352 
§ 26. In the case of some gaseous bodies, only one value of 
the refractive index is known (corresponding to one definite wave¬ 
length) and the dispersion has not been investigated. Now from (3) 
and (4), § 15., we have 
^ = 1 \l J _)_ (3tf/2A) 
so that, if we assume that the vA -law holds, the value of Z 0 for 
such gases can be calculated. It is true that the vzl-law has been 
found not to hold in some cases; hence it is evident that in general 
this method of calculating À 0 cannot be relied upon. The equation, 
however, may perhaps be serviceable in suggesting new lines of 
research. 
The following is given for the sake of illustration. The refractive 
indices are taken from measurements due to Sir William Ramsay 
and Dr Morris W. Travers *) and refer to X — 5,893. 
Helium . . . 
v = 1,0000363 . . 
. X 0 = 0,63 
Neon . . . 
1,0000687 . . 
0,86 
Argon . . . 
1,000284 . . 
1,69 
Krypton . . . 
1,000425 . . 
2,03 
Xenon . . . 
1,000689 . . 
2,49 
The following indices have been reduced to 0° C. and 760 millims. 
[see § 23.] from measurements due to Mr Clive Cuthbertson 2 ) and 
to Messrs C. Cuthbertson and E. P. Metcalfe 3 ); 
they are for sodium- 
light throughout. 
Phosphorus . . . 
v = 1,002394 . . 
Ni 
o 
II 
I — 4 
rf* 4 
Zinc . . . 
1,001030 . . 
2,20 
Arsenic . . . 
1,003104 . . 
1,62 
Cadmium . . . 
1,001337 
2,46 
Tellurium . . . 
1,002495 . . 
2,03 
Mercury . . . 
1,000933 . . 
2,25 
It may be well to repeat. 
by w r ay of caution, 
that the values of 
are not given here as plausible theoretical results. The point which 
x ) Phil. Transactions 
Roy. Soc. L. (A). Vol. 
197. p. 47. 1901 (see ib. 
p. 81). 
2 ) Phil. Transactions 
Roy. Soc. L. (A). Vol. 
204. p. 823. 1905. 
3 ) Phil. Transactions 
Roy. Soc. L. (A). Vol. 
207. p. 135. 1907. 
