TRANSACTIONS OF SECTION A. 455 
seen, the wire is in precisely the same position relative to the drift in both cases, 
and we are therefore justified in assuming that its elastic properties remain un- 
altered. Hence we are driven to the conclusion that equality of time periods 
could only be effected by a change of the correct amount in m, The moment of 
inertia would be the same in the two cases; that is, 
mr = const. 
Hence 2mrér +7?dm = 0 
dm _ 98h _ 
m ” 
or 
a.e., the necessary relative change in mass is double the relative change in length. 
Now, in the first case (7.e., when the length of the bar is parallel to the drift), 
the particle is executing vibrations in a direction perpendicular to the drift, and, 
in the second case, it is moving parallel to the drift. In the second case the 
mass is less than in the first, and in order to account for equality of time periods 
it is necessary to suppose that the mass of a particle is greater when moving 
perpendicularly to the ether drift than when moving parallel to it by an amount 
8? times the mass of the particle. 
It will be interesting to examine whether or not this result is in accordance 
with the values which have heen calculated for the transverse and longitudinal 
masses of a moving electron. If m, is the longitudinal mass and m, the transverse 
mass, the above reasoning leads to 
mi 14 [ey 
my 
On referring to Lorentz’s calculation we find 
NE Eee 2. 
m (=e 
and from Abraham’s calculations we obtain 
14+2@ 
co pics Nees 
“te @m=l- “8 
m 1+ = B? 5 
in both cases neglecting powers of 8 higher than the second. In both cases the 
transverse mass is found to he less than the longitudinal mass, and not greater, 
as required for a negative result in the above experiment. In these circum- 
stances it is apparent that either these theories of transverse and longitudinal 
mass do not adequately represent the physical facts, or the experiment suggested 
in this paper is one which would, if practicable, yield a positive result, 
7. On the Nature of Ionisation.! By Professor H. E. Armstrone, F.R.S. 
8. Note on the Echelon Spectroscope and the Resolution of the Green 
Mercwry Line. By H. STansriep. 
An echelon spectroscope constructed by Messrs. Adam Hilger for Professor 
Schuster was described. Photographs of the green mercury line obtained with 
the instrument showed all the known components of the green line and a number 
of fainter components which had not previously been described. 
Results obtained recently by different observers with echelon spectroscopes 
showed good agreement amongst themselves, and agreed with the values for three 
1 See the Hectrician, August 9, 1907, 
