Stoney— 0/? the Interrupted Spectra of Gases. Ill 
lengths in vacuo, which are proportionate to the periodic times. Kow, 
by interpolating between Ketteler's observations^ on the dispersion of 
air, we find — 
f^h = I'OOO 29952 
i«^= 1-000 29685 
= 1-000 29383 
for the refractive indices of air of standard pressure and temperature 
for the rays h, F, and C. From these we deduce that if the air be at 
14° of temperature, the refractive indices will become 
^7, = 1-000 2845 
^,.= 1-000 2820 
^0= 1-000 2791 
Multiplying the foregoing wave-lengths by these values we find for 
the wave-lengths in vacuo, 
h =4102-37, 
i^=4862-ll, 
(7 = 6563-93, 
which are the 32nd, 27th, and 20th harmonics of a fundamental vibra- 
tion, whose wave-length in vacuo is 
0-13127714 of a millimetre, 
as appears from the following table : — 
Observed -wave-lengtlis 
reduced to wave- 
lengths in vacuo. 
Calculated values. 
Differences. 
Xth-metres, 
h =4102-37 
i^=4862-ll 
C= 6563-93 
Xth-metres. 
3-\xl31277-14 = 4102-41 
^Vxl31277-14 = 4862-12 
2^x131277-14 = 6563-86 
Xth-metres. 
+ 0-04 
40-01 
-0-07 
Thus, the outstanding difi'erences are all fractions of an eleventh-metre, 
an eleventh-metre being the limit within whi^^ Angstrom thinks that 
his measures may be depended on. 
The wave-length 0-13127714 of a millimetre corresponds to the 
periodic time 4-4 fourteenth- seconds, if we assume the velocity of 
light to be 298,000,000 metres per second. 
Hence, we may conclude, with a good deal of confidence, that 4-4 
XIYth-seconds, is very nearly the periodic time of one of the motions 
within the molecules of Hydrogen. 
The other harmonics of this fundamental motion in the molecules 
of Hydrogen, viz. : the 19th, 21st, 22nd, &c., harmonics, are not found 
* "Philosophical Magazine," Part ii. for 1866, p. 345. 
