438 REPORT— 1904. 
That is a frictional force opposite to the direction of motion. It is proportional 
to the square of the curvature of the path, or to the square of the intensity of the 
transverse magnetic field producing the circular motion. Near the velocity of 
light, when « is very small, the force increases rapidly. 
b. Uniform Motion along an Helix.—Put g=9,+9,, where g, is the constant 
velocity along the axis of the screw, g, the projection of g to a plane perpendicular 
to the axis. Let 7 be the radius of thecircle, described with velocity g,. 
Consider that g=4, is perpendicular both to g, and g,, and that 
2 
§-%=-9,%, 
you get 
4 
§4=0,49= -2; 
hence 
RK ie ~b/ oe fF _§g ah, 
: KT GK 7 
Putting 
: hg, cs =B,, B,°+B,?=P, 
ce ec 
you can write 
RK," = —b4, faite —ba, 92° 1-B,*). 
Rage 
The first term gives a force resisting the rectilinear motion 9g, along the axis 
of the helix, the second term a force resisting the circular motion g,. When the 
electron moves rapidly through a homogeneous magnetic field both forces come 
into play. 
5. Quantitative Determination of the Anomalous Dispersion of Sodiwm 
Vapour.' By Professor R. W. Woop. 
6. On the Dynamical Significance of Kundt’s Law of Anomalous Dispersion. 
Ly Professor J. Larmor, Sec. B.S. 
It was pointed out that the energy of a train of approximately homogeneous 
radiation must be propagated forwards; on the principles of O. Reynolds and 
Rayleigh this requires that the group-velocity must be positive, provided there is 
no absorption. Thus, outside an absorption-band the index of refraction must 
always increase with increasing frequency of the wave-train. Inside the absorp- 
tion-band the argument does not apply, but the curve of dispersion there bends 
round so as to connect the two arcs, both with upward trend, on the two sides of 
the band. These are the features of actual dispersion-curves to which Kundt 
drew attention. 
7. On the Relation of the Rintgen Radiation to Ordinary Light. 
By Professor J. Larmor, Sec. RWS. 
Arguments were offered in support of the view advanced by Sir George Stokes 
(‘ Wilde Lecture,’ Lit. and Phil. Soc., Manchester, 1897) that a single radiant 
pulse, incident on a molecular medium, would not suffer regular refraction. As 
natural radiation consists of a succession of pulses, propagated from molecular 
shocks occurring at the surface of the incandescent solid or liquid radiator, it 
becomes necessary on this view to specify some kind of regularity in the shocks, 
in order to explain refraction and dispersion. It is held that a statistical regu- 
* Appeared in full in the Phil, Mag., viii. p. 293. 
