519 
NATURE 
[AucusT 17, 1899 
to this used by Kayser and Runge, but they found 
that some of their constants varied little from ele- 
ment to element. In that way they not only obtained 
‘the first term of a series, but the whole series throughout 
the entire length of the spectrum, and where observations 
had been made in the case of the different elements they 
could of course check their calculations by the actual 
observations so made, and see how the theory seemed to 
be justified as the work was extended. The first line in 
a series must be considered to be comparable to a funda- 
mental note in music. It represents really the longest 
light wave in the same way that the fundamental note in 
music represents the longest sound wave. Both series of 
wesults, obtained in the way I have described by Kayser and 
Runge and by Rydberg, show us that, in many cases, 
we may be almost certain to obtain from the higgledy- 
piggledy arrangement of the lines in the spectrum of any 
one substance two or three beautiful regular series like 
those that I have already shown you in the case of helium 
and asterium. There is a little difference in the nomen- 
clature employed by the investigators to whom I have 
weferred, as shown in the annexed table. 
Series Nomenc.ature. 
Intensity. Kayser and Runge. Rydberg. 
Strongest Principal series Principal series 
Weaker ... Ist subordinate series | Nebulous series 
Weakest ... 2ndsubordinate series Sharp series 
The strongest lines which they observed at the temper- 
atures they worked with, they put into what they call a 
“* principal series,” and then the weaker lines were dis- 
tributed among other two series. Kayser and Runge 
called them the “first-” and “second-subordinate ” 
series ; Rydberg calls them the “ nebulous-series ” and the 
“sharp-series.” It is important to remember this in case 
you come across any reference to these matters, in order 
that you may see what the exact equivalent is. The 
lines of the principal series almost always reverse them- 
selves very easily indeed—that is to say, that the absorp- 
tion is indicated by them more readily than it is by the 
other lines. Then, when we come to the second sub- 
‘ordinate or sharp series, it is found that these sometimes 
broaden out towards the red end of the spectrum. 
This work, of course, has required considerable in- 
vestigation ; the first attempts were not quite satisfactory, 
‘because the observations on which it was based had 
not been of sufficient accuracy. With greater disper- 
sion it has been found that some of the lines which 
were supposed at first to be single are really double ; so 
that it is quite usual now when we consider this question 
of series to suppose that in some cases the series are 
composed of single lines, in other cases of doubles, and 
in other cases of triplets; and it was at first, indeed, 
imagined that in these differences we were face to face 
with a very important physical difference between the 
various elements, but Rydberg has suggested that possibly 
after all it may be a difference merely in the seeing. 
He says : + 
“The difference between the doubles and triplets is 
‘only relative. This opinion is confirmed by the fact that 
the triplets appear often in the form of doubles, the most 
refrangible component not having sufficient intensity to 
become visible. Further, the relative intensity of the 
components of the doubles seems equal to that of the two 
less refrangible components of the triplets. 
“ For these reasons I have dared to propose the hypo- 
thesis that the two kinds of component rays are of the 
same order, or that the doubles are only triplets of which 
1 Kon. Su. Vet. Ak. Hand., vol. 23, ii. p. 135. 
NO. 1555, VOL. 60] 
the most refrangible component is too feeble to be seen, 
or has perhaps the absolute value of zero. . . .” 
If the lines are more difficult to see, and if the sub-series 
of lines get stronger towards either the red end or the blue 
end, then we are more likely to see one line than two, 
and more likely to see two lines than three. 
I have already referred to the many years old sug- 
gestion that a line is a remnant of a fluting. If you 
could see the whole fluting, you would see what is 
represented in the upper horizon of the diagram ; if you 
Fic. 5.—Diagram illustrating Rydberg s idea of the appearance of 
triplets, doubles, &c. 
could not see the whole of it, you would get what is 
represented in the second horizon, that is to say, a triplet. 
If the third line were very difficult of observation you 
would only see a doublet, and if the inside line were 
weaker than the other you would only see a single line. 
Single lines Doubles Triplets 
i wally... . | . 
Principal Subordinate] Principal |Subordinate| Principal |Subordinate 
series | series series series series series 
Helium Helium Oxygen | Oxygen 
Asterium | Asterium | Hydrogen (?)| Hydrogen | Sulphur | Sulphur 
Lithium (?) Lithium (?) | Selenium | Selenium 
| Sodium Sodium Magnesium 
| Potassium Potassium Calcium 
| Rubidium Strontium 
| Cobalt Zinc 
| Copper Cadmium 
Silver Mercury 
Aluminium 
| Indium | 
Thallium 
| 
There is only a very small number of the chemical 
elements which give us single lines ; in the principal 
series, so far, we only know of helium and asterium: 
in the subordinate series we only know of asterium. The 
number of doubles, you will observe, is very much greater, 
but it is not so great in relation to the principal series as 
it is in the case of the subordinate series; but although 
we have nine elements giving us triplets in the sub- 
ordinate series, we have only three which give them in 
the principal series. 
(To be continued.) 
THE DOVER MEETING OF THE BRITISH 
ASSOCIATION. 
HE final arrangements for this year’s meeting are 
now sufficiently completed for a fairly accurate 
forecast to be made. Whether the meeting will be large 
or small it is still too early to judge, but whether large or 
small it will certainly be a very interesting one. As to 
accommodation in the town, there is little doubt but that 
at the time of meeting ample accommodation will be 
available, though the committee have had great difficulties 
in inducing hotel keepers and lodging-house owners to 
