THE SUN. 
487 
having the solar spectrum in the middle it serves as a scale (Plate XXXVIII.). Such 
spectra directly compared are shown in the accompanying Plates (XXXVIII. -XL.). 
Fig. 4. — Slit plate with sliding shutter (actual size). 
A, shutter ; B B, grooves in which it slides ; C, opening ; D cl, spring ; cl’, holes to regulate motion of shutter ; 
e, slit seen through opening ; e, slit behind shutter. 
III. ON THE COINCIDENCES OF SPECTRAL LINES. 
In former references to this subject I have shown: — 
1st. That the lines of metallic spectra do not all extend to the same distance from the 
pole which is emitting the light. In other words, some lines are short, others are long. 
2nd. That in those cases where in the solar spectrum the whole number of lines given 
by a metal do not appear, those that do thus appear are the longest lines. 
3rd. That it was possible to ascertain roughly by the number of lines left in a spectrum 
when a metal was alloyed, the percentage of it existing in the alloy. 
4th. That greater tension of the incandescent vapour giving a spectrum would pro- 
bably increase the number of lines in a spectrum. 
The examination of the various spectra of metals and alloys undertaken in connexion 
with these researches afforded evidence not only of the great impurity of most of the 
metals used, but of the fact that many, if not all, of the coincidences observed by Thalen 
and others might be explained in the light of former work. 
An examination of Angstrom’s map of the solar spectrum shows many cases in which 
a line has been observed to be common to two or more spectra ; and this is especially 
the case with the lines of iron, titanium, and calcium, nearly every other solar metallic 
spectrum exhibiting one or more cases of coincidence with the latter. 
Now it is first to be remarked that these cases of coincidence can scarcely be acci- 
dental. 
In those cases which have been examined, it has frequently been found that a line 
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