56 
DR. W. M. HICKS: A CRITICAL STUDY OF SPECTRAL SERIES. 
but the nature of the tliree lines are different and 1076 is about The V 2 should 
be 542 and the observed is within limits of error. A1 has a weak doublet 
1 . . . . 1857-56 53816-42 
100-66 
3 . . . . 1854-09 53917-08 
The separation is probably witliin error limits of v = 112, but is as it stands also 
about -roi'- The evidence is therefore against the existence of the terms in question, 
but the possil)ility of the lines splitting up into collaterals of smaller intensity, as is 
the case of HgP, sliould be borne in mind. 
If the Cd triplet is correctly assigned, viz., n = 47694, the number to be deducted 
from the s-sequence should be -5275 in place of -5243. This is as much above -5243 
as tliat is above -5222. 
We may now })roceed to consider Pasohen’s Identification of lines for the MgP and 
CaP series in the light of the regularities discovered above. In applying them it is 
necessary to decide on the limits to employ, using, at least at first, IIydberg’s law as 
to their connection. The limit calcidated for MgS from the first three lines gives 
(see Tal)le I.) 39758-18 + 1-12, and from the first four (be., term in 7n"“) 39752-83 + 2-73, 
from wliich Ryduergs law gives (MgSi(l) = 19285-44) respectively 20472-74 and 
20167 - 39 . The value 39752 agrees best with the measurements of the higher orders, 
wliilst 20464-43 is the actual limit calculated from the three first lines of Paschen. 
Taking then S(co) = 39752-83 and P(oo) = 20467-39, the numbers for comparison 
are given in the table above, using for Pi (2) Pasciien’s bolometer reading (7655-3), 
as it gives tlie most favourable comparison. It will be seen that the differences are 
respectively 2739 (138), 1097 (900), 4792 (2741), 7626 (5803). These by rather 
forcing the limits of variation can he made to give typical differences of about, say, 
2700, 2000. A closer agreement can, of course, be found by allowing Rydberg’s law 
to be only approximate. The result of the discussion, therefore, does not contradict 
Pasohen’s identitication. 
In view, liowever, of the suspicion raised above that the difference for Mg and Ca 
might be expected to be less than '5, it will be interesting to see if lines satisfying 
tliis can be found. Out of observed lines, however, I have only been able to find two 
which might fit in with this system, viz., 15768-3 = P 3 ( 2 ), 15759-1 = P 2 ( 2 ). These 
give values of the denominator of -786281 and -786645 with a difference 364, fitting 
in well with that of P 3 (l) and P 2 (l), but there is no appearance of a Pj (2), which 
should have a diflPerence of about 4725 over the denominator of S (l). 
For Ca Paschen gives for the first triplet 19856-9, 19935-8, 19946-8, and for the 
first of the next triplet 9546-8. Using limits on the same basis as for Mg it is found 
the P(oo) l676()-56, S(co) = 33983-45. The first S triplet, which Rydberg’s law 
makes also the first of the P, gives separations 2792 and 1368. Paschen’s first 
triplet gives 2600 and 359, which is not in order. If, however, the line 19917-5 
